Forces: friction, falling and floating

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

1. Kofi kicks a ball. Which way does the arrow point?

   Tap your answer.

   1. A Back to the foot

      Not this one. The arrow shows the way the force acts on the ball. The kick sends the ball away from the foot. Try another.

   2. B The way the ball is kicked

      Yes, that is right. An arrow points the way the force acts. The kick acts on the ball in the way it moves.

   3. C Up

      Not this one. The kick sends the ball sideways, so the arrow points sideways. Try another.

   4. D Down to the ground

      Not this one. Down is the way the Earth pulls. The kick is a different force, and it acts sideways. Try another.

   See the answer

   The way the ball is kicked

   An arrow points the way the force acts. The kick acts on the ball in the way it moves.

   • If you chose A: The arrow shows the way the force acts on the ball. The kick sends the ball away from the foot.
   • If you chose C: The kick sends the ball sideways, so the arrow points sideways.
   • If you chose D: Down is the way the Earth pulls. The kick is a different force, and it acts sideways.

2. Which of these is a pull?

   Tap your answer.

   1. A A magnet draws a nail towards it

      Yes, that is right. A pull brings a thing closer. The magnet pulls the nail towards itself.

   2. B Wind blows a leaf along

      Not this one. The wind pushes the leaf away from itself. That is a push. Try another.

   3. C A hand squashes clay

      Not this one. The hand pushes on the clay. That is a push. Try another.

   4. D A child kicks a ball

      Not this one. The foot pushes the ball away. That is a push. Try another.

   See the answer

   A magnet draws a nail towards it

   A pull brings a thing closer. The magnet pulls the nail towards itself.

   • If you chose B: The wind pushes the leaf away from itself. That is a push.
   • If you chose C: The hand pushes on the clay. That is a push.
   • If you chose D: The foot pushes the ball away. That is a push.

3. Amina squeezes a sponge. What does her force change?

   Tap your answer.

   1. A Its speed

      Not this one. The sponge does not start to move. Squeezing changes how it looks. Try another.

   2. B Its weight

      Not this one. A squeeze does not change how heavy a dry sponge is. It changes its shape. Try another.

   3. C Its shape

      Yes, that is right. The sponge gets squashed. A force can change the shape of a thing.

   See the answer

   Its shape

   The sponge gets squashed. A force can change the shape of a thing.

   • If you chose A: The sponge does not start to move. Squeezing changes how it looks.
   • If you chose B: A squeeze does not change how heavy a dry sponge is. It changes its shape.

4. Look at the picture. Which push is bigger?

   Tap your answer.

   1. A They are the same

      Not this one. The arrows differ in length, so the forces differ in size. Try another.

   2. B Push B

      Yes, that is right. A longer arrow means a bigger force. The arrow for push B is longer.

   3. C Push A

      Not this one. A short arrow means a smaller force. Try another.

   See the answer

   Push B

   A longer arrow means a bigger force. The arrow for push B is longer.

   • If you chose A: The arrows differ in length, so the forces differ in size.
   • If you chose C: A short arrow means a smaller force.

5. A book lies still on a table. The Earth pulls it down. What else acts on it?

   Tap your answer.

   1. A Nothing. The book is still.

      Not this one. A still thing can have forces on it. The table pushes up and the two forces balance. Try another.

   2. B The book pushes the table, and the table does nothing.

      Not this one. The table pushes back up on the book. That push is why the book does not fall through. Try another.

   3. C The table pushes it up.

      Yes, that is right. The table pushes up as hard as the Earth pulls down. The forces are balanced.

   See the answer

   The table pushes it up.

   The table pushes up as hard as the Earth pulls down. The forces are balanced.

   • If you chose A: A still thing can have forces on it. The table pushes up and the two forces balance.
   • If you chose B: The table pushes back up on the book. That push is why the book does not fall through.

6. Draw a box. Add a push to the right and a pull to the left. Make the pull longer.

   See the answer

   A box. A short arrow points right. A longer arrow points left.

   An arrow points the way the force acts. A longer arrow is a bigger force.

7. A ball rolls straight along the ground. Ravi taps it from the side, and it turns. What did the tap change?

   Tap your answer.

   1. A Its shape

      Not this one. A hard ball keeps its shape when it is tapped. The tap changed the way it moves. Try another.

   2. B Its weight

      Not this one. A tap does not change how heavy the ball is. It changes the way the ball moves. Try another.

   3. C Its direction

      Yes, that is right. The ball moved straight, and the tap made it turn. A force can change the direction of a thing.

   4. D Nothing. Only a kick can change how a ball moves.

      Not this one. Any push or pull can change how a thing moves. Even a tap can. Try another.

   See the answer

   Its direction

   The ball moved straight, and the tap made it turn. A force can change the direction of a thing.

   • If you chose A: A hard ball keeps its shape when it is tapped. The tap changed the way it moves.
   • If you chose B: A tap does not change how heavy the ball is. It changes the way the ball moves.
   • If you chose D: Any push or pull can change how a thing moves. Even a tap can.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

1. Look at the table. How long is the band with 2 stones?

   Tap your answer.

   1. A 2 cm

      Not this one. You read the number of stones, or the empty band. Find the row for 2 stones. Try another.

   2. B 5 cm

      Not this one. That is the length with 3 stones. Find the row for 2 stones. Try another.

   3. C 4 cm

      Yes, that is right. The row for 2 stones says 4 cm.

   4. D 3 cm

      Not this one. That is the length with 1 stone. The first row is 0 stones. So 2 stones is row three. Try another.

   See the answer

   4 cm

   The row for 2 stones says 4 cm.

   • If you chose A: You read the number of stones, or the empty band. Find the row for 2 stones.
   • If you chose B: That is the length with 3 stones. Find the row for 2 stones.
   • If you chose D: That is the length with 1 stone. The first row is 0 stones. So 2 stones is row three.

2. The band is 2 cm long with an empty cup. It is 5 cm long with three stones. How much longer is it with the stones?

   Tap your answer.

   1. A 5 cm

      Not this one. That is the whole length with the stones. Take away the 2 cm it had before. Try another.

   2. B 7 cm

      Not this one. You added the two lengths. Take the short one away from the long one. Try another.

   3. C 2 cm

      Not this one. That is how long the band was at the start. Find how much it grew. Try another.

   4. D 3 cm

      Yes, that is right. The band grew from 2 cm to 5 cm. 5 − 2 = 3 cm.

   See the answer

   3 cm

   The band grew from 2 cm to 5 cm. 5 − 2 = 3 cm.

   • If you chose A: That is the whole length with the stones. Take away the 2 cm it had before.
   • If you chose B: You added the two lengths. Take the short one away from the long one.
   • If you chose C: That is how long the band was at the start. Find how much it grew.

3. In what unit do scientists measure force?

   Tap your answer.

   1. A Newtons

      Yes, that is right. Force is measured in newtons. We write N.

   2. B Kilograms

      Not this one. Kilograms tell how much stuff a thing has. A force is a push or a pull. Try another.

   3. C Centimetres

      Not this one. Centimetres tell how long the band is. That shows the pull, but it is not the unit of force. Try another.

   See the answer

   Newtons

   Force is measured in newtons. We write N.

   • If you chose B: Kilograms tell how much stuff a thing has. A force is a push or a pull.
   • If you chose C: Centimetres tell how long the band is. That shows the pull, but it is not the unit of force.

4. Ravi hangs a pebble in the cup. The band is 3 cm long. A bigger stone makes it 5 cm. Which pull is bigger?

   Tap your answer.

   1. A The pebble pulls more.

      Not this one. The band stretches more for a bigger pull. The longer band shows the bigger pull. Try another.

   2. B They pull the same.

      Not this one. The lengths are different, so the pulls are different. Try another.

   3. C The bigger stone pulls more.

      Yes, that is right. The band is longer with the bigger stone. More stretch means more pull.

   See the answer

   The bigger stone pulls more.

   The band is longer with the bigger stone. More stretch means more pull.

   • If you chose A: The band stretches more for a bigger pull. The longer band shows the bigger pull.
   • If you chose B: The lengths are different, so the pulls are different.

5. From an earlier chapter

   A stone hangs still from a band. The Earth pulls the stone down. What does the band do?

   Tap your answer.

   1. A It pulls the stone up as hard as the Earth pulls down.

      Yes, that is right. The two forces are balanced, so the stone stays still.

   2. B Nothing. The stone is still.

      Not this one. A still thing can have forces on it. They are balanced. Go to S2.5.1. Try another.

   3. C It pushes the stone down.

      Not this one. A stretched band pulls back towards where it started. That is up. Go to S2.5.1. Try another.

   See the answer

   It pulls the stone up as hard as the Earth pulls down.

   The two forces are balanced, so the stone stays still.

   • If you chose B: A still thing can have forces on it. They are balanced. Go to S2.5.1.
   • If you chose C: A stretched band pulls back towards where it started. That is up. Go to S2.5.1.

6. Draw a force meter with two stones in the cup. Write the reading from the table.

   See the answer

   A band hangs from a pencil. Two stones are in the cup. The reading is 4 cm.

   The table says 4 cm for 2 stones.

7. Priya hangs a stone from a force meter. Look at the picture. What does the meter read?

   Tap your answer.

   1. A 3 kg

      Not this one. This meter measures a pull. Its scale is in newtons, N. Kilograms tell how much stuff a thing has. Try another.

   2. B 3 N

      Yes, that is right. The pointer stands at the mark 3. The scale is in newtons. The stone pulls with 3 N.

   3. C 5 N

      Not this one. That is the biggest number on the scale. Read the number at the pointer, not at the end. Try another.

   4. D 0 N

      Not this one. The pointer stands at 0 N when nothing hangs. The stone pulls it down to a bigger number. Try another.

   See the answer

   3 N

   The pointer stands at the mark 3. The scale is in newtons. The stone pulls with 3 N.

   • If you chose A: This meter measures a pull. Its scale is in newtons, N. Kilograms tell how much stuff a thing has.
   • If you chose C: That is the biggest number on the scale. Read the number at the pointer, not at the end.
   • If you chose D: The pointer stands at 0 N when nothing hangs. The stone pulls it down to a bigger number.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

1. Which surface gives the most friction?

   Tap your answer.

   1. A A smooth floor

      Not this one. A smooth surface has fewer bumps. It gives less friction. Try another.

   2. B A rough mat

      Yes, that is right. A rough surface has big bumps that catch. It gives the most friction.

   3. C A sheet of glass

      Not this one. Glass is very smooth. It gives little friction. Try another.

   See the answer

   A rough mat

   A rough surface has big bumps that catch. It gives the most friction.

   • If you chose A: A smooth surface has fewer bumps. It gives less friction.
   • If you chose C: Glass is very smooth. It gives little friction.

2. Look at the picture. How much further did it slide on the floor than on sand?

   Tap your answer.

   1. A 78 cm

      Yes, that is right. It slid 90 cm on the floor. It slid 12 cm on sand. 90 − 12 = 78 cm.

   2. B 102 cm

      Not this one. You added the two slides. Take the short one away from the long one. Try another.

   3. C 90 cm

      Not this one. That is the slide on the floor. Take away the slide on the sand. Try another.

   See the answer

   78 cm

   It slid 90 cm on the floor. It slid 12 cm on sand. 90 − 12 = 78 cm.

   • If you chose B: You added the two slides. Take the short one away from the long one.
   • If you chose C: That is the slide on the floor. Take away the slide on the sand.

3. A ball rolls over grass and stops. What slows it?

   Tap your answer.

   1. A Its push runs out.

      Not this one. On smoother ground the same push goes further. So the ground is slowing it. Try another.

   2. B The ball gets tired.

      Not this one. A ball is not alive. A force slows it. Try another.

   3. C Friction from the grass, and the air

      Yes, that is right. The grass rubs against the ball, and the air pushes against it. These forces slow it.

   See the answer

   Friction from the grass, and the air

   The grass rubs against the ball, and the air pushes against it. These forces slow it.

   • If you chose A: On smoother ground the same push goes further. So the ground is slowing it.
   • If you chose B: A ball is not alive. A force slows it.

4. Why does your shoe not slip when you walk?

   Tap your answer.

   1. A The air holds the shoe.

      Not this one. The air pushes on you a little. The ground grips your shoe. Try another.

   2. B Friction between the shoe and the ground

      Yes, that is right. Your shoe pushes back on the ground. Friction holds the shoe, so you move forward.

   3. C Friction makes us slip.

      Not this one. Friction stops slipping. On a wet floor there is little friction. That is when you slip. Try another.

   See the answer

   Friction between the shoe and the ground

   Your shoe pushes back on the ground. Friction holds the shoe, so you move forward.

   • If you chose A: The air pushes on you a little. The ground grips your shoe.
   • If you chose C: Friction stops slipping. On a wet floor there is little friction. That is when you slip.

5. From an earlier chapter

   Amina pulls a box with her force meter. What unit would a science book use?

   Tap your answer.

   1. A Kilograms

      Not this one. Kilograms tell how much stuff a thing has. Force is measured in newtons. Go to S2.5.2. Try another.

   2. B Metres

      Not this one. Metres measure length. Force is measured in newtons. Go to S2.5.2. Try another.

   3. C Newtons

      Yes, that is right. Force is measured in newtons. We write N.

   See the answer

   Newtons

   Force is measured in newtons. We write N.

   • If you chose A: Kilograms tell how much stuff a thing has. Force is measured in newtons. Go to S2.5.2.
   • If you chose B: Metres measure length. Force is measured in newtons. Go to S2.5.2.

6. Draw a box sliding to the right. Draw the friction arrow.

   See the answer

   A box with an arrow pointing left, labelled friction.

   Friction acts against the slide. The box slides right, so the arrow points left.

7. Look at the two surfaces, close up. Which block is harder to slide?

   Tap your answer.

   1. A The block on the rough surface

      Yes, that is right. The big bumps of a rough surface catch on the block. That gives more friction, so the block is harder to slide.

   2. B The block on the smooth surface

      Not this one. A smooth surface has tiny bumps, so little catches. That block slides more easily. Try another.

   3. C Both are the same, because both are blocks.

      Not this one. The surfaces differ, so the friction differs. Big bumps catch more than tiny bumps. Try another.

   See the answer

   The block on the rough surface

   The big bumps of a rough surface catch on the block. That gives more friction, so the block is harder to slide.

   • If you chose B: A smooth surface has tiny bumps, so little catches. That block slides more easily.
   • If you chose C: The surfaces differ, so the friction differs. Big bumps catch more than tiny bumps.

8. Which sentence about friction is true?

   Tap your answer.

   1. A Friction is always bad, so we should get rid of it.

      Not this one. We need friction to walk, hold a cup and stop a bicycle. Try another.

   2. B Friction helps us to walk and to hold things.

      Yes, that is right. Friction lets a shoe grip the ground. It lets a hand grip a cup. It also slows things down.

   3. C Friction happens only when a thing moves fast.

      Not this one. Friction acts when two surfaces rub or try to slide. It acts at any speed. Try another.

   4. D Friction pushes a thing the way it slides.

      Not this one. Friction acts against the slide. It does not help the thing along. Try another.

   See the answer

   Friction helps us to walk and to hold things.

   Friction lets a shoe grip the ground. It lets a hand grip a cup. It also slows things down.

   • If you chose A: We need friction to walk, hold a cup and stop a bicycle.
   • If you chose C: Friction acts when two surfaces rub or try to slide. It acts at any speed.
   • If you chose D: Friction acts against the slide. It does not help the thing along.

9. Look at the picture. A bottle top slid down the same ramp onto three surfaces. Which surface gave the most friction?

   Tap your answer.

   1. A floor

      Not this one. The top slid furthest on the floor. A long slide means little friction. Try another.

   2. B cloth

      Not this one. The cloth is in the middle. One surface slowed the top even more. Try another.

   3. C They are all the same

      Not this one. The slides are different lengths, so the friction is different. Try another.

   4. D sand

      Yes, that is right. The top slid the shortest way on sand. The surface that stops it soonest gives the most friction.

   See the answer

   sand

   The top slid the shortest way on sand. The surface that stops it soonest gives the most friction.

   • If you chose A: The top slid furthest on the floor. A long slide means little friction.
   • If you chose B: The cloth is in the middle. One surface slowed the top even more.
   • If you chose C: The slides are different lengths, so the friction is different.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

1. Which gives less friction under a heavy box?

   Tap your answer.

   1. A A rough mat

      Not this one. A rough mat catches the box. It gives more friction. Try another.

   2. B Round rollers

      Yes, that is right. Rolling rubs much less than sliding. Rollers give less friction.

   3. C A layer of sand

      Not this one. Sand makes a surface rougher. It gives more friction. Try another.

   See the answer

   Round rollers

   Rolling rubs much less than sliding. Rollers give less friction.

   • If you chose A: A rough mat catches the box. It gives more friction.
   • If you chose C: Sand makes a surface rougher. It gives more friction.

2. Why does a bicycle chain get a drop of oil?

   Tap your answer.

   1. A The oil makes the chain grip the wheel.

      Not this one. Oil does the opposite. It makes surfaces slide, so there is less friction. Try another.

   2. B The oil makes more heat.

      Not this one. Friction makes heat. Oil gives less friction, so less heat. Go to S2.5.3. Try another.

   3. C The oil gives less friction between the moving parts.

      Yes, that is right. The oil lets the parts slide past each other. There is less friction.

   See the answer

   The oil gives less friction between the moving parts.

   The oil lets the parts slide past each other. There is less friction.

   • If you chose A: Oil does the opposite. It makes surfaces slide, so there is less friction.
   • If you chose B: Friction makes heat. Oil gives less friction, so less heat. Go to S2.5.3.

3. Why do boots for mud have grooves?

   Tap your answer.

   1. A Mud goes into the grooves, so the rubber can grip.

      Yes, that is right. Mud and water move into the grooves. The rubber touches the ground and grips.

   2. B The grooves make the boot smooth.

      Not this one. Grooves are gaps. They give mud and water a place to go. Try another.

   3. C The grooves make the boot lighter.

      Not this one. The grooves are there for grip, not for weight. Try another.

   See the answer

   Mud goes into the grooves, so the rubber can grip.

   Mud and water move into the grooves. The rubber touches the ground and grips.

   • If you chose B: Grooves are gaps. They give mud and water a place to go.
   • If you chose C: The grooves are there for grip, not for weight.

4. Look at the picture. Which row has the least friction?

   Tap your answer.

   1. A sliding

      Not this one. The friction arrow in this row is the longest. It has the most friction. Try another.

   2. B oil

      Not this one. The oil row has a short arrow. The rollers row has a shorter one. Try another.

   3. C They are the same

      Not this one. The arrows differ in length, so the friction differs. Try another.

   4. D rollers

      Yes, that is right. The friction arrow is the shortest in the rollers row.

   See the answer

   rollers

   The friction arrow is the shortest in the rollers row.

   • If you chose A: The friction arrow in this row is the longest. It has the most friction.
   • If you chose B: The oil row has a short arrow. The rollers row has a shorter one.
   • If you chose C: The arrows differ in length, so the friction differs.

5. From an earlier chapter

   A box slides to the right and stops. Which way does the friction arrow point?

   Tap your answer.

   1. A Right, with the slide

      Not this one. Friction acts against the slide. It does not help the box along. Go to S2.5.3. Try another.

   2. B Left, against the slide

      Yes, that is right. Friction pushes against the way a thing slides. The box slides right, so friction points left.

   3. C Up

      Not this one. Friction acts along the surface, against the way the box slides. Go to S2.5.3. Try another.

   See the answer

   Left, against the slide

   Friction pushes against the way a thing slides. The box slides right, so friction points left.

   • If you chose A: Friction acts against the slide. It does not help the box along. Go to S2.5.3.
   • If you chose C: Friction acts along the surface, against the way the box slides. Go to S2.5.3.

6. Write two ways to get less friction. Write two ways to get more.

   See the answer

   Less: oil, or wheels and rollers. More: grooved soles, or sand on a slippery path.

   Oil and rollers help things slide. Grooves and sand help things grip.

7. Why is a wet, smooth floor slippery?

   Tap your answer.

   1. A The floor is cold, so the shoe slides.

      Not this one. Cold is not the reason. The water between shoe and floor gives less friction. Try another.

   2. B The water makes the floor rough.

      Not this one. A film of water makes a floor smoother, not rougher. Try another.

   3. C The water makes the shoe stick.

      Not this one. It is the other way round. Water gets between shoe and floor. The shoe slides. Try another.

   4. D Water between the shoe and the floor gives less friction.

      Yes, that is right. A film of water lets the shoe slide. With less friction your foot slips.

   See the answer

   Water between the shoe and the floor gives less friction.

   A film of water lets the shoe slide. With less friction your foot slips.

   • If you chose A: Cold is not the reason. The water between shoe and floor gives less friction.
   • If you chose B: A film of water makes a floor smoother, not rougher.
   • If you chose C: It is the other way round. Water gets between shoe and floor. The shoe slides.

8. A climber puts chalk on her hands. Why?

   Tap your answer.

   1. A Chalk gives the hands more grip.

      Yes, that is right. Chalk gives more friction between the hands and the rock. The hands do not slip.

   2. B Chalk makes the hands slide more easily.

      Not this one. Chalk is not oil. It helps the hands hold on without slipping. Try another.

   3. C Chalk makes the hands wet, so they stick.

      Not this one. Chalk dries the hands. Sweat is slippery. Dry hands grip the rock better. Try another.

   See the answer

   Chalk gives the hands more grip.

   Chalk gives more friction between the hands and the rock. The hands do not slip.

   • If you chose B: Chalk is not oil. It helps the hands hold on without slipping.
   • If you chose C: Chalk dries the hands. Sweat is slippery. Dry hands grip the rock better.

9. Look at the picture. Leila walks on a rubber mat and then on a wet floor. Where is she more likely to slip?

   Tap your answer.

   1. A On the rubber mat

      Not this one. The friction arrow is long on the mat. A mat gives a lot of friction, so the shoe grips. Try another.

   2. B On the wet floor

      Yes, that is right. The friction arrow is the shortest on the wet floor. With little friction a shoe slips.

   3. C She is as likely to slip on one as on the other.

      Not this one. The arrows differ in length, so the friction differs. Try another.

   See the answer

   On the wet floor

   The friction arrow is the shortest on the wet floor. With little friction a shoe slips.

   • If you chose A: The friction arrow is long on the mat. A mat gives a lot of friction, so the shoe grips.
   • If you chose C: The arrows differ in length, so the friction differs.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

1. Which way does gravity pull things?

   Tap your answer.

   1. A Away from the Earth

      Not this one. Things fall to the Earth. They do not float away from it. Gravity pulls in. Try another.

   2. B Towards the Sun

      Not this one. The Sun is very far away. The Earth pulls the things near it much harder. Try another.

   3. C Towards the centre of the Earth

      Yes, that is right. Wherever you stand, gravity pulls you towards the middle of the Earth.

   See the answer

   Towards the centre of the Earth

   Wherever you stand, gravity pulls you towards the middle of the Earth.

   • If you chose A: Things fall to the Earth. They do not float away from it. Gravity pulls in.
   • If you chose B: The Sun is very far away. The Earth pulls the things near it much harder.

2. Mei lives on the far side of the Earth. Why does she not fall off?

   Tap your answer.

   1. A Gravity pulls her towards the centre of the Earth too.

      Yes, that is right. For everyone on the Earth, down means towards the centre.

   2. B She holds on tight to the ground.

      Not this one. Holding on is not what keeps her there. Gravity pulls her to the Earth. Try another.

   3. C The air above her pushes her down.

      Not this one. Air does not hold her down. The pull of the Earth does. Try another.

   See the answer

   Gravity pulls her towards the centre of the Earth too.

   For everyone on the Earth, down means towards the centre.

   • If you chose B: Holding on is not what keeps her there. Gravity pulls her to the Earth.
   • If you chose C: Air does not hold her down. The pull of the Earth does.

3. What is weight?

   Tap your answer.

   1. A How much room a thing takes up

      Not this one. That is its size. Weight is a pull, a force. Try another.

   2. B The pull of gravity on a thing

      Yes, that is right. Weight is a force. It is the pull of gravity on a thing.

   3. C How fast a thing falls

      Not this one. Speed is how fast. Weight is the pull that makes it fall. Try another.

   See the answer

   The pull of gravity on a thing

   Weight is a force. It is the pull of gravity on a thing.

   • If you chose A: That is its size. Weight is a pull, a force.
   • If you chose C: Speed is how fast. Weight is the pull that makes it fall.

4. Amina drops a big stone and a small stone. She drops both from the same height. What happens?

   Tap your answer.

   1. A The big stone lands first.

      Not this one. The big stone is pulled harder. But it has more to move. They land together. Try another.

   2. B The small stone lands first.

      Not this one. Small things do not fall faster. Both stones land together. Try another.

   3. C It depends on how heavy the stones are.

      Not this one. Stones of any weight fall together. Weight does not change how fast they fall. Try another.

   4. D They land together.

      Yes, that is right. Big and small stones fall at the same rate. Try it on soft ground.

   See the answer

   They land together.

   Big and small stones fall at the same rate. Try it on soft ground.

   • If you chose A: The big stone is pulled harder. But it has more to move. They land together.
   • If you chose B: Small things do not fall faster. Both stones land together.
   • If you chose C: Stones of any weight fall together. Weight does not change how fast they fall.

5. A stone weighs 12 newtons on the Earth. The Moon pulls about 1/6 as hard. About how many newtons does it weigh on the Moon?

   See the answer

   2 newtons

   The Moon pulls about one sixth as hard. So 12 divided by 6 gives 2. The stone is still the same amount of stuff.

6. From an earlier chapter

   Which unit do scientists use to measure a force such as weight?

   Tap your answer.

   1. A The centimetre

      Not this one. A centimetre measures length. A force is measured in newtons. Go to S2.5.2. Try another.

   2. B The litre

      Not this one. A litre measures how much liquid. A force is measured in newtons. Go to S2.5.2. Try another.

   3. C The kilogram

      Not this one. A kilogram tells how much stuff is in a thing. A force is measured in newtons. Go to S2.5.2. Try another.

   4. D The newton

      Yes, that is right. Forces are measured in newtons. Weight is a force.

   See the answer

   The newton

   Forces are measured in newtons. Weight is a force.

   • If you chose A: A centimetre measures length. A force is measured in newtons. Go to S2.5.2.
   • If you chose B: A litre measures how much liquid. A force is measured in newtons. Go to S2.5.2.
   • If you chose C: A kilogram tells how much stuff is in a thing. A force is measured in newtons. Go to S2.5.2.

7. There is no air on the Moon. Does gravity pull a ball dropped there?

   Tap your answer.

   1. A No. Gravity needs air to work.

      Not this one. Gravity does not need air. The Moon pulls a ball down, with no air at all. Try another.

   2. B No. Only the Earth has gravity.

      Not this one. Every big world has gravity, and so does the Moon. Its pull is weaker than the Earth's. Try another.

   3. C Yes, but the pull is weaker than on the Earth.

      Yes, that is right. Gravity pulls things with or without air. The Moon is smaller than the Earth, so its pull is weaker.

   4. D Yes, and it is just as strong as on the Earth.

      Not this one. The Moon pulls about one sixth as hard as the Earth. A stone weighs less there. Try another.

   See the answer

   Yes, but the pull is weaker than on the Earth.

   Gravity pulls things with or without air. The Moon is smaller than the Earth, so its pull is weaker.

   • If you chose A: Gravity does not need air. The Moon pulls a ball down, with no air at all.
   • If you chose B: Every big world has gravity, and so does the Moon. Its pull is weaker than the Earth's.
   • If you chose D: The Moon pulls about one sixth as hard as the Earth. A stone weighs less there.

8. Kofi hangs a stone from a force meter and keeps it still. Look at the picture. What is the weight of the stone?

   Tap your answer.

   1. A 4 N

      Yes, that is right. The pointer stands at 4 and the scale is in newtons. The weight of the stone is 4 N.

   2. B 4 kg

      Not this one. Weight is a force, so it is read in newtons. Kilograms tell how much stuff is in the stone. Try another.

   3. C 0 N

      Not this one. A still stone still has forces on it. They are balanced. Gravity pulls it down with the number on the scale. Try another.

   4. D 8 N

      Not this one. The meter pulls up and gravity pulls down with equal forces. They are not added. The weight is the one pull, 4 N. Try another.

   See the answer

   4 N

   The pointer stands at 4 and the scale is in newtons. The weight of the stone is 4 N.

   • If you chose B: Weight is a force, so it is read in newtons. Kilograms tell how much stuff is in the stone.
   • If you chose C: A still stone still has forces on it. They are balanced. Gravity pulls it down with the number on the scale.
   • If you chose D: The meter pulls up and gravity pulls down with equal forces. They are not added. The weight is the one pull, 4 N.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

1. Why does flat paper fall more slowly than a paper ball?

   Tap your answer.

   1. A More of the sheet meets air. So air pushes up more.

      Yes, that is right. The sheet and the ball weigh the same. The sheet meets more air, so it falls more slowly.

   2. B Gravity pulls a flat sheet less.

      Not this one. It is the same paper. Gravity pulls it the same, flat or crumpled. Go to S2.5.5. Try another.

   3. C The paper ball is heavier.

      Not this one. Crumpling does not change the weight. The ball just meets less air. Try another.

   See the answer

   More of the sheet meets air. So air pushes up more.

   The sheet and the ball weigh the same. The sheet meets more air, so it falls more slowly.

   • If you chose B: It is the same paper. Gravity pulls it the same, flat or crumpled. Go to S2.5.5.
   • If you chose C: Crumpling does not change the weight. The ball just meets less air.

2. Mei makes two paper spinners. One has long wings. One has short wings. Which falls more slowly?

   Tap your answer.

   1. A The one with short wings

      Not this one. Short wings meet less air. The air pushes up less, so it falls faster. Try another.

   2. B The one with long wings

      Yes, that is right. Long wings meet more air. The air pushes up harder, so the spinner falls more slowly.

   3. C They fall the same

      Not this one. Wing length changes how much air they meet. Make two and test them. Try another.

   See the answer

   The one with long wings

   Long wings meet more air. The air pushes up harder, so the spinner falls more slowly.

   • If you chose A: Short wings meet less air. The air pushes up less, so it falls faster.
   • If you chose C: Wing length changes how much air they meet. Make two and test them.

3. You push your flat hand through air. Then you push it through water. Where is the push back bigger?

   Tap your answer.

   1. A In air

      Not this one. You can feel air push, but water pushes back much harder. Try another.

   2. B It is the same in both

      Not this one. Water resistance is much stronger than air resistance. Try another.

   3. C Neither pushes back.

      Not this one. Air is real stuff. Swing a flat card fast. You feel it push on your hand. Try another.

   4. D In water

      Yes, that is right. Water resistance is stronger than air resistance. You feel it at once.

   See the answer

   In water

   Water resistance is stronger than air resistance. You feel it at once.

   • If you chose A: You can feel air push, but water pushes back much harder.
   • If you chose B: Water resistance is much stronger than air resistance.
   • If you chose C: Air is real stuff. Swing a flat card fast. You feel it push on your hand.

4. Why do fish have smooth, pointed shapes?

   Tap your answer.

   1. A Gravity pulls a pointed shape less.

      Not this one. Gravity does not care about shape. Water resistance does. Go to S2.5.5. Try another.

   2. B A pointed shape makes water push back harder.

      Not this one. It is the other way round. A flat front meets more water and gets pushed back harder. Try another.

   3. C Water slides past them, so the push back is small.

      Yes, that is right. A pointed front meets less water. The water resistance is smaller. A fish moves on with less effort.

   See the answer

   Water slides past them, so the push back is small.

   A pointed front meets less water. The water resistance is smaller. A fish moves on with less effort.

   • If you chose A: Gravity does not care about shape. Water resistance does. Go to S2.5.5.
   • If you chose B: It is the other way round. A flat front meets more water and gets pushed back harder.

5. From an earlier chapter

   A bottle top slides on a smooth floor. It slides on a rough mat. Which makes it stop sooner?

   Tap your answer.

   1. A The rough mat

      Yes, that is right. A rough surface gives more friction. More friction stops the bottle top sooner.

   2. B The smooth floor

      Not this one. A smooth floor gives less friction. The top slides further. Go to S2.5.3. Try another.

   3. C Both stop in the same place

      Not this one. Rough surfaces give more friction than smooth ones. The mat stops it sooner. Go to S2.5.3. Try another.

   See the answer

   The rough mat

   A rough surface gives more friction. More friction stops the bottle top sooner.

   • If you chose B: A smooth floor gives less friction. The top slides further. Go to S2.5.3.
   • If you chose C: Rough surfaces give more friction than smooth ones. The mat stops it sooner. Go to S2.5.3.

6. Draw a flat sheet of paper falling. Draw an arrow for gravity and an arrow for air resistance.

   See the answer

   A sheet with an arrow down for gravity. A long arrow up for the air. It is almost as long as the gravity arrow.

   Gravity pulls the sheet down. The air pushes up against it, so it falls slowly.

7. Why does a parachute slow a fall?

   Tap your answer.

   1. A Gravity pulls less on a person with a parachute.

      Not this one. Gravity pulls the same. The wide canopy is what changes the fall. Go to S2.5.5. Try another.

   2. B The wide canopy meets lots of air, so air pushes up hard.

      Yes, that is right. A wide canopy meets much more air than a person alone. The air resistance is big, so the fall is slow.

   3. C A parachute is lighter than air.

      Not this one. A parachute still falls, so it is not lighter than air. It meets a lot of air, and the air pushes up. Try another.

   See the answer

   The wide canopy meets lots of air, so air pushes up hard.

   A wide canopy meets much more air than a person alone. The air resistance is big, so the fall is slow.

   • If you chose A: Gravity pulls the same. The wide canopy is what changes the fall. Go to S2.5.5.
   • If you chose C: A parachute still falls, so it is not lighter than air. It meets a lot of air, and the air pushes up.

8. A parachutist falls at a steady speed. The picture shows only the weight. What does the air do?

   Tap your answer.

   1. A Nothing. Air cannot push on a parachute.

      Not this one. Air is real stuff. A wide canopy meets a lot of it. The air pushes back. Try another.

   2. B It pushes down, so the parachutist falls faster.

      Not this one. Air resistance acts against the way a thing moves. The parachutist moves down, so it pushes up. Try another.

   3. C It pushes up much harder than the weight pulls down.

      Not this one. Then the parachutist would be slowed more and more. A steady speed needs the two forces equal. Try another.

   4. D It pushes up as hard as the weight pulls down.

      Yes, that is right. A steady speed means the forces are balanced. The air pushes up as hard as gravity pulls down.

   See the answer

   It pushes up as hard as the weight pulls down.

   A steady speed means the forces are balanced. The air pushes up as hard as gravity pulls down.

   • If you chose A: Air is real stuff. A wide canopy meets a lot of it. The air pushes back.
   • If you chose B: Air resistance acts against the way a thing moves. The parachutist moves down, so it pushes up.
   • If you chose C: Then the parachutist would be slowed more and more. A steady speed needs the two forces equal.

9. A swimmer moves through water. Look at the picture. Which way does the water push on the swimmer?

   Tap your answer.

   1. A Forwards, the way the swimmer moves

      Not this one. Water does not carry a swimmer along here. It pushes against the movement. Try another.

   2. B Up, towards the sky

      Not this one. Water does push up on a swimmer. Water resistance acts against the movement. Try another.

   3. C Backwards, against the way the swimmer moves

      Yes, that is right. Water resistance pushes against the way a thing moves. The swimmer moves forwards, so the push is backwards.

   4. D Down, towards the bottom

      Not this one. Down is the way the Earth pulls. Water resistance acts against the movement. Try another.

   See the answer

   Backwards, against the way the swimmer moves

   Water resistance pushes against the way a thing moves. The swimmer moves forwards, so the push is backwards.

   • If you chose A: Water does not carry a swimmer along here. It pushes against the movement.
   • If you chose B: Water does push up on a swimmer. Water resistance acts against the movement.
   • If you chose D: Down is the way the Earth pulls. Water resistance acts against the movement.

10. A feather and a stone fall from the same height. The stone lands first. Why?

    Tap your answer.

    1. A Gravity does not pull on a feather.

       Not this one. Gravity pulls on everything. The feather falls more slowly because the air slows it a lot. Try another.

    2. B The air slows the feather much more than it slows the stone.

       Yes, that is right. A feather is light and wide. The air slows it a lot. A stone is hardly slowed.

    3. C Heavy things always fall faster.

       Not this one. A big stone and a small stone land together. The feather is slowed by air, not just by its weight. Try another.

    See the answer

    The air slows the feather much more than it slows the stone.

    A feather is light and wide. The air slows it a lot. A stone is hardly slowed.

    • If you chose A: Gravity pulls on everything. The feather falls more slowly because the air slows it a lot.
    • If you chose C: A big stone and a small stone land together. The feather is slowed by air, not just by its weight.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

1. A big log and a small stone go into water. What happens?

   Tap your answer.

   1. A Both sink, because both are heavy.

      Not this one. A log is heavy, but it floats. What a thing is made of matters, not weight alone. Try another.

   2. B The log floats and the stone sinks.

      Yes, that is right. Wood floats. Stone sinks. Weight alone does not decide.

   3. C The stone floats, because it is lighter.

      Not this one. The stone is lighter than the log, but it still sinks. Light things can sink. Try another.

   See the answer

   The log floats and the stone sinks.

   Wood floats. Stone sinks. Weight alone does not decide.

   • If you chose A: A log is heavy, but it floats. What a thing is made of matters, not weight alone.
   • If you chose C: The stone is lighter than the log, but it still sinks. Light things can sink.

2. You push a closed empty bottle under water and let go. It pops up. Why?

   Tap your answer.

   1. A The water pushes it up harder than its weight pulls down.

      Yes, that is right. The upthrust is bigger than the weight. So the bottle rises until it floats.

   2. B Gravity stops pulling it under water.

      Not this one. Gravity still pulls it. The upthrust from the water is bigger. Go to S2.5.5. Try another.

   3. C The air above the water pushes it up.

      Not this one. The push comes from the water, not the air. Try another.

   See the answer

   The water pushes it up harder than its weight pulls down.

   The upthrust is bigger than the weight. So the bottle rises until it floats.

   • If you chose B: Gravity still pulls it. The upthrust from the water is bigger. Go to S2.5.5.
   • If you chose C: The push comes from the water, not the air.

3. A block of wood floats and stays still. What is true?

   Tap your answer.

   1. A The upthrust is bigger than its weight.

      Not this one. Then the block would rise. It stays still, so the two forces match. Try another.

   2. B The upthrust is smaller than its weight.

      Not this one. Then the block would sink. It floats, so the upthrust matches its weight. Try another.

   3. C There is no upthrust, because the block floats.

      Not this one. Water pushes up on a floating block. That push is what holds it up. Try another.

   4. D The upthrust matches its weight.

      Yes, that is right. A floating thing stays still. The push up equals the pull down.

   See the answer

   The upthrust matches its weight.

   A floating thing stays still. The push up equals the pull down.

   • If you chose A: Then the block would rise. It stays still, so the two forces match.
   • If you chose B: Then the block would sink. It floats, so the upthrust matches its weight.
   • If you chose C: Water pushes up on a floating block. That push is what holds it up.

4. Mei tests 12 things in water. 7 of them float. The rest sink. How many sink?

   See the answer

   5

   The rest sink, so 12 − 7 = 5.

5. From an earlier chapter

   A small apple weighs about 1 newton. Kofi holds a bag of 8 small apples. About how many newtons is the weight of the bag?

   See the answer

   8 newtons

   Each apple weighs about 1 newton. Eight apples weigh about 8 newtons.

6. Draw a stick floating in water. Draw two arrows on it, one for weight and one for upthrust.

   See the answer

   A stick in the top of the water. An arrow up and an arrow down, the same length.

   A floating stick stays still. The upthrust up equals the weight down.

7. A big log floats. A big stone sinks. A tiny pin sinks. What does this show?

   Tap your answer.

   1. A Heavy things float and light things sink.

      Not this one. The big stone is heavy, and it sinks. Weight alone does not decide. Try another.

   2. B Big things float and small things sink.

      Not this one. The big stone is big, and it sinks. Size alone does not decide. Try another.

   3. C Weight alone does not decide. What a thing is made of matters.

      Yes, that is right. A heavy log floats and a light pin sinks. Weight is not all. What it is made of matters, and so does its shape.

   4. D The log is lighter than the pin.

      Not this one. The log is much heavier than the pin. It floats because of what it is made of, and its shape. Try another.

   See the answer

   Weight alone does not decide. What a thing is made of matters.

   A heavy log floats and a light pin sinks. Weight is not all. What it is made of matters, and so does its shape.

   • If you chose A: The big stone is heavy, and it sinks. Weight alone does not decide.
   • If you chose B: The big stone is big, and it sinks. Size alone does not decide.
   • If you chose D: The log is much heavier than the pin. It floats because of what it is made of, and its shape.

8. A block of wood floats. Look at the picture. How big is the upthrust on it?

   Tap your answer.

   1. A 4 N

      Yes, that is right. A floating block stays still, so the upthrust matches the weight. The weight is 4 N, so the upthrust is 4 N.

   2. B 0 N

      Not this one. Water pushes up on a floating block. With no push the block would sink. Try another.

   3. C 8 N

      Not this one. The upthrust matches the weight. It is not doubled. Try another.

   4. D 2 N

      Not this one. Part of the block is above the water. The upthrust still matches the whole weight. With less, the block would sink deeper. Try another.

   See the answer

   4 N

   A floating block stays still, so the upthrust matches the weight. The weight is 4 N, so the upthrust is 4 N.

   • If you chose B: Water pushes up on a floating block. With no push the block would sink.
   • If you chose C: The upthrust matches the weight. It is not doubled.
   • If you chose D: Part of the block is above the water. The upthrust still matches the whole weight. With less, the block would sink deeper.

9. A stone is let go under water. Look at the picture. What happens to it?

   Tap your answer.

   1. A It floats.

      Not this one. A thing floats when the upthrust matches its weight. Here the upthrust is smaller. Try another.

   2. B It sinks, because its weight is bigger than the upthrust.

      Yes, that is right. The pull down is bigger than the push up. So the stone goes down.

   3. C It stays still in the middle.

      Not this one. It would stay still only if the two forces were equal. Try another.

   See the answer

   It sinks, because its weight is bigger than the upthrust.

   The pull down is bigger than the push up. So the stone goes down.

   • If you chose A: A thing floats when the upthrust matches its weight. Here the upthrust is smaller.
   • If you chose C: It would stay still only if the two forces were equal.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

1. A clay lump sinks. The same clay shaped as a boat floats. Why?

   Tap your answer.

   1. A The boat pushes aside more water, so the water pushes up harder.

      Yes, that is right. The clay weighs the same either way. The hollow boat gets a bigger push up, so it floats.

   2. B The boat weighs less than the lump.

      Not this one. It is the same clay, so the weight is the same. Only the shape changed. Try another.

   3. C Water pushes up on the boat, but not on the lump.

      Not this one. Water pushes up on both. On the lump the push is too small. Go to S2.5.7. Try another.

   See the answer

   The boat pushes aside more water, so the water pushes up harder.

   The clay weighs the same either way. The hollow boat gets a bigger push up, so it floats.

   • If you chose B: It is the same clay, so the weight is the same. Only the shape changed.
   • If you chose C: Water pushes up on both. On the lump the push is too small. Go to S2.5.7.

2. Why does a big steel ship float?

   Tap your answer.

   1. A Steel is lighter than water.

      Not this one. A steel nail sinks. The hollow shape lets the ship float. Try another.

   2. B It is hollow, so it pushes aside a lot of water.

      Yes, that is right. The water it pushes aside weighs as much as the whole ship. So the ship floats.

   3. C Big things float.

      Not this one. A big stone sinks. Size alone does not decide. Go to S2.5.7. Try another.

   See the answer

   It is hollow, so it pushes aside a lot of water.

   The water it pushes aside weighs as much as the whole ship. So the ship floats.

   • If you chose A: A steel nail sinks. The hollow shape lets the ship float.
   • If you chose C: A big stone sinks. Size alone does not decide. Go to S2.5.7.

3. Two clay boats are alike, but one has higher sides. Which holds more beans before it sinks?

   Tap your answer.

   1. A The boat with lower sides

      Not this one. Water pours over low sides sooner. The boat fills and sinks. Try another.

   2. B They hold the same

      Not this one. Shape changes how much a boat holds. Higher sides keep water out for longer. Try another.

   3. C The boat with higher sides

      Yes, that is right. A boat with higher sides can sit deeper before water pours in. It pushes aside more water, so it can hold more.

   See the answer

   The boat with higher sides

   A boat with higher sides can sit deeper before water pours in. It pushes aside more water, so it can hold more.

   • If you chose A: Water pours over low sides sooner. The boat fills and sinks.
   • If you chose B: Shape changes how much a boat holds. Higher sides keep water out for longer.

4. Ana's clay boat holds 18 beans before it sinks. Ravi's holds 24. How many more beans does Ravi's boat hold?

   See the answer

   6 more beans

   Take 18 from 24. That leaves 6.

5. From an earlier chapter

   Kofi pushes a heavy box over a floor. Which change gives less friction?

   Tap your answer.

   1. A Put a rough mat under the box.

      Not this one. A rough mat gives more friction, not less. Rolling rubs less than sliding. Go to S2.5.4. Try another.

   2. B Spread sand under the box.

      Not this one. Sand makes the floor rougher. It gives more friction, not less. Go to S2.5.4. Try another.

   3. C Put the box on round pencils.

      Yes, that is right. Rolling rubs much less than sliding. The pencils work as rollers.

   See the answer

   Put the box on round pencils.

   Rolling rubs much less than sliding. The pencils work as rollers.

   • If you chose A: A rough mat gives more friction, not less. Rolling rubs less than sliding. Go to S2.5.4.
   • If you chose B: Sand makes the floor rougher. It gives more friction, not less. Go to S2.5.4.

6. Draw a boat floating on water. Draw an arrow for weight and an arrow for upthrust.

   See the answer

   A boat on the water. An arrow down for weight. An arrow up for upthrust. Both the same length.

   A floating boat stays still. The upthrust matches the weight.

7. A ship takes on more load. Look at the ship. What happens to it in the water?

   Tap your answer.

   1. A It stays level, because the load is inside the ship.

      Not this one. The load adds weight. The upthrust must grow to match it, so the ship sits lower. Try another.

   2. B It sits lower and pushes aside more water.

      Yes, that is right. More load means more weight. The ship sinks in until the upthrust matches the new weight.

   3. C It sits higher, because the load pushes it up.

      Not this one. A load adds weight. It pulls the ship down, not up. Try another.

   4. D It sinks at once.

      Not this one. A ship can carry a load until water pours over its edge. First it sits lower. Try another.

   See the answer

   It sits lower and pushes aside more water.

   More load means more weight. The ship sinks in until the upthrust matches the new weight.

   • If you chose A: The load adds weight. The upthrust must grow to match it, so the ship sits lower.
   • If you chose C: A load adds weight. It pulls the ship down, not up.
   • If you chose D: A ship can carry a load until water pours over its edge. First it sits lower.

8. What fills most of the inside of a steel ship?

   Tap your answer.

   1. A Air

      Yes, that is right. The hull is hollow and full of air. That is why it pushes aside so much water.

   2. B Water

      Not this one. A hull full of water would sink. The hull is hollow and full of air. Try another.

   3. C Solid steel

      Not this one. A ship of solid steel would sink like a nail. The hull has a space inside it. Try another.

   See the answer

   Air

   The hull is hollow and full of air. That is why it pushes aside so much water.

   • If you chose B: A hull full of water would sink. The hull is hollow and full of air.
   • If you chose C: A ship of solid steel would sink like a nail. The hull has a space inside it.

9. Tariq squeezes foil into a tight ball. It sinks. He folds a second sheet of the same size into a boat. It floats. Why?

   Tap your answer.

   1. A The foil boat weighs less than the ball.

      Not this one. The two sheets are the same size, so they weigh the same. Only the shape changed. Try another.

   2. B Water pushes up on the boat but not on the ball.

      Not this one. Water pushes up on both. On the ball the push is too small. Try another.

   3. C A thin sheet of foil always floats.

      Not this one. The ball is made of the same thin foil, and it sinks. Shape decides. Try another.

   4. D The boat pushes aside more water, so the upthrust is bigger.

      Yes, that is right. The foil weighs the same in both shapes. The hollow boat pushes aside more water. So the water pushes up harder.

   See the answer

   The boat pushes aside more water, so the upthrust is bigger.

   The foil weighs the same in both shapes. The hollow boat pushes aside more water. So the water pushes up harder.

   • If you chose A: The two sheets are the same size, so they weigh the same. Only the shape changed.
   • If you chose B: Water pushes up on both. On the ball the push is too small.
   • If you chose C: The ball is made of the same thin foil, and it sinks. Shape decides.