Chapter 3 of 8

All 8 chapters
  1. 1Forces are pushes and pulls, readunderstood
  2. 2Measuring forces, readunderstood
  3. 3Friction, readunderstood
  4. 4More friction or less, readunderstood
  5. 5Gravity and falling, readunderstood
  6. 6Air and water resistance, readunderstood
  7. 7Floating and sinking, readunderstood
  8. 8Shape and floating, readunderstood

S2.5.3 Chapter 3

Friction

Why does a sliding thing slow down and stop?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

  • You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Compare how far an object slides on smooth and rough surfaces
  • Explain how friction helps us walk and slows things down

Slide a book across a table. It slows down and stops. Nobody caught it. Why did it stop?

When two surfaces rub, there is a force called friction. Friction pushes against the way a thing slides.

Draw it first

Draw friction on a sliding box

  1. A box rests on the ground line. A long arrow points right from the box, labelled push. A shorter arrow points left from the box, labelled friction. Small bumps under the ground line are labelled rough ground.push

    1 Draw a box on a ground line. Draw a long arrow pointing right. Write push.

  2. A box rests on the ground line. A long arrow points right from the box, labelled push. A shorter arrow points left from the box, labelled friction. Small bumps under the ground line are labelled rough ground.pushfriction

    2 Draw a shorter arrow pointing left. Write friction.

  3. A box rests on the ground line. A long arrow points right from the box, labelled push. A shorter arrow points left from the box, labelled friction. Small bumps under the ground line are labelled rough ground.pushfrictionrough ground

    3 Draw little bumps under the ground. Write rough ground.

The friction arrow points the other way from the push. Friction acts against the slide.

A rough surface has bumps you can feel. A smooth surface feels flat. Even a smooth surface has tiny bumps.

The bumps on two surfaces catch on each other. A rough surface gives more friction than a smooth one.

Two blocks, each resting on a surface seen close up. The left surface, labelled smooth, has tiny bumps. The right surface, labelled rough, has big bumps that reach into the block. Each block is labelled block.blocksmoothblockrough
Close up, a rough surface has big bumps that catch. A smooth surface has tiny ones.

A ball rolling on the ground slows down too. The ground and the air push against it.

Try it

How far does it slide?

You need: a bottle top or a flat stone, a book and a pile of books, for a ramp, a smooth floor or table, a cloth or mat, a little dry sand or fine soil, a ruler or a string marked in cm, paper and a pencil.

Be safe: Keep sand and soil away from your eyes. Wash your hands afterwards.

  1. Lean the book on the pile to make a ramp. Put its foot on the smooth floor.
  2. Put the bottle top flat at a mark on the ramp. Let it go. Do not push it. If it will not slide, make the ramp steeper.
  3. Measure how far it slides past the foot of the ramp. Write it down.
  4. Lay the cloth flat past the ramp. Do it again from the same mark.
  5. Spread a thin layer of sand. Do it again from the same mark.

What you will see: The bottle top slides far on the smooth floor. It slides less on the cloth. It slides least on the sand. Each time it started the same way. Only the surface changed. In our test it slid 90, 40 and 12 cm. Your numbers will differ.

If you cannot try it, this is what happens.
Three tracks start at a dashed line labelled start. The track labelled floor is the longest and ends with a bottle top and the label 90 cm. The track labelled cloth is shorter and ends with the label 40 cm. The track labelled sand is the shortest and ends with the label 12 cm.startfloor90 cmcloth40 cmsand12 cm

A wrong idea many people have

A thing stops because its push runs out.

Why it feels right: You gave it a push. It slowed down and stopped. It seems the push was used up.

The test that breaks it: Use the ramp again. Let the top go from the same mark. Do it on the floor, then on the cloth. The start is the same. If the push ran out, the slide would be the same. It is not.

What is true: The surface slows the bottle top. More friction means a shorter slide.

See it three ways

As a picture

A person with one arrow that points right from them, labelled friction. It points the way the person walks.friction

In words

When you walk, your shoe pushes back on the ground. Friction stops the shoe slipping. The ground pushes you forward.

Friction helps us. You could not walk on a floor with no friction. On a wet, smooth floor you slip, because there is little friction.

Friction lets you hold a cup. It lets the brakes of a bicycle slow the wheel. The brake pads rub the wheel.

Friction also makes heat. Rub your hands together fast. They get warm.

Key words

friction
the force between two surfaces that rub. It acts against the slide.
rough
a surface with bumps you can feel.
smooth
a surface that feels flat.

Back to your guess

The answer is D: The ground rubs against it.

The ground rubs against it. The rubbing is a force called friction. It pushes against the way the thing slides.

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

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

    3. 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?

    Three tracks start at a dashed line labelled start. The track labelled floor is the longest and ends with a bottle top and the label 90 cm. The track labelled cloth is shorter and ends with the label 40 cm. The track labelled sand is the shortest and ends with the label 12 cm.startfloor90 cmcloth40 cmsand12 cm
    Tap your answer.

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

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

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

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

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

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

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

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

    3. 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?

    Two blocks, each resting on a surface seen close up. The left surface, labelled rough, has big bumps that reach into the block. The right surface, labelled smooth, has tiny bumps. Each block is labelled block.blockroughblocksmooth
    Tap your answer.

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

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

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

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

    4. 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?

    Three tracks start at a dashed line labelled start. The track labelled floor is the longest and ends with a bottle top and the label 90 cm. The track labelled cloth is shorter and ends with the label 40 cm. The track labelled sand is the shortest and ends with the label 12 cm.startfloor90 cmcloth40 cmsand12 cm
    Tap your answer.

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

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

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

    4. 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.

Keep it

Copy this into your exercise book.

Friction is a force between two surfaces that rub. It acts against the slide. Rough surfaces give more friction than smooth ones. Friction helps us walk and hold things. It also slows things down.

Draw: A box sliding right. A push arrow right. A shorter friction arrow left.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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