Chapter 2 of 8

All 8 chapters
  1. 1The skeleton, readunderstood
  2. 2Joints, readunderstood
  3. 3Muscles move bones, readunderstood
  4. 4Teeth, readunderstood
  5. 5Food groups, readunderstood
  6. 6A balanced diet, readunderstood
  7. 7Exercise and rest, readunderstood
  8. 8Keeping food safe, readunderstood

S2.3.2 Chapter 2

Joints

Where does my body bend and turn?

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.

By the end you can

  • Find joints in your body that bend and joints that turn
  • Compare how the knee and the shoulder move

Bend your elbow. Bend your fingers. Bend your knee. Where does your body bend?

A joint is a place where two bones meet. You bend and turn at your joints.

A door swings open and shut on its hinge. It cannot swing sideways. Your elbow works the same way.

A door seen from above. It is hinged at the left end. Closed, it lies flat, labelled door closed. Open, it is a dotted line swung up from the hinge, labelled door open. Labels: hinge, door closed, door open.hingedoor closeddoor open
A door turns on its hinge. It swings one way and back.

A hinge joint bends one way, then straightens. Your elbows, knees and finger joints are hinge joints.

Draw it first

Draw two kinds of joint

  1. Two joints drawn in four steps. On the left is a hinge joint: two bones meet at a round joint, labelled hinge. A dotted bone and an arrow show it bends one way, labelled bends one way. On the right is a ball and socket joint: a ball at the end of a bone sits in a cup, labelled ball and socket. Dotted bones show it turns in many directions. Labels: hinge, bends one way, ball and socket.hinge

    1 Draw two bones that meet at a round joint. Write hinge.

  2. Two joints drawn in four steps. On the left is a hinge joint: two bones meet at a round joint, labelled hinge. A dotted bone and an arrow show it bends one way, labelled bends one way. On the right is a ball and socket joint: a ball at the end of a bone sits in a cup, labelled ball and socket. Dotted bones show it turns in many directions. Labels: hinge, bends one way, ball and socket.hingebends one way

    2 Draw a dotted bone bent to one side. Draw an arrow. Write bends one way.

  3. Two joints drawn in four steps. On the left is a hinge joint: two bones meet at a round joint, labelled hinge. A dotted bone and an arrow show it bends one way, labelled bends one way. On the right is a ball and socket joint: a ball at the end of a bone sits in a cup, labelled ball and socket. Dotted bones show it turns in many directions. Labels: hinge, bends one way, ball and socket.hingebends one wayball and socket

    3 Draw a ball at the end of a bone, in a cup. Write ball and socket.

  4. Two joints drawn in four steps. On the left is a hinge joint: two bones meet at a round joint, labelled hinge. A dotted bone and an arrow show it bends one way, labelled bends one way. On the right is a ball and socket joint: a ball at the end of a bone sits in a cup, labelled ball and socket. Dotted bones show it turns in many directions. Labels: hinge, bends one way, ball and socket.hingebends one wayball and socket

    4 Draw dotted bones in two other directions.

Your shoulder is different. A round bone end sits in a cup of another bone. This is a ball and socket joint.

A ball and socket joint turns in many directions. Swing your arm in a big circle. Your hip works the same way.

Some joints do not move. The bones of your skull are joined tight. Your jaw is a joint that does move. It lets you chew and talk.

The skull from the side. Zigzag lines show where the bones of the skull are joined tight. The jaw is hatched and hangs on a hinge. A dotted jaw shows it swung open, with an arrow. Labels: joined tight, jaw moves.joined tightjaw moves

Find four joints on the body. A ring marks a joint that turns many ways. A square marks a joint that bends one way.

A body drawn as bones. A ring marks the shoulder and the hip, which turn many ways. A square marks the elbow and the knee, which bend one way. Labels: shoulder, elbow, hip, knee, turns many ways, bends one way.shoulderkneeelbowhipturns many waysbends one way

See it three ways

As a picture

A table with two columns, Joint and How it moves. Row one: knee, with bends one way. Row two: elbow, with bends one way. Row three: shoulder, with turns many ways. Row four: hip, with turns many ways.JointHow it moveskneebends one wayelbowbends one wayshoulderturns many wayshipturns many ways

In words

The knee is a hinge joint. It bends one way. The shoulder is a ball and socket joint. It turns many ways. Your arm swings in a circle. Your lower leg does not.

Try it

Compare your knee and your shoulder

You need: your own body, a chair, some clear space.

Be safe: Move slowly. Stop if anything hurts. Keep clear of other people.

  1. Sit on a chair. Hold your thigh still with both hands. Do not let it roll.
  2. Swing your lower leg. Which ways can it go? Try to draw a circle in the air with it.
  3. Stand up with room around you. Hold your arm straight.
  4. Draw a big circle in the air with your whole arm.
  5. Bend the middle joint of a finger. Does it bend one way or many ways?

What you will see: Your lower leg swings back and forth. It cannot draw a circle. Your whole arm draws a big circle. The middle joint of your finger bends one way, like a hinge.

If you cannot try it, this is what happens.
Two pictures. On the left, a straight line with arrows pointing up and down, labelled knee and back and forth. On the right, a big circle, labelled shoulder and a big circle. Labels: knee, back and forth, shoulder, a big circle.kneeback and forthshouldera big circle

A wrong idea many people have

The knee can move every way, like the shoulder.

Why it feels right: Your whole leg can swing in many ways. So it seems the knee does it.

The test that breaks it: Sit and hold your thigh still with both hands. Try to draw a circle in the air with your lower leg. You cannot. It only swings back and forth. Now draw a big circle with your whole arm. The shoulder can.

What is true: The knee is a hinge joint. It bends one way. The hip is a ball and socket joint. It lets the whole leg swing.

Key words

joint
a place where two bones meet.
hinge joint
a joint that bends one way, like a door. The knee is one.
ball and socket joint
a joint that turns in many directions. The shoulder is one.

Back to your guess

The answer is C: I bend where two bones meet.

You bend where two bones meet. Those places are called joints. Each bone is stiff.

Check yourself

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

  1. Which joint bends one way, like a door on its hinge?

    Tap your answer.

    1. Not this one. The shoulder turns in many directions. It is a ball and socket joint. Try another.

    2. Yes, that is right. The knee is a hinge joint. It bends one way and then straightens.

    3. Not this one. The hip turns in many directions. It is a ball and socket joint. Try another.

    See the answer

    The knee

    The knee is a hinge joint. It bends one way and then straightens.

    • If you chose A: The shoulder turns in many directions. It is a ball and socket joint.
    • If you chose C: The hip turns in many directions. It is a ball and socket joint.

  2. Amina swings her whole arm in a big circle. Which joint lets her do it?

    Tap your answer.

    1. Not this one. The elbow is a hinge joint. It bends one way and cannot make a circle. Try another.

    2. Not this one. The knee is in the leg. It bends one way, like the elbow. Try another.

    3. Yes, that is right. The shoulder is a ball and socket joint. It turns in many directions.

    See the answer

    The shoulder

    The shoulder is a ball and socket joint. It turns in many directions.

    • If you chose A: The elbow is a hinge joint. It bends one way and cannot make a circle.
    • If you chose B: The knee is in the leg. It bends one way, like the elbow.

  3. What is a joint?

    Tap your answer.

    1. Yes, that is right. Two bones meet at a joint. The bones do not bend. The joint lets them move.

    2. Not this one. A muscle is not a joint. A joint is where bones meet. Try another.

    3. Not this one. A joint is not inside one bone. It is where two bones meet. Try another.

    See the answer

    A place where two bones meet

    Two bones meet at a joint. The bones do not bend. The joint lets them move.

    • If you chose B: A muscle is not a joint. A joint is where bones meet.
    • If you chose C: A joint is not inside one bone. It is where two bones meet.

  4. Ravi says, "My knee moves every way." What do you tell him?

    Tap your answer.

    1. Not this one. Only the shoulder and hip are ball and socket joints. The knee is a hinge. Try another.

    2. Yes, that is right. The knee is a hinge joint. The hip is a ball and socket joint.

    3. Not this one. The knee and elbow are hinge joints. They bend one way only. Try another.

    See the answer

    The knee bends one way. The hip lets the whole leg swing.

    The knee is a hinge joint. The hip is a ball and socket joint.

    • If you chose A: Only the shoulder and hip are ball and socket joints. The knee is a hinge.
    • If you chose C: The knee and elbow are hinge joints. They bend one way only.

  5. From an earlier chapter

    Which bones make a cage round the heart and lungs?

    Tap your answer.

    1. Not this one. The skull is the hard case round the brain. Go to S2.3.1. Try another.

    2. Yes, that is right. The ribs are curved bones. They protect the heart and lungs.

    3. Not this one. The thigh bone is in the leg. It is the longest bone in the body. Go to S2.3.1. Try another.

    See the answer

    The ribs

    The ribs are curved bones. They protect the heart and lungs.

    • If you chose A: The skull is the hard case round the brain. Go to S2.3.1.
    • If you chose C: The thigh bone is in the leg. It is the longest bone in the body. Go to S2.3.1.

  6. Draw a hinge joint and a ball and socket joint. Write a name under each.

    See the answer

    Two bones meet at a round joint. A ball sits in a cup.

    A hinge joint bends one way. A ball and socket joint turns in many directions.

  7. How are the bones of the skull joined?

    Tap your answer.

    1. Not this one. The skull is many bones. The zigzag lines are joints where they meet. Try another.

    2. Not this one. The joints of the skull are tight. Only the jaw moves. Try another.

    3. Yes, that is right. The skull is many bones joined tight. It protects the brain. Only the jaw joint moves.

    See the answer

    The bones are joined tight and do not move.

    The skull is many bones joined tight. It protects the brain. Only the jaw joint moves.

    • If you chose A: The skull is many bones. The zigzag lines are joints where they meet.
    • If you chose B: The joints of the skull are tight. Only the jaw moves.

  8. Leila sits and swings her lower leg back and forth. Which joint is working?

    Tap your answer.

    1. Not this one. The hip lets the whole leg swing. The lower leg swings at the knee. Try another.

    2. Not this one. A bone is stiff. It does not bend. The bend is at a joint. Try another.

    3. Not this one. The ankle is a hinge too, but it moves the foot. The lower leg swings at the knee. Try another.

    4. Yes, that is right. The knee is a hinge joint. It bends one way and straightens. So the lower leg swings back and forth.

    See the answer

    The knee, a hinge joint

    The knee is a hinge joint. It bends one way and straightens. So the lower leg swings back and forth.

    • If you chose A: The hip lets the whole leg swing. The lower leg swings at the knee.
    • If you chose B: A bone is stiff. It does not bend. The bend is at a joint.
    • If you chose C: The ankle is a hinge too, but it moves the foot. The lower leg swings at the knee.

Keep it

Copy this into your exercise book.

A joint is where two bones meet. A hinge joint bends one way, like the knee and elbow. A ball and socket joint turns many ways. The shoulder and hip are two.

Draw: A hinge joint with one arrow. A ball and socket joint with three arrows.

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

No internet tomorrow? Save the whole book as one file (704 KB). It opens on any phone.