Chapter 5 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

S2.5.5 Chapter 5

Gravity and falling

Why does a stone fall down when I let go?

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

  • Explain that gravity pulls things towards the centre of the Earth
  • Explain that weight is the pull of gravity on an object

You let go of a stone. It falls. Nobody pushes it. What makes it fall?

The Earth pulls it. This pull is gravity. Gravity is the pull of the Earth on things.

Gravity is a force. It is a pull. You cannot see it. You can see what it does.

Draw it first

Draw how gravity pulls

  1. A big circle, labelled Earth. Four small circles sit above, below, left and right of it. The left one is labelled stone. An arrow from each stone points at the Earth. Dashed lines run from the edge of the Earth to a black dot in the middle, labelled centre.Earth

    1 Draw a big circle. Write Earth.

  2. A big circle, labelled Earth. Four small circles sit above, below, left and right of it. The left one is labelled stone. An arrow from each stone points at the Earth. Dashed lines run from the edge of the Earth to a black dot in the middle, labelled centre.Earthstone

    2 Draw a stone above, below and at each side. Draw an arrow from each to the Earth.

  3. A big circle, labelled Earth. Four small circles sit above, below, left and right of it. The left one is labelled stone. An arrow from each stone points at the Earth. Dashed lines run from the edge of the Earth to a black dot in the middle, labelled centre.Earthstonecentre

    3 Draw a dashed line from each arrow to the middle. Write centre.

Gravity pulls things towards the centre of the Earth. The centre is the middle.

Down means towards the centre of the Earth, wherever you stand.

Mei lives on the far side of the Earth from Kofi. She does not fall off. Gravity pulls her towards the centre too.

Gravity gives things weight. Weight is the pull of gravity on a thing.

See it three ways

As a picture

A ball with one arrow pointing down from it. The arrow is labelled weight.weight

In words

You let go of a ball. Gravity pulls it down. The size of that pull is its weight.

In symbols

Weight is a force, so we measure it in newtons. A small apple weighs about 1 newton. Ten small apples weigh about 10 newtons.

People say a bag weighs 2 kilograms. That is fine in daily life. In science, kilograms tell how much stuff is in a thing. Weight is a force, so it is in newtons.

A box on a table does not fall. The table pushes up on it. The push up equals the pull down. So the box stays still.

A box with an arrow pointing up from it, labelled table pushes, and an arrow pointing down from it, labelled weight. The two arrows are the same length.table pushesweight
Two equal forces balance. The box stays still.

The Moon has gravity too, but it is weaker. A stone would weigh about 1/6 as much there. It is still the same amount of stuff.

Two force meters, each with a scale from 0 to 6 and the label N for newtons. Each holds the same stone in a bag, labelled stone. The left one is labelled on the Earth, and its pointer stands at 6. The right one is labelled on the Moon, and its pointer stands at 1.on the Earth0123456Nstoneon the Moon0123456Nstone
The same stone weighs 6 N on the Earth. On the Moon it weighs about 1 N.

Try it

Do big stones fall faster?

You need: a stone as big as your fist, a stone as big as a bean, soft ground, sand or a thick pile of cloth, a friend to watch and listen, a sheet of paper.

Be safe: Hold stones no higher than your waist. Drop them onto soft ground only. Keep your feet back. Nobody stands below or close. Never drop anything from a window, a roof or a wall.

  1. Lay the cloth or find soft ground. Make sure nobody stands near.
  2. Hold the big stone in one hand. Hold the small stone in the other. Keep them at the same height.
  3. Open both hands at the same moment.
  4. Listen. Do you hear one thud or two? Try three times.
  5. Now drop a flat sheet of paper. Then crumple it into a tight ball and drop it.

What you will see: The two stones land together. You hear one thud. The flat paper floats down slowly. The tight paper ball falls fast. It is the same paper, so its weight is the same.

If you cannot try it, this is what happens.
A big stone, labelled big stone, and a small stone, labelled small stone, are held at the same height. A dashed line runs straight down from each to the line labelled ground. Under the picture is the label they land together.big stonesmall stonegroundthey land together

A wrong idea many people have

Heavy things fall faster than light things.

Why it feels right: A stone drops fast. A leaf or a sheet of paper drifts down slowly.

The test that breaks it: Drop a big stone and a small stone together onto soft ground. Hold them at the same height. They land together. Now drop a flat sheet of paper. Then drop it crumpled into a ball. The ball falls much faster. It weighs the same as the sheet.

What is true: Gravity makes big and small stones fall together. Air slows a flat, light sheet. The next chapter shows how.

Key words

gravity
the pull of the Earth on things. It pulls towards the centre.
centre
the middle of the Earth.
weight
the pull of gravity on a thing. It is measured in newtons.

Back to your guess

The answer is B: The Earth pulls it down.

The Earth pulls it down. This pull is gravity. It pulls big stones and small stones the same way. Gravity needs no air.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    4. 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?

    A force meter hangs by a ring. Its scale has the marks 0, 1, 2, 3, 4 and 5, with the label N for newtons. A pointer stands at the mark 4. A stone, labelled stone, hangs in a bag from the hook.012345Nstone
    Tap your answer.

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

    2. 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. 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. 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.

Keep it

Copy this into your exercise book.

Gravity is the pull of the Earth on things. It pulls towards the centre of the Earth. Weight is the pull of gravity on a thing, in newtons. Big and small stones fall together.

Draw: Draw the Earth and four stones. Draw an arrow from each stone towards the centre.

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 (691 KB). It opens on any phone.