Chapter 7 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

S2.5.7 Chapter 7

Floating and sinking

Why does a big log float but a tiny stone sink?

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

  • Predict and test which objects float or sink in water
  • Explain that water pushes up on things put into it

A big log floats on a river. A tiny stone sinks. Why?

To float is to stay up on the water. To sink is to go down to the bottom.

Water pushes up on anything you put in it. This push is called upthrust.

Push a closed, empty bottle down into a bucket of water. Feel it push back on your hand. The more you push in, the harder it pushes back.

A closed empty bottle, labelled bottle, is held under the water of a bowl, labelled water. An arrow points up from it, labelled upthrust. An arrow of the same length points down from it, labelled hand and weight.waterbottleupthrusthand and weight
The water pushes the held bottle up.

Draw it first

Draw a floating thing and a sinking thing

  1. A bowl of water, labelled water. A piece of wood, labelled wood, sits in the top of the water. An arrow points up from it, labelled upthrust, and an arrow of the same length points down from it, labelled weight. A stone, labelled stone, is under the water. Its arrow up is short and its arrow down is long.water

    1 Draw a bowl and fill it with water. Write water.

  2. A bowl of water, labelled water. A piece of wood, labelled wood, sits in the top of the water. An arrow points up from it, labelled upthrust, and an arrow of the same length points down from it, labelled weight. A stone, labelled stone, is under the water. Its arrow up is short and its arrow down is long.waterwoodupthrustweight

    2 Draw a piece of wood in the top of the water. Draw two arrows of the same length, up and down. Write upthrust and weight.

  3. A bowl of water, labelled water. A piece of wood, labelled wood, sits in the top of the water. An arrow points up from it, labelled upthrust, and an arrow of the same length points down from it, labelled weight. A stone, labelled stone, is under the water. Its arrow up is short and its arrow down is long.waterwoodupthrustweightstone

    3 Draw a stone under the water. Draw a short arrow up and a long arrow down.

The wood sinks into the water a little. It stops when the upthrust equals its weight. Then it floats.

A stone is different. Under water, the upthrust is still less than its weight. So it sinks.

So heavy things do not always sink. A big log is heavy and it floats. A small stone, a nail or a pin is light. It sinks.

It depends on what a thing is made of, and its shape. You will see the shape part in the next chapter.

See it three ways

As a picture

A table with two columns. The first column, headed floats, lists big log, leaf and closed bottle. The second column, headed sinks, lists pin, small stone and nail.floatssinksbig logpinleafsmall stoneclosed bottlenail

In words

Some big things float. Some tiny things sink. What a thing is made of decides, and so does its shape.

Try it

Which things float?

You need: a bowl or a bucket of water, a wooden stick, a leaf, a closed empty plastic bottle with its lid, a metal spoon, a small stone, paper and a pencil.

Be safe: Use a bowl or a bucket only. Never play near deep water. Wash your hands after.

  1. Draw a table on paper. Write the names of the things down the side.
  2. Guess for each thing. Will it float or sink? Write your guess.
  3. Put one thing into the water, gently.
  4. Write what happened: floats or sinks.
  5. Compare your guesses with what happened.

What you will see: Most sticks and leaves float. The closed bottle floats. The spoon and the small stone sink. Some guesses may be wrong. That is how you learn.

If you cannot try it, this is what happens.
A table with the heads thing and result. Rows: wooden stick, floats. Leaf, floats. Closed bottle, floats. Metal spoon, sinks. Small stone, sinks.thingresultwooden stickfloatsleaffloatsclosed bottlefloatsmetal spoonsinkssmall stonesinks

A wrong idea many people have

Heavy things sink and light things float.

Why it feels right: A stone is heavy and sinks. A feather is light and floats.

The test that breaks it: Find a thick piece of wood and a small metal key. Hold one in each hand. The wood feels heavier. Put both in water. The wood floats. The key sinks.

What is true: Weight alone does not decide. It depends on what a thing is made of, and its shape.

Key words

float
to stay up on the water.
sink
to go down to the bottom of the water.
upthrust
the push up that water gives to a thing in it.

Back to your guess

The answer is C: It depends on what a thing is made of, and its shape.

It depends on what a thing is made of, and its shape. A big log is heavy, but it floats.

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

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

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

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

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

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

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

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

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

    3. 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. 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?

    A block of wood, labelled wood, floats in a bowl of water, labelled water. Part of it stands above the water. One arrow points down from it, labelled weight 4 N.waterwoodweight 4 N
    Tap your answer.

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

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

    4. 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?

    A stone, labelled stone, is under the water of a bowl, labelled water. An arrow points up from it, labelled upthrust 3 N. A longer arrow points down from it, labelled weight 5 N.waterstoneupthrust 3 Nweight 5 N
    Tap your answer.

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

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

    3. 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.

Keep it

Copy this into your exercise book.

Water pushes up on things in it. This is upthrust. A thing floats if upthrust matches its weight before it goes under. Heavy things can float and light things can sink. It depends on what a thing is made of.

Draw: Draw a bowl of water. Draw a stick floating and a stone sinking, with 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 (691 KB). It opens on any phone.