Chapter 8 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

S2.5.8 Chapter 8

Shape and floating

How can a ship made of steel float?

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

  • Make a lump of clay float by shaping it into a boat
  • Explain why a big steel ship floats

You press a lump of clay into water. It sinks. Now you shape the same clay into a boat. It floats.

The clay did not change. Its weight is the same. Only its shape changed.

A hollow shape has a space inside it. A clay boat is hollow.

A hollow boat pushes aside much more water than a lump does. A thing that pushes aside more water gets a harder push up.

Draw it first

Draw why a clay boat floats

  1. Two bowls with a dashed line, labelled water level. In the left bowl a black lump of clay, labelled clay lump, is under the line. In the right bowl a clay boat, labelled clay boat, has its floor under the line. An arrow of the same length points down from each, labelled weight. On the lump the arrow up, labelled upthrust, is short, and the word sinks is under the bowl. On the boat the arrow up is as long as the arrow down, and the word floats is under the bowl.water levelclay lumpclay boat

    1 Draw a lump of clay and a clay boat in two bowls. Mark the water level.

  2. Two bowls with a dashed line, labelled water level. In the left bowl a black lump of clay, labelled clay lump, is under the line. In the right bowl a clay boat, labelled clay boat, has its floor under the line. An arrow of the same length points down from each, labelled weight. On the lump the arrow up, labelled upthrust, is short, and the word sinks is under the bowl. On the boat the arrow up is as long as the arrow down, and the word floats is under the bowl.water levelclay lumpclay boatweightweight

    2 Draw an arrow down from each. Make them the same length. Write weight.

  3. Two bowls with a dashed line, labelled water level. In the left bowl a black lump of clay, labelled clay lump, is under the line. In the right bowl a clay boat, labelled clay boat, has its floor under the line. An arrow of the same length points down from each, labelled weight. On the lump the arrow up, labelled upthrust, is short, and the word sinks is under the bowl. On the boat the arrow up is as long as the arrow down, and the word floats is under the bowl.water levelclay lumpclay boatweightweightupthrustupthrustsinksfloats

    3 Draw a short arrow up from the lump. Draw a long arrow up from the boat. Write upthrust.

The lump pushes aside a little water. The upthrust is less than the weight. So the lump sinks.

The boat pushes aside a lot of water. The upthrust grows until it matches the weight. So the boat floats.

See it three ways

As a picture

Under the heading water pushed aside, a small hatched box, labelled lump, and a box twice as big, labelled boat.water pushed asidelumpboat

In words

The boat pushes aside more water than the lump. So the water pushes up harder on the boat.

In symbols

Say the clay weighs 1 newton, like a small apple. As a lump, the water pushes up about half a newton. It sinks. As a boat, the water pushes up 1 newton. It floats.

A big steel ship floats for the same reason. Its body is called the hull. The hull is hollow and full of air.

A steel ship afloat, cut through the middle. The hull, labelled steel hull, is a thin wall around a space full of air, labelled air. The water, labelled water, reaches part way up the hull. An arrow points up from the ship, labelled upthrust. An arrow of the same length points down from it, labelled weight.airsteel hullwaterupthrustweight
A hollow steel hull. Weight and upthrust are equal.

The ship pushes aside a huge amount of water. That water weighs as much as the whole ship. So the upthrust matches the ship, and it floats.

A lump of steel sinks. Shape the same steel into a hollow hull and it floats.

A ship carries a load. The load adds weight. The ship sinks a little lower and pushes aside more water.

Two steel ships in water, one above the other. The first is labelled empty ship and sits high in the water. The second is labelled loaded ship. It carries crates, labelled load, and sits lower in the water. Each ship has a hull labelled steel hull, air inside labelled air, and water labelled water.empty shipairsteel hullwaterloaded shipairsteel hullwaterload
The load adds weight. The ship sits lower and pushes aside more water.

If the load is too big, water pours over the edge. The ship fills and sinks.

Try it

Make a clay boat

You need: clay from the ground or a river bank, a bowl of water, a handful of dry beans or small stones, kitchen foil, if you have no clay.

Be safe: Use a bowl only. Do not eat the clay or the beans. Wash your hands after.

  1. Roll a ball of clay as big as a small egg. Put it on the water. It sinks.
  2. Take it out. Shape the same clay into a boat.
  3. Make the sides high and thin. Press the clay so it has no holes.
  4. Put the boat on the water. Does it float? If water comes in, make the sides higher.
  5. Put in beans, one at a time. Count them.
  6. Stop when the boat sinks. Write down the number.

What you will see: The lump sinks. The boat floats and holds some beans. A boat with higher sides holds more. A foil boat floats too. A tight foil ball may sink.

If you cannot try it, this is what happens.
Two bowls with a dashed line for the water level. In the left bowl a black lump of clay, labelled clay lump, lies on the bottom. In the right bowl a clay boat, labelled clay boat, floats and holds a pile of black beans, labelled beans.beansclay lumpclay boat

A wrong idea many people have

A ship made of steel must sink, because steel sinks.

Why it feels right: A steel nail sinks. A ship is made of steel, and it is huge.

The test that breaks it: Squeeze a sheet of kitchen foil into a tight ball. It may sink. Take a second sheet the same size. Fold it into a boat with high sides. It floats. Foil is a thin metal, so it shows the same idea.

What is true: Shape counts. A hollow ship pushes aside a lot of water. The water pushes up on it as hard as its weight.

Key words

hollow
having a space inside it, like a bowl.
hull
the body of a ship or a boat.
load
the things a boat carries. A load adds weight.

Back to your guess

The answer is D: The ship is hollow and pushes aside a lot of water.

The ship is hollow. It pushes aside a lot of water. So the water pushes up very hard.

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

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

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

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

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

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

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

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

    3. 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?

    A steel ship afloat, cut through the middle. Its hull is labelled steel hull, the space inside is labelled air, and the water is labelled water.airsteel hullwater
    Tap your answer.

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

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

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

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

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

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

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

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

    4. 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.

Keep it

Copy this into your exercise book.

A hollow shape pushes aside more water. The water pushes up harder. That is why a clay boat floats and a lump sinks. A steel ship is hollow. It pushes aside a lot of water, so it floats.

Draw: Draw a lump sinking and a boat floating. Add arrows for weight and upthrust.

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

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