Chapter 6 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

S2.5.6 Chapter 6

Air and water resistance

Why does paper fall slowly when a stone falls fast?

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 paper spinners and compare how fast they fall
  • Explain why fish and boats have smooth, pointed shapes

A stone falls fast. A sheet of paper falls slowly. Gravity pulls both. So why the difference?

You cannot see air, but it is there. It pushes on things that move through it.

Air resistance is the push of air against a moving thing.

Draw it first

Draw why a flat sheet falls slowly

  1. A flat sheet of paper, labelled flat paper, and a paper ball, labelled paper ball. An arrow points down from each, labelled gravity. A long arrow points up from the sheet, labelled big push up. A short arrow points up from the ball, labelled small push up. Under them are the labels falls slowly and falls faster.gravitygravityflat paperpaper ball

    1 Draw a flat sheet and a paper ball. Draw a gravity arrow down from each.

  2. A flat sheet of paper, labelled flat paper, and a paper ball, labelled paper ball. An arrow points down from each, labelled gravity. A long arrow points up from the sheet, labelled big push up. A short arrow points up from the ball, labelled small push up. Under them are the labels falls slowly and falls faster.gravitygravityflat paperpaper ballbig push upsmall push up

    2 Draw a long arrow up from the sheet. Draw a short arrow up from the ball.

  3. A flat sheet of paper, labelled flat paper, and a paper ball, labelled paper ball. An arrow points down from each, labelled gravity. A long arrow points up from the sheet, labelled big push up. A short arrow points up from the ball, labelled small push up. Under them are the labels falls slowly and falls faster.gravitygravityflat paperpaper ballbig push upsmall push upfalls slowlyfalls faster

    3 Write falls slowly under the sheet. Write falls faster under the ball.

A flat sheet meets a lot of air. The air pushes up hard. The sheet falls slowly.

Crumple the same paper. It weighs the same. But less of it meets the air. The push up is smaller. It falls faster.

A parachute has a wide canopy. It meets a lot of air, so the air pushes up hard.

A parachutist hangs by strings from a wide canopy, labelled parachute. An arrow points up from the canopy, labelled air resistance. An arrow of the same length points down from the person, labelled weight.parachuteair resistanceweight
A parachutist falling at a steady speed.

This parachutist falls at a steady speed. The air pushes up as hard as gravity pulls down. The forces are balanced.

Water pushes on moving things too. Water resistance is the push of water against a moving thing. It is much stronger than air resistance.

A swimmer feels water resistance. The water pushes back against the swimmer all the time.

A swimmer, labelled swimmer, moves right through water. An arrow above them points right, labelled moves. An arrow points left from the back of the swimmer, labelled water resistance.swimmermoveswater resistance
The water pushes back on the swimmer. It acts against the movement.

Fish, boats and flying birds have smooth, pointed shapes. We say they are streamlined. Water and air slide past them.

See it three ways

As a picture

Two bodies move right through water. The top one has a flat front and the bottom one has a pointed front. In front of each body an arrow points right, labelled moves. From the back of each body an arrow points left, for the push of the water. It is long for the flat front, labelled big push. It is short for the pointed front, labelled small push. Labels: flat front, pointed front.movesbig pushflat frontmovessmall pushpointed front

In words

A flat front meets a lot of water. A pointed front meets less. The push back is smaller. The body moves on with less effort.

Try it

Make paper spinners

You need: two paper strips, each 4 cm wide and 16 cm long, a grown up with scissors, two paper clips, a friend to watch.

Be safe: A grown up uses the scissors. Do not climb on a chair or a wall to drop them.

  1. A grown up cuts each strip down the middle. Cut from the top to half way.
  2. Fold the two top flaps out. Fold one forward and one back. These are the wings.
  3. Fold the bottom of each strip up by 2 cm. Slide a paper clip on.
  4. A grown up cuts the wings of one spinner to half length.
  5. Hold both spinners high above your head, at the same height.
  6. Let go of both at the same moment. Which lands last? Try three times.

What you will see: Both spinners turn as they fall. The spinner with long wings falls more slowly. It lands last. Long wings meet more air.

If you cannot try it, this is what happens.
Two paper spinners were let go at the same time from the dashed line, labelled start. The spinner with short wings, labelled short wings, has landed on the line labelled ground. The spinner with long wings, labelled long wings, is still in the air.startlong wingsshort wingsground

A wrong idea many people have

Air is nothing, so it cannot push.

Why it feels right: You cannot see air. Your hand moves through it with no trouble.

The test that breaks it: Hold a flat piece of card in front of you. Swing it fast, flat side first. Feel the push on your hand. Now swing it edge first. The push is much smaller.

What is true: Air is real stuff. It pushes against things that move through it. A wide, flat thing meets more air.

Key words

air resistance
the push of air against a moving thing.
water resistance
the push of water against a moving thing.
streamlined
a smooth, pointed shape that meets little air or water.

Back to your guess

The answer is C: Air pushes up against the paper.

Air pushes up against the paper. Gravity pulls the paper, but the air slows its fall.

Check yourself

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

  1. Why does flat paper fall more slowly than a paper ball?

    Tap your answer.

    1. Yes, that is right. The sheet and the ball weigh the same. The sheet meets more air, so it falls more slowly.

    2. Not this one. It is the same paper. Gravity pulls it the same, flat or crumpled. Go to S2.5.5. Try another.

    3. Not this one. Crumpling does not change the weight. The ball just meets less air. Try another.

    See the answer

    More of the sheet meets air. So air pushes up more.

    The sheet and the ball weigh the same. The sheet meets more air, so it falls more slowly.

    • If you chose B: It is the same paper. Gravity pulls it the same, flat or crumpled. Go to S2.5.5.
    • If you chose C: Crumpling does not change the weight. The ball just meets less air.

  2. Mei makes two paper spinners. One has long wings. One has short wings. Which falls more slowly?

    Tap your answer.

    1. Not this one. Short wings meet less air. The air pushes up less, so it falls faster. Try another.

    2. Yes, that is right. Long wings meet more air. The air pushes up harder, so the spinner falls more slowly.

    3. Not this one. Wing length changes how much air they meet. Make two and test them. Try another.

    See the answer

    The one with long wings

    Long wings meet more air. The air pushes up harder, so the spinner falls more slowly.

    • If you chose A: Short wings meet less air. The air pushes up less, so it falls faster.
    • If you chose C: Wing length changes how much air they meet. Make two and test them.

  3. You push your flat hand through air. Then you push it through water. Where is the push back bigger?

    Tap your answer.

    1. Not this one. You can feel air push, but water pushes back much harder. Try another.

    2. Not this one. Water resistance is much stronger than air resistance. Try another.

    3. Not this one. Air is real stuff. Swing a flat card fast. You feel it push on your hand. Try another.

    4. Yes, that is right. Water resistance is stronger than air resistance. You feel it at once.

    See the answer

    In water

    Water resistance is stronger than air resistance. You feel it at once.

    • If you chose A: You can feel air push, but water pushes back much harder.
    • If you chose B: Water resistance is much stronger than air resistance.
    • If you chose C: Air is real stuff. Swing a flat card fast. You feel it push on your hand.

  4. Why do fish have smooth, pointed shapes?

    Tap your answer.

    1. Not this one. Gravity does not care about shape. Water resistance does. Go to S2.5.5. Try another.

    2. Not this one. It is the other way round. A flat front meets more water and gets pushed back harder. Try another.

    3. Yes, that is right. A pointed front meets less water. The water resistance is smaller. A fish moves on with less effort.

    See the answer

    Water slides past them, so the push back is small.

    A pointed front meets less water. The water resistance is smaller. A fish moves on with less effort.

    • If you chose A: Gravity does not care about shape. Water resistance does. Go to S2.5.5.
    • If you chose B: It is the other way round. A flat front meets more water and gets pushed back harder.

  5. From an earlier chapter

    A bottle top slides on a smooth floor. It slides on a rough mat. Which makes it stop sooner?

    Tap your answer.

    1. Yes, that is right. A rough surface gives more friction. More friction stops the bottle top sooner.

    2. Not this one. A smooth floor gives less friction. The top slides further. Go to S2.5.3. Try another.

    3. Not this one. Rough surfaces give more friction than smooth ones. The mat stops it sooner. Go to S2.5.3. Try another.

    See the answer

    The rough mat

    A rough surface gives more friction. More friction stops the bottle top sooner.

    • If you chose B: A smooth floor gives less friction. The top slides further. Go to S2.5.3.
    • If you chose C: Rough surfaces give more friction than smooth ones. The mat stops it sooner. Go to S2.5.3.

  6. Draw a flat sheet of paper falling. Draw an arrow for gravity and an arrow for air resistance.

    See the answer

    A sheet with an arrow down for gravity. A long arrow up for the air. It is almost as long as the gravity arrow.

    Gravity pulls the sheet down. The air pushes up against it, so it falls slowly.

  7. Why does a parachute slow a fall?

    Tap your answer.

    1. Not this one. Gravity pulls the same. The wide canopy is what changes the fall. Go to S2.5.5. Try another.

    2. Yes, that is right. A wide canopy meets much more air than a person alone. The air resistance is big, so the fall is slow.

    3. Not this one. A parachute still falls, so it is not lighter than air. It meets a lot of air, and the air pushes up. Try another.

    See the answer

    The wide canopy meets lots of air, so air pushes up hard.

    A wide canopy meets much more air than a person alone. The air resistance is big, so the fall is slow.

    • If you chose A: Gravity pulls the same. The wide canopy is what changes the fall. Go to S2.5.5.
    • If you chose C: A parachute still falls, so it is not lighter than air. It meets a lot of air, and the air pushes up.

  8. A parachutist falls at a steady speed. The picture shows only the weight. What does the air do?

    A parachutist hangs by strings from a wide canopy, labelled parachute. One arrow points down from the person, labelled weight.parachuteweight
    Tap your answer.

    1. Not this one. Air is real stuff. A wide canopy meets a lot of it. The air pushes back. Try another.

    2. Not this one. Air resistance acts against the way a thing moves. The parachutist moves down, so it pushes up. Try another.

    3. Not this one. Then the parachutist would be slowed more and more. A steady speed needs the two forces equal. Try another.

    4. Yes, that is right. A steady speed means the forces are balanced. The air pushes up as hard as gravity pulls down.

    See the answer

    It pushes up as hard as the weight pulls down.

    A steady speed means the forces are balanced. The air pushes up as hard as gravity pulls down.

    • If you chose A: Air is real stuff. A wide canopy meets a lot of it. The air pushes back.
    • If you chose B: Air resistance acts against the way a thing moves. The parachutist moves down, so it pushes up.
    • If you chose C: Then the parachutist would be slowed more and more. A steady speed needs the two forces equal.

  9. A swimmer moves through water. Look at the picture. Which way does the water push on the swimmer?

    A swimmer, labelled swimmer, moves right through water. An arrow above them points right, labelled moves.swimmermoves
    Tap your answer.

    1. Not this one. Water does not carry a swimmer along here. It pushes against the movement. Try another.

    2. Not this one. Water does push up on a swimmer. Water resistance acts against the movement. Try another.

    3. Yes, that is right. Water resistance pushes against the way a thing moves. The swimmer moves forwards, so the push is backwards.

    4. Not this one. Down is the way the Earth pulls. Water resistance acts against the movement. Try another.

    See the answer

    Backwards, against the way the swimmer moves

    Water resistance pushes against the way a thing moves. The swimmer moves forwards, so the push is backwards.

    • If you chose A: Water does not carry a swimmer along here. It pushes against the movement.
    • If you chose B: Water does push up on a swimmer. Water resistance acts against the movement.
    • If you chose D: Down is the way the Earth pulls. Water resistance acts against the movement.

  10. A feather and a stone fall from the same height. The stone lands first. Why?

    Tap your answer.

    1. Not this one. Gravity pulls on everything. The feather falls more slowly because the air slows it a lot. Try another.

    2. Yes, that is right. A feather is light and wide. The air slows it a lot. A stone is hardly slowed.

    3. Not this one. A big stone and a small stone land together. The feather is slowed by air, not just by its weight. Try another.

    See the answer

    The air slows the feather much more than it slows the stone.

    A feather is light and wide. The air slows it a lot. A stone is hardly slowed.

    • If you chose A: Gravity pulls on everything. The feather falls more slowly because the air slows it a lot.
    • If you chose C: A big stone and a small stone land together. The feather is slowed by air, not just by its weight.

Keep it

Copy this into your exercise book.

Air and water push against things that move through them. This is air resistance and water resistance. A wide, flat thing meets more, so it falls or moves slower. Fish and boats are pointed.

Draw: Draw a flat sheet and a paper ball. Add a long arrow up and a short arrow up.

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.