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Primary Science · Year 3Chapter 9 · Forces
Year 3 · Chapter 9

Forces

By the end of this chapter you will be able to

  • Say that a force is a push or a pull, and show the direction of a push or a pull with an arrow
  • Explain that forces act on objects and can make them start, stop, speed up, slow down or change direction
  • Compare how a toy car moves on different surfaces in a fair test, and describe the pattern in the results
  • Explain that friction is a force that slows down moving things, and give examples where it helps and where it gets in the way
  • Name air resistance and water resistance as forces that slow down moving things
  • Say that many forces need contact between two objects, and that magnetic forces can act at a distance

You should already know

  • Fair tests, and changing only one thing at a time (Year 3 Science, Chapter 1)
  • Light and shadows, and a fair test on shadow size (Year 3 Science, Chapter 8)

Where this leads

  • Magnets, and how they attract and repel each other (Year 3 Science, Chapter 10)
  • Forces in more detail, including gravity and resistance (Year 5 Science)

National Curriculum for England, Key Stage 2 science, Year 3. Content: F1 compare how things move on different surfaces, F2 notice that some forces need contact between 2 objects, but magnetic forces can act at a distance.

Exercise P  Do you remember?

1
FWhich part of a flowering plant holds the plant up and carries water to the leaves? Write the word.
2
FWhat is the name for the transfer of pollen from one flower to another? Write the word.
3
FDo animals, including humans, make their own food? Write yes or no.
4
FSoil is made from broken-down rock and ___ matter. Write the word.
5
FWhich part of the skeleton protects the brain? Write the word.
9.1

Pushes and pulls

Year 3
F: a force is a push or a pull; a push moves something away from the person, a pull moves something towards them
Video lesson: Pushes and pullsComing soon
Story. Sam pushes a swing away from him, and it goes higher. Ruby pulls her sledge back towards her. Mr Patel says, “Both of those are forces. One is a push, and one is a pull.”

A force is a push or a pull. When you push a door, you give it a force. When you pull a drawer, you give it a force too.

A push moves something away from the person pushing. A pull moves something towards the person pulling.

In a diagram, a force is drawn as an arrow. The arrow points the way the force acts. For a push, it points away from the person. For a pull, it points towards the person.

PushPullthe box moves away from youthe box moves towards you
Left: a push, away from you. Right: a pull, towards you. The arrows show the direction of each force.

The table sorts four actions into pushes and pulls.

ActionPush or pull?
Pushing a pram away from youpush
Pulling a sledge towards youpull
Pushing a swing to make it gopush
Pulling a drawer openpull

Key ideas

  • A force is a push or a pull.
  • A push moves something away from the person pushing.
  • A pull moves something towards the person pulling.
  • We draw a force as an arrow that points the way the force acts.
Worked example 9.1.1
Look at the table in section 9.1. Is pushing a swing a push or a pull? Write the word.

Method. Find the row about the swing.

  1. Find the row. The row says pushing a swing to make it go.
    Pushing a swing
  2. Read the answer. The answer in that row is push.
    push
  3. Write the answer.
    push

Check. Pushing moves the swing away from you. ✓

Worked example 9.1.2
Ruby stands still and pushes a box away from her. Does the force on the box move it away from Ruby, or towards Ruby? Write away or towards.

Method. Think about the direction of a push.

  1. Think about the word push. A push moves something away from the person pushing.
    away
  2. Apply it to Ruby. Ruby pushes the box, so the box moves away from Ruby.
    away from Ruby
  3. Write the answer.
    away

Check. The arrow for the push points away from Ruby. ✓

Watch out. Think about which way the person is pushing, not which way the person is facing.

Worked example 9.1.3
Leo pulls a bag towards himself. Write the direction of the force on the bag: towards Leo or away from Leo.

Method. Think about the direction of a pull.

  1. Think about a pull. A pull moves something towards the person pulling.
    towards
  2. Apply it to Leo. Leo pulls the bag, so the force on the bag points towards Leo.
    towards Leo
  3. Write the answer.
    towards Leo

Check. The arrow for a pull points towards the person. ✓

Watch out

Watch out: for a pull, the arrow points towards the person doing the pulling, not away from them.

Try it at home or school (with a grown-up). Sit on a chair with a grown-up, in a clear space. Use one hand to push a cushion or a soft toy away from you. Then use the same hand to pull it back towards you. Say which movement was a push and which was a pull, and which way each arrow would point.

Exam tip

Grown-up tip. Ask: “Which way did it move? Did you push it or pull it? Which way does the arrow point?” Let your child use the words push, pull and force.

Exercise 9A  Pushes and pulls

1
FIs pushing a door away from you a push or a pull? Write the word.
2
FIs pulling a bucket of water towards you a push or a pull? Write the word.
3
FFill in the gap: a force is a push or a ___. Write the word.
4
FRuby pulls a sledge towards herself. Does the pull move the sledge away from Ruby, or towards Ruby? Write the word.
5
FWhich way does a push move something: away from the person or towards the person? Write away or towards.
6
FLook at the table in section 9.1. How many of the four actions are pushes? Write the number.
7
FLook at the table in section 9.1. Write the action in the table that is a pull on a drawer.
8
PSLeo sits on the floor. He pushes a toy car away from him with one hand. Write the direction of the force on the car: towards Leo or away from Leo.
9
PSWorking scientifically. Predict which way a toy car will move if you push it gently away from you. Write one prediction.
10
PSPriya says a push always moves something towards her. Is Priya right? Write no or yes.
11
RExplain how you can tell a push arrow from a pull arrow by its direction. Write one sentence.
9.2

Forces change how things move

Year 3
F: forces act on objects and can make them start, stop, speed up, slow down or change direction
Video lesson: Forces change how things moveComing soon
Story. Mo kicks a football that is lying still on the grass, and it starts to roll. Priya stops it with her foot. Mr Patel says, “The kick and the stop were both forces. Each one changed how the ball moved.”

A force acts on an object. When a force acts on an object, it can change the way the object moves. A force can make an object:

The picture shows each of these, with the force in orange and the motion in blue.

Starta push starts it movingStopa hand stops itSpeed upa push makes it fasterSlow downa force slows it downChange directiona kick sends it a new wayforcemotion
A force can start, stop, speed up, slow down or change the direction of a ball.

The table shows what a force can do, with an example for each.

What the force doesExample
starts an object movingA kick starts a football moving
stops a moving objectA hand stops a rolling ball
makes an object go fasterA push makes a trolley speed up
makes an object go more slowlyFriction slows a sliding box
changes the direction of an objectA kick from the side sends a ball off in a new direction

Key ideas

  • A force acts on an object.
  • A force can start an object moving, or stop a moving object.
  • A force can make an object speed up, slow down or change direction.
  • A force that pushes against the direction of motion slows the object down, or stops it.
Worked example 9.2.1
Look at the table in section 9.2. Which effect is shown by a hand that stops a rolling ball? Write the words.

Method. Find the row about the hand.

  1. Find the row. The row says a hand stops a rolling ball.
    A hand stops a rolling ball
  2. Read the effect. The effect in that row is stops a moving object.
    stops a moving object
  3. Write the answer.
    stops a moving object

Check. The hand pushes against the ball, so the ball stops. ✓

Watch out. The first column gives the effect. The second column gives an example.

Worked example 9.2.2
A still trolley is pushed, and it starts to roll. Which effect does the push have? Write the words.

Method. Think about the trolley before and after the push.

  1. Before the push. The trolley was still.
    still
  2. After the push. The push made the still trolley start moving.
    starts moving
  3. Write the answer.
    starts an object moving

Check. A force can start a still object moving. ✓

Worked example 9.2.3
A ball rolls on grass. The grass pushes against it, and the ball slows down. Does this force make the ball go faster or more slowly? Write the words.

Method. Think about the direction of the force.

  1. Find the direction of the force. The grass pushes against the ball, which is the opposite way to its motion.
    against the motion
  2. Decide the effect. A force that acts against the motion makes the ball go more slowly.
    more slowly
  3. Write the answer.
    more slowly

Check. Friction from the grass slows the ball. ✓

Watch out

Watch out: a force does not have to make an object go faster. A force that pushes against the direction of motion slows the object down, or stops it.

Try it at home or school (with a grown-up). Roll a small ball gently along a clear floor indoors, with a grown-up watching. Give it one gentle push to start it. Then stop it with your foot or a hand. Say which force started the ball and which force stopped it. Keep well away from stairs, doors and the edge of a table.

Exam tip

Grown-up tip. Ask: “What made the ball start moving? What made it stop? Did it speed up, slow down or change direction?” Let your child use the words force, start and stop.

Exercise 9B  Forces change how things move

1
FCan a force start a still ball moving? Write yes or no.
2
FLook at the table in section 9.2. Which effect is shown by a kick that starts a football moving? Write the words.
3
FIs a hand that stops a rolling ball applying a force to the ball? Write yes or no.
4
FFill in the gap: a force can make a moving object go more ___. Write the word.
5
FFill in the gap: a force can make an object change ___. Write one word about the way it moves.
6
FLook at the table in section 9.2. Which effect does a push on a trolley have when it speeds up? Write the words.
7
FA ball is rolling. Mei kicks it sideways, and it moves off in a new direction. Write the effect of the kick, in two words.
8
PSPriya says a force can only make things move faster. Is Priya right? Write no or yes.
9
PSAli pushes a still box, and it starts to move. Write the word for the effect: start, stop, speed up, slow down or change direction.
10
PSWorking scientifically. Predict what will happen to a rolling ball if a hand is held in its path. Write one prediction.
11
RWrite one sentence that explains how a force can change the motion of a ball.
9.3

Contact and magnetic forces

Year 3
F2: some forces need contact between 2 objects, but magnetic forces can act at a distance
Video lesson: Contact and magnetic forcesComing soon
Story. Leo pushes a box across the floor with his hand. Then he holds a magnet near a steel paper clip, and the clip moves towards it across a small gap. “How can it move if nothing touches it?” Leo asks. Mr Patel says, “Some forces need contact, and some can act at a distance.”

Most forces need contact. This means the two objects must touch. When your hand pushes a box, your hand touches the box, and the force acts through that contact.

Magnetic forces are different. A magnet can pull a steel paper clip across a small gap, without touching it. We say that magnetic forces can act at a distance.

TouchingNot touchingthe hand and box touchthe magnet pulls across a gap
Left: a hand touches a box and pushes it. Right: a magnet pulls a paper clip across a small gap, without touching it.

The table shows six actions. Some need the two objects to touch, and some do not.

ActionDo the two objects need to touch?
A hand pushes a toy car across a tableyes
A magnet pulls a steel paper clip across a small gapno
A rope pulls a sledge up a snowy hillyes
A foot kicks a footballyes
A book slows and stops on a table because of rubbingyes
A magnet pulls a steel nail across a small gapno

Key ideas

  • Many forces need contact: the two objects must touch.
  • Magnetic forces can act at a distance, so a magnet can pull a steel paper clip across a small gap.
  • A push, a pull and friction all need the objects to touch.
  • A steel nail can also be pulled by a magnet across a small gap.
Worked example 9.3.1
Look at the table in section 9.3. Does a rope pulling a sledge need the rope and the sledge to touch? Write yes or no.

Method. Find the row about the rope.

  1. Find the row. The row says a rope pulls a sledge up a snowy hill.
    rope and sledge
  2. Read the answer. The answer in the second column is yes.
    yes
  3. Write the answer.
    yes

Check. The rope must be attached to the sledge for the pull to work. ✓

Worked example 9.3.2
Look at the table in section 9.3. Does a magnet need to touch a steel nail for it to be pulled across a small gap? Write yes or no.

Method. Find the row about the steel nail.

  1. Find the row. The row says a magnet pulls a steel nail across a small gap.
    steel nail
  2. Read the answer. The answer in the second column is no.
    no
  3. Write the answer.
    no

Check. A magnetic force can act at a distance. ✓

Worked example 9.3.3
Look at the table in section 9.3. How many of the six actions need the two objects to touch? Write the number.

Method. Count the rows that say yes.

  1. Read the second column. The rows that say yes are the hand and toy car, the rope and sledge, the foot and football, and the book on the table.
    four yes
  2. Count them. There are four actions that need contact.
    4
  3. Write the answer.
    4

Check. The two magnet rows say no. ✓

Watch out

Watch out: a magnetic force can act at a distance, but most other forces, such as a push, a pull or friction, need the objects to touch.

Try it at home or school (with a grown-up). With a grown-up, hold a magnet near a steel paper clip on a table, and move the magnet slowly until the clip moves. Notice how close the magnet must be. Then push a small toy with your finger, and notice that your finger must touch it. Keep magnets away from your mouth, and away from phones and bank cards.

Exam tip

Grown-up tip. Ask: “Did the magnet and the clip touch? Did your finger touch the toy?” Let your child use the words contact and act at a distance.

Exercise 9C  Contact and magnetic forces

1
FDoes a hand pushing a box need the hand and the box to touch? Write yes or no.
2
FCan a magnet pull a steel paper clip across a small gap? Write yes or no.
3
FFill in the gap: magnetic forces can act at a ___. Write the word.
4
FFill in the gap: many forces need ___ between two objects. Write the word.
5
FLook at the table in section 9.3. Does a foot kicking a football need contact? Write yes or no.
6
FLook at the table in section 9.3. How many of the six actions need contact? Write the number.
7
FLook at the table in section 9.3. Write the action in the table that uses a steel nail.
8
FDoes a rope pulling a sledge need the rope and the sledge to touch? Write yes or no.
9
PSPriya says every force needs the two objects to touch. Is Priya right? Write no or yes.
10
PSWorking scientifically. Plan a test to find out how far a magnet can pull a steel paper clip. Write one thing that must stay the same each time.
11
RExplain why a magnet can pull a paper clip without touching it, but a hand must touch a box to push it. Write one sentence.
9.4

Moving on different surfaces

Year 3
F1: compare how things move on different surfaces, using a fair test
Video lesson: Moving on different surfacesComing soon
Story. Ruby and Aisha roll the same toy car down the same ramp, first on a smooth plastic tray, and then on sandpaper. “It goes much further on the tray,” says Aisha. Ruby says, “Let’s test it properly, so we know.”

A surface is the top layer that an object moves on, such as a floor, a carpet, a tray or a sheet of sandpaper. Different surfaces make an object move in different ways. A car on a smooth surface can roll a long way. The same car on a rough surface stops much sooner.

To compare surfaces, do a fair test. Use the same car, the same ramp, the same starting place and the same release. Change only the surface. Measure how far the car travels on each surface, and repeat each test three times.

startsurface being testeddistance travellednot to scale
A toy car rolls down a ramp and along a surface. The distance it travels on the surface is measured. Not to scale.

Ruby and Aisha recorded their results in the table. The average is the three trials added together, then divided by three.

SurfaceTrial 1 (cm)Trial 2 (cm)Trial 3 (cm)Average (cm)
Smooth plastic tray178182180180
Wooden floor149151150150
Thick carpet82788080
Sandpaper28323030
050100150average distance (cm)180 cmPlastic tray150 cmWooden floor80 cmCarpet30 cmSandpaper
The average distance on each surface, from the table: plastic tray 180 cm, wooden floor 150 cm, carpet 80 cm, sandpaper 30 cm.

Key ideas

  • Compare surfaces with a fair test: change only the surface.
  • Keep the same car, ramp, starting place and release each time.
  • Repeat each test three times, and find the average.
  • A rough surface makes the car travel a shorter distance than a smooth surface.
Worked example 9.4.1
Look at the table in section 9.4. Which surface made the car travel furthest, on average? Write the surface.

Method. Look at the averages.

  1. Find the largest average. The largest average is 180 cm, on the smooth plastic tray.
    180 cm
  2. Write the surface. So the car travelled furthest on the smooth plastic tray.
    Smooth plastic tray
  3. Write the answer.
    Smooth plastic tray

Check. The three trials on the tray were 178, 182 and 180 cm. ✓

Worked example 9.4.2
Look at the table in section 9.4. What is the average distance on the thick carpet? Write the number of centimetres.

Method. Add the three trials, then divide by three.

  1. Add the trials. The carpet trials are 82, 78 and 80 cm. Added together, they make 240 cm.
    82 + 78 + 80 = 240
  2. Divide by three. Divide 240 by 3 to get the average.
    240 divided by 3 is 80
  3. Write the answer.
    80

Check. The table shows an average of 80 cm for the carpet. ✓

Watch out. Divide by three, because there are three trials.

Worked example 9.4.3
Look at the table in section 9.4. How much further did the car travel on the smooth plastic tray than on the sandpaper, on average? Write the number of centimetres.

Method. Subtract the smaller average from the larger one.

  1. Read the two averages. The tray averages 180 cm, and the sandpaper averages 30 cm.
    180 and 30
  2. Subtract. The difference is 180 minus 30, which is 150.
    180 - 30 = 150
  3. Write the answer.
    150

Check. Check: 30 + 150 = 180. ✓

Watch out. Subtract the smaller number from the larger one.

Watch out

Watch out: if you change the car, the ramp or the release as well as the surface, the test is not fair. You will not know which change made the difference.

Try it at home or school (with a grown-up). With a grown-up, roll a toy car or a small tin down a ramp made from a book, with the other end resting on a tray. Test two surfaces: the tray and a towel. Use the same release every time. Measure how far the car goes with a ruler, three times on each surface, and find the average. Keep the ramp low, and do not roll the car near stairs or the edge of a table.

Exam tip

Grown-up tip. Ask: “What did you keep the same? Which surface made the car go furthest?” Let your child find the average by adding the three trials and dividing by three.

Exercise 9D  Moving on different surfaces

1
FLook at the bar chart in section 9.4. Which surface gives the shortest bar? Write the name.
2
FWrite the word for the top layer that an object moves on, such as a floor or a carpet.
3
FLook at the table in section 9.4. What is the average distance on the wooden floor? Write the number of centimetres.
4
FLook at the table in section 9.4. What is the average distance on the sandpaper? Write the number of centimetres.
5
FLook at the table in section 9.4. Is the average on the wooden floor more or less than the average on the thick carpet? Write more or less.
6
FFill in the gap: a rough surface makes the car travel a ___ distance. Write shorter or longer.
7
FLook at the table in section 9.4. What is the shortest of the three trials on the wooden floor? Write the number of centimetres.
8
FLook at the table in section 9.4. Add the three trials for the thick carpet. What is the total? Write the number of centimetres.
9
PSLook at the table in section 9.4. Which two surfaces have averages of more than 100 cm? Write the two names.
10
PSWorking scientifically. Predict whether the car will go further on a towel or on a wooden floor. Write towel or wooden floor, and give one reason.
11
RAli says the rougher the surface, the further the car goes. Is Ali right? Write no or yes, and give one reason.
9.5

Friction

Year 3
F1: friction as a force that slows moving things, with examples where it helps and where it gets in the way
Video lesson: FrictionComing soon
Story. Yusuf runs across the grass, and his trainers grip the ground so that he does not slip. Then he rolls a ball across the grass, and it slows down and stops. Mr Patel says, “Both of those are caused by friction.”

Friction is a force that acts between two surfaces that touch and rub against each other. Friction slows down moving things. It always acts against the direction of movement.

Rough surfaces make more friction than smooth surfaces. A block sliding on sandpaper slows down much faster than the same block sliding on a smooth tray.

Rough surfaceSmooth surfacemore friction: a longer arrowless friction: a shorter arrow
Two blocks. The block on the rough surface has a long friction arrow. The block on the smooth surface has a short friction arrow.

Friction can help us, or it can get in the way. The table gives some examples.

ExampleHelps or gets in the way?
Shoes grip the floor, so you do not sliphelps
Brakes press on a bicycle wheel to slow it downhelps
A sledge slows down on the grassgets in the way
Car tyres grip the road, so the car can stophelps
A toy car slows down on thick carpetgets in the way
Moving parts of a machine rub together and wear outgets in the way

Key ideas

  • Friction is a force that slows down moving things.
  • Friction acts between two surfaces that touch and rub.
  • Friction acts against the direction of movement.
  • Rough surfaces make more friction than smooth surfaces.
  • Friction can help, for example by stopping us from slipping, or it can get in the way.
Worked example 9.5.1
Look at the table in section 9.5. Does friction help or get in the way when a sledge slows down on the grass? Write the words.

Method. Find the row about the sledge.

  1. Find the row. The row says a sledge slows down on the grass.
    sledge
  2. Read the answer. The answer in the second column is gets in the way.
    gets in the way
  3. Write the answer.
    gets in the way

Check. Friction slows the sledge, which is not what we want here. ✓

Worked example 9.5.2
Look at the table in section 9.5. Which example uses friction to stop someone from slipping? Write the example.

Method. Look for the example about slipping.

  1. Read the table. The row about shoes says they grip the floor, so you do not slip.
    grip the floor
  2. Write the example. So the example is shoes gripping the floor.
    Shoes grip the floor
  3. Write the answer.
    Shoes grip the floor, so you do not slip

Check. Friction helps your shoes grip the floor. ✓

Worked example 9.5.3
Fill in the gap in this sentence about friction: friction acts between two surfaces that are touching and ___. Write the word.

Method. Read the key idea in section 9.5.

  1. Read the key idea. Friction acts between two surfaces that touch and rub.
    touch and rub
  2. Find the missing word. The gap comes after touching and, so the missing word is the word for rubbing.
    rub
  3. Write the answer.
    rubbing

Check. Touching and rubbing both describe the two surfaces. ✓

Watch out

Watch out: friction is not always bad. Without friction, your shoes would slip on the floor, so friction helps you walk.

Try it at home or school (with a grown-up). Choose one object, such as a plastic lid. With a grown-up, give it the same gentle push on a smooth floor, and then on a towel or a rug. Which surface makes it slide less far? Repeat each test three times, and find the average distance. Keep to a clear floor, with no one walking on it.

Exam tip

Grown-up tip. Ask: “Where does the friction come from? Which surface made the lid slide less far?” Let your child use the words friction, rough and smooth.

Exercise 9E  Friction

1
FWhat is the name of the force that slows down moving things when two surfaces rub together? Write the word.
2
FDoes friction speed up or slow down a moving object? Write the words.
3
FFill in the gap: friction acts against the direction of ___. Write the word.
4
FWhich surface makes more friction: sandpaper or a smooth plastic tray? Write the name.
5
FLook at the table in section 9.5. Does friction help or get in the way when brakes press on a bicycle wheel? Write the words.
6
FLook at the table in section 9.5. How many of the six examples say that friction helps? Write the number.
7
FWrite one example from the table in section 9.5 where friction gets in the way.
8
PSA toy car is pushed across a thick carpet and slows down quickly. Does friction help or get in the way? Write the words.
9
PSYusuf says grit on an icy road makes the road less grippy. Is Yusuf right? Write no or yes.
10
PSWorking scientifically. Plan a fair test to find out whether friction is bigger on a towel or on a plastic tray. Write one thing that must stay the same.
11
RExplain why a car needs friction between its tyres and the road in order to stop. Write one sentence.
9.6

Air resistance and water resistance

Year 3
F: forces act on moving objects; air resistance and water resistance slow down moving things
Video lesson: Air resistance and water resistanceComing soon
Story. Priya drops two sheets of the same paper from the same height. One stays flat, and the other is crumpled into a tight ball. The flat sheet floats down slowly, and the ball drops quickly. “Why does the flat sheet fall more slowly?” she asks. Mr Patel says, “Air is pushing against it. That is air resistance.”

Air resistance is a force that acts on moving things in the air. It pushes against them and slows them down. A bigger flat surface catches more air, so it has more air resistance. A parachute is wide, so it catches a lot of air and falls slowly.

Water resistance is a force that acts on moving things in water. It pushes against them and slows them down. A flat plate moved sideways through water feels harder to move than the same plate moved edge-first.

Flat sheetCrumpled ballbig flat shape: more airresistance, falls slowlysmall shape: less airresistance, falls quickly
Two shapes falling. The flat sheet has four air arrows pushing up, so it has more air resistance. The crumpled ball has one air arrow.
water resistancemotiona flat plate moved sideways through water
A flat plate moves sideways through water. Water resistance pushes back against the motion.

The table shows the results of a fair test. Priya dropped each shape from the same height, three times, and timed how long it took to reach the floor.

TrialFlat sheet (seconds)Crumpled ball (seconds)
Trial 12.00.6
Trial 22.20.5
Trial 32.10.7
Average2.10.6

Key ideas

  • Air resistance is a force that slows down moving things in air.
  • Air resistance pushes against a falling object, in the opposite direction to its motion.
  • A bigger flat surface catches more air, so it has more air resistance.
  • Water resistance is a force that slows down moving things in water.
Worked example 9.6.1
Look at the table in section 9.6. Which shape falls more slowly, on average: the flat sheet or the crumpled ball? Write the shape.

Method. Compare the averages.

  1. Read the averages. The flat sheet averages 2.1 seconds, and the crumpled ball averages 0.6 seconds.
    2.1 and 0.6
  2. Choose the slower one. The longer time is for the flat sheet, so it falls more slowly.
    flat sheet
  3. Write the answer.
    flat sheet

Check. The flat sheet catches more air, so it has more air resistance. ✓

Watch out. The shape that takes longer to reach the floor falls more slowly.

Worked example 9.6.2
Look at the table in section 9.6. How many seconds longer did the flat sheet take than the crumpled ball, on average? Write the number.

Method. Subtract the averages.

  1. Read the averages. The flat sheet averages 2.1 seconds, and the crumpled ball averages 0.6 seconds.
    2.1 and 0.6
  2. Subtract. The difference is 2.1 minus 0.6, which is 1.5.
    2.1 - 0.6 = 1.5
  3. Write the answer.
    1.5

Check. Check: 0.6 + 1.5 = 2.1. ✓

Watch out. Take the smaller time away from the larger time.

Worked example 9.6.3
A parachute is wide and catches a lot of air. Does a parachute have more air resistance or less air resistance than a small tight ball? Write more or less.

Method. Think about the air that each one catches.

  1. Think about the parachute. A parachute is wide, so it catches a lot of air.
    catches a lot of air
  2. Compare. A small tight ball catches less air, so the parachute has more air resistance.
    more
  3. Write the answer.
    more

Check. More air caught means more air resistance. ✓

Watch out

Watch out: the crumpled ball is not heavier than the flat sheet. It is made of the same paper, so only its shape is different.

Try it at home or school (with a grown-up). With a grown-up, take two sheets of the same paper. Drop one flat, and crumple the other into a tight ball. Drop both from the same height, one at a time, onto a rug or a soft floor. Watch which one falls more slowly. Repeat three times for each shape, and find the averages. Stand on the floor, and never climb on a chair to drop them.

Exam tip

Grown-up tip. Ask: “Which one fell more slowly? Which one caught more air?” Let your child explain the air pushing up, in their own words.

Exercise 9F  Air resistance and water resistance

1
FWhat is the name of the force that slows down a moving object in air? Write the words.
2
FWhat is the name of the force that slows down a moving object in water? Write the words.
3
FFill in the gap: air resistance pushes ___ a falling object. Write the word.
4
FWhich falls more slowly: a flat sheet of paper or a crumpled ball of the same paper? Write the shape.
5
FLook at the table in section 9.6. What is the average time for the crumpled ball? Write the number of seconds.
6
FLook at the table in section 9.6. What is the time for the flat sheet in trial 2? Write the number of seconds.
7
FDoes a bigger flat surface catch more air or less air? Write more or less.
8
FTrue or false: water resistance is a force that acts on moving things in water. Write true or false.
9
PSMo says a crumpled ball of paper is heavier than the same paper when it is flat, so it falls faster. Is Mo right? Write no or yes.
10
PSWorking scientifically. Predict whether a flat plate or a thin edge of the same plate will feel harder to move through water. Write one prediction.
11
RExplain why a parachute falls more slowly than a small ball. Write one sentence.

Chapter summary

SkillMethod
Pushes and pullsA force is a push or a pull. A push moves something away from the person, and a pull moves it towards them. Forces are drawn as arrows.
Changing motionA force can start, stop, speed up, slow down or change the direction of an object.
Contact and magnetsMany forces need the two objects to touch. Magnetic forces can act at a distance, across a small gap.
SurfacesIn a fair test on surfaces, change only the surface. A rough surface makes the car travel a shorter distance.
FrictionFriction is a force between surfaces that rub. It slows down moving things, and it can help us or get in the way.
Air and water resistanceAir resistance and water resistance are forces that push against moving things and slow them down.

Can you…?

  • I can say that a force is a push or a pull, and draw its direction as an arrow
  • I can name four things a force can do to a moving or still object
  • I can say which forces need contact, and that a magnet can act at a distance
  • I can plan and read a fair test on how things move on different surfaces
  • I can say what friction is, and give an example where it helps and one where it gets in the way
  • I can name air resistance and water resistance, and say what they do to moving things

Key terms

force
a push or a pull
push
a force that moves something away from the person pushing
pull
a force that moves something towards the person pulling
arrow
a drawing that shows the direction in which a force acts
contact
when two objects touch each other
act at a distance
to affect something without touching it, as a magnet can pull a steel paper clip across a small gap
surface
the top layer that an object moves on, such as a floor or a carpet
friction
a force that acts between two surfaces that rub, and slows down moving things
air resistance
a force from the air that pushes against moving things and slows them down
water resistance
a force from the water that pushes against moving things and slows them down
average
the total of several results, divided by how many results there are
fair test
a test where only one thing is changed, and everything else is kept the same

Exercise 9Z  Check-up

1
Look at the picture of a push and a pull. In the picture of the pull, does the arrow point towards the person or away from them? Write the word.
PushPullthe box moves away from youthe box moves towards you
2
Name the force that slows a box sliding across a table. Write the word.
3
Fill in the gap: a force can make a still object start ___. Write the word.
4
Can a magnetic force act at a distance? Write yes or no.
5
Look at the bar chart of the average distance on each surface. Which surface made the car travel the shortest distance? Write the name.
050100150average distance (cm)180 cmPlastic tray150 cmWooden floor80 cmCarpet30 cmSandpaper
6
Write two examples of friction helping us.
7
Write the name of the force that slows a moving object in water.
8
Ruby pulls a sledge towards herself. Does she push or pull? Write the word.
9
Which needs the two objects to touch: a hand pushing a box, or a magnet pulling a steel paper clip across a small gap? Write the one that needs contact.
10
Explain why a flat sheet of paper falls more slowly than a crumpled ball of the same paper. Write one sentence.

Total 18 marks. Allow about 18 minutes.

Exercise 9X  Practice test questions

1
FWrite the word that means a push or a pull.[1 mark]
2
FRuby pulls her sledge towards herself. Write the word for this kind of force: push or pull.[1 mark]
3
FLook at the picture of five ways a force changes a ball. Write the labels of two pictures: one where the force makes the ball start moving, and one where the force makes it change direction.
Starta push starts it movingStopa hand stops itSpeed upa push makes it fasterSlow downa force slows it downChange directiona kick sends it a new wayforcemotion
[2 marks]
4
FHere are three actions. A hand pushes a toy car across a table. A magnet pulls a steel paper clip across a small gap. A rope pulls a sledge up a snowy hill. Write the action that does not need the two objects to touch.[1 mark]
5
FHere are three trials for a toy car on thick carpet: 82 cm, 78 cm and 80 cm. Work out the average distance. Write the number of centimetres.[2 marks]
6
FWrite one example of friction helping us, and one example of friction getting in the way.[2 marks]
7
FA flat sheet of paper and a crumpled ball of the same paper are dropped from the same height. Which one falls more slowly, and which force slows it more? Write the shape, then the force.[2 marks]
8
RRuby says a bigger flat surface has more air resistance. Is Ruby right? Write yes or no, and explain why in one sentence.[3 marks]
9
FA flat plate is moved sideways through water. Write the name of the force that slows it down, and say whether that force acts with or against the motion.[2 marks]
10
FMo pushes a still ball, and it starts to move. Priya then stops the ball with her hand. Write what each push did to the ball.[2 marks]
← Chapter 8: Light and shadowsChapter 10: Magnets →

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