Levers, pulleys and gears
By the end of this chapter you will be able to
- Name the parts of a lever: the pivot, the effort and the load
- Explain that a longer effort arm makes the effort easier, and test this with a fair test
- Explain how a movable pulley makes lifting easier, and how a fixed pulley only changes the direction of the pull
- Explain how a small gear turning a large gear changes speed, and why the large gear turns more slowly
- Choose a mechanism for a job, and give a reason based on evidence
You should already know
- Pushes and pulls, and how forces change things (Year 3 Science, Chapter 9)
- Gravity, friction and resistance (Year 5 Science, Chapter 9)
Where this leads
- Working scientifically, planning and reporting (Year 5 Science, Chapter 1)
- Forces in Year 6 Science, which build on mechanisms
National Curriculum for England, Key Stage 2 science, Year 5. Content: F3 recognise that some mechanisms, including levers, pulleys and gears, allow a smaller force to have a greater effect.
Exercise P Do you remember?
Method. Think about what pulls objects towards the Earth.
- Think about the force. Gravity pulls objects towards the Earth.pulls objects
- Name the force. The force is gravity.gravity
- Write the answer. gravity
Check. Gravity acts on every object on the Earth. ✓
Method. Think about the words for a force.
- Think about the definition. A force is a push or a pull.push or pull
- Choose a word. A push is a kind of force, so the answer is push.push
- Write the answer. push
Check. A pull is also a force. ✓
Method. Think about surfaces that rub.
- Think about rubbing. Friction acts between two surfaces that rub.rub
- Name the force. The force is friction.friction
- Write the answer. friction
Check. Friction acts against the motion. ✓
Method. Think about the wheel that lifts things with a rope.
- Think about the wheel. A wheel with a rope running over it is used to lift things.lift things
- Name it. The word is pulley.pulley
- Write the answer. pulley
Check. Pulleys are a kind of simple machine. ✓
Method. Think about fair tests.
- Think about a fair test. A fair test keeps everything else the same.the same
- Fill the gap. So the missing word is same.same
- Write the answer. same
Check. A fair test changes one thing only. ✓
Levers
A lever is a rigid bar that turns about a fixed point, called the pivot or fulcrum. The force you push with is the effort. The force that is lifted or moved is the load.
A lever allows a smaller force to have a greater effect, when the effort is further from the pivot than the load. The distance from the pivot to the effort is the effort arm. The longer the effort arm, the easier the load is to lift.
In a fair test, the load arm stayed at 10 cm, and the load was 6 identical weights. The effort was changed by moving the effort arm. The table shows how many identical weights were needed at the effort end to lift the load.
| Effort arm (cm) | Weights needed to lift the load |
|---|---|
| 10 | 6 |
| 20 | 3 |
| 30 | 2 |
Key ideas
- A lever is a rigid bar that turns about a pivot.
- The effort is the force you apply; the load is the force you want to lift or move.
- A longer effort arm means a smaller effort is needed to lift the same load.
- A lever allows a smaller force to have a greater effect.
Method. Find the row for 20 cm.
- Find the row. The row for an effort arm of 20 cm needs 3 weights.3 weights
- Read the number. So 3 weights are needed.3
- Write the answer. 3
Check. With the effort arm twice as long, half the weights are needed. ✓
Method. Find the smallest number of weights.
- Read the table. The fewest weights is 2, at an effort arm of 30 cm.2 weights
- Name the arm. So the effort arm is 30 cm.30
- Write the answer. 30
Check. The longest arm needs the smallest effort. ✓
Method. Read the first row of the table.
- Read the row. The first row, with an effort arm of 10 cm, needs 6 weights.6 weights
- Write the number. So the answer is 6.6
- Write the answer. 6
Check. Six weights is the same as the load itself. ✓
Watch out
Watch out: a longer effort arm does not change the load. It makes the effort easier, but the effort moves further to lift the load.
Try it (with your teacher or a grown-up). Put a ruler across a pencil, so that it balances. Put one coin at the short end, and a coin at the far end. Slide the ruler, or the coins, until it balances. Which end needs to be further from the pivot to lift the heavier object?
Exam tip
Grown-up tip. Ask: “Which end is further from the pivot? Which one needs the smaller push?” Let your child use the words effort and load.
Exercise 10A Levers
Method. Read the definition in section 10.1.
- Read the definition. A lever is a rigid bar that turns about a fixed point, the pivot.fixed point
- Name it. The word is pivot.pivot
- Write the answer. pivot
Check. The pivot is also called the fulcrum. ✓
Method. Read section 10.1.
- Read the definition. The force you push with is the effort.the effort
- Name it. The word is effort.effort
- Write the answer. effort
Check. The load is the force that is lifted or moved. ✓
Method. Read section 10.1.
- Read the definition. The force that is lifted or moved is the load.the load
- Name it. The word is load.load
- Write the answer. load
Check. The effort is the force you apply. ✓
| Effort arm (cm) | Weights needed to lift the load |
|---|---|
| 10 | 6 |
| 20 | 3 |
| 30 | 2 |
Method. Read the first row.
- Read the row. The effort arm of 10 cm needs 6 weights.6 weights
- Write the number. So the answer is 6.6
- Write the answer. 6
Check. The load arm and the first effort arm are both 10 cm. ✓
| Effort arm (cm) | Weights needed to lift the load |
|---|---|
| 10 | 6 |
| 20 | 3 |
| 30 | 2 |
Method. Read the row for 30 cm.
- Read the row. The effort arm of 30 cm needs 2 weights.2 weights
- Write the number. So the answer is 2.2
- Write the answer. 2
Check. A longer effort arm needs fewer weights. ✓
| Effort arm (cm) | Weights needed to lift the load |
|---|---|
| 10 | 6 |
| 20 | 3 |
| 30 | 2 |
Method. Subtract the 30 cm number from the 10 cm number.
- Read the numbers. At 10 cm, 6 weights are needed. At 30 cm, 2 weights are needed.6 and 2
- Subtract. The difference is 6 minus 2, which is 4 weights.6 - 2 = 4
- Write the answer. 4
Check. Check: 2 + 4 = 6. ✓
| Effort arm (cm) | Weights needed to lift the load |
|---|---|
| 10 | 6 |
| 20 | 3 |
| 30 | 2 |
Method. Find the arm that is twice 10 cm.
- Find the arm. Twice 10 cm is 20 cm.20 cm
- Check the weights. The 20 cm row needs 3 weights, which is half of 6.3 weights
- Write the answer. 20
Check. Doubling the arm halves the weights needed. ✓
Method. Think about where the push is.
- Think about the push. The child pushes down on the far end, which is the long arm.far end
- Name the part. So the effort is on the long arm.long arm
- Write the answer. long arm
Check. The effort is on the long arm, far from the pivot. ✓
Method. Read the labels in the picture.
- Read the labels. The long arm is on the effort side, labelled long arm.long arm
- Name the arm. So the effort arm is the longer one.effort
- Write the answer. effort
Check. The effort arm is the longer one in the picture. ✓
Method. Read the key ideas in section 10.1.
- Read the key idea. A lever allows a smaller force to have a greater effect.smaller force
- Fill the gap. So the missing word is smaller.smaller
- Write the answer. smaller
Check. The longer effort arm makes the effort easier. ✓
Method. Think about the pattern in the table.
- Think about the pattern. The longer the effort arm, the fewer weights are needed.fewer weights
- Make the prediction. So a 20 cm arm needs fewer weights than a 10 cm arm.fewer
- Write the answer. fewer
Check. Check with your teacher on a simple lever. ✓
Method. Think about the turning.
- Think about the arm. The effort is further from the pivot.further from the pivot
- Write the reason. So a smaller effort gives the same turning effect, lifting the load.turning effect
- Write the answer. Accept any sentence that says the longer arm gives the effort a greater turning effect, so a smaller effort lifts the same load.
Check. A longer arm gives the effort more turning effect. ✓
Method. Think about the fair test.
- Think about the test. Only the effort arm was changed, and the load was kept the same.only the effort arm
- Write the reason. So any difference in the weights is caused by the arm length.caused by the arm length
- Write the answer. Accept any sentence that says only the effort arm changed, and the load stayed the same, so the difference in weights is due to the arm length.
Check. A fair test changes one thing, here the arm length. ✓
Pulleys
A pulley is a wheel with a groove, which a rope runs over. There are two kinds that are useful for this chapter:
- a fixed pulley, which is fixed to a support. It changes only the direction of the pull. Pulling down lifts the load up, but the effort is the same as the weight of the load.
- a movable pulley, which moves with the load. The load is shared between the two parts of the rope, so the effort needed is about half the load.
The table shows how many identical weights were needed to lift the same load, for three set-ups.
| Set-up | Weights needed to lift the same load |
|---|---|
| no pulley, lifting the load straight up | 8 |
| a fixed pulley, fixed to a support | 8 |
| a movable pulley, which moves with the load | 4 |
Key ideas
- A fixed pulley changes the direction of the pull, but not the size of the effort.
- A movable pulley shares the load between two ropes, so the effort is about half.
- Pulleys help to lift heavy loads with less effort.
- The pulley picture shows the effort pulling down, and the load going up.
Method. Find the movable pulley row.
- Find the row. The movable pulley row shows 4 weights.4 weights
- Read the number. So the answer is 4.4
- Write the answer. 4
Check. A movable pulley needs about half the weights. ✓
Method. Find the no pulley row.
- Find the row. The no pulley row shows 8 weights.8 weights
- Read the number. So the answer is 8.8
- Write the answer. 8
Check. The fixed pulley row also shows 8. ✓
Method. Subtract the movable pulley number from the no pulley number.
- Read the two numbers. No pulley needs 8 weights, and a movable pulley needs 4.8 and 4
- Subtract. The difference is 8 minus 4, which is 4 weights fewer.8 - 4 = 4
- Write the answer. 4
Check. Check: 4 + 4 = 8. ✓
Watch out
Watch out: a fixed pulley does not make the lift easier. It only changes the direction of the pull. Only a movable pulley shares the load.
Try it (with your teacher or a grown-up). With a grown-up, tie a rope to the back of a chair, and loop it over a broom handle or a wooden dowel that is held up. Pull the rope down to lift a small bag. Then tie the bag to a second loop of rope so that it moves with the bag. Compare how hard each one is to lift.
Exam tip
Grown-up tip. Ask: “Which set-up was easier? Did the pulley change the direction, or the size of the pull?” Keep the bag light and the set-up safe.
Exercise 10B Pulleys
Method. Read section 10.2.
- Read the definition. A pulley is a wheel with a groove, which a rope runs over.a wheel
- Name it. The word is pulley.pulley
- Write the answer. pulley
Check. Pulleys are used to lift loads. ✓
Method. Read the bullet points in section 10.2.
- Read the bullet. A fixed pulley changes only the direction of the pull.changes the direction
- Choose the word. So the answer is fixed.fixed
- Write the answer. fixed
Check. A fixed pulley is fixed to a support. ✓
Method. Read the bullet points in section 10.2.
- Read the bullet. A movable pulley moves with the load, and shares it between two parts of the rope.shares the load
- Choose the word. So the answer is movable.movable
- Write the answer. movable
Check. Sharing the load halves the effort needed. ✓
| Set-up | Weights needed to lift the same load |
|---|---|
| no pulley, lifting the load straight up | 8 |
| a fixed pulley, fixed to a support | 8 |
| a movable pulley, which moves with the load | 4 |
Method. Read the fixed pulley row.
- Read the row. The fixed pulley row shows 8 weights.8
- Write the number. So the answer is 8.8
- Write the answer. 8
Check. A fixed pulley does not reduce the effort. ✓
| Set-up | Weights needed to lift the same load |
|---|---|
| no pulley, lifting the load straight up | 8 |
| a fixed pulley, fixed to a support | 8 |
| a movable pulley, which moves with the load | 4 |
Method. Read the movable pulley row.
- Read the row. The movable pulley row shows 4 weights.4
- Write the number. So the answer is 4.4
- Write the answer. 4
Check. The movable pulley halves the 8 weights. ✓
| Set-up | Weights needed to lift the same load |
|---|---|
| no pulley, lifting the load straight up | 8 |
| a fixed pulley, fixed to a support | 8 |
| a movable pulley, which moves with the load | 4 |
Method. Compare the numbers.
- Compare the numbers. The movable pulley needs 4 weights, and no pulley needs 8.4 and 8
- Decide. Fewer weights means it is easier, so the answer is yes.yes
- Write the answer. yes
Check. The movable pulley needs less effort. ✓
| Set-up | Weights needed to lift the same load |
|---|---|
| no pulley, lifting the load straight up | 8 |
| a fixed pulley, fixed to a support | 8 |
| a movable pulley, which moves with the load | 4 |
Method. Compare the numbers in the table.
- Compare the rows. The no pulley and fixed pulley rows both show 8 weights.8 and 8
- Name the two. So no pulley and a fixed pulley needed the same number.no pulley and a fixed pulley
- Write the answer. no pulley and a fixed pulley
Check. A fixed pulley only changes the direction. ✓
Method. Read section 10.2.
- Read the bullet. The effort needed is about half the load.half
- Fill the gap. So the missing word is half.half
- Write the answer. half
Check. The load is shared by two parts of the rope. ✓
Method. Read the labels.
- Read the labels. The right-hand pulley is labelled movable pulley.right-hand
- Name the pulley. So the movable pulley is the one on the right.movable
- Write the answer. movable
Check. The movable pulley moves with the load. ✓
Method. Think about the rope.
- Think about the rope. The load is shared between two parts of the rope.shared
- Make the prediction. So the load feels lighter to lift.lighter
- Write the answer. lighter
Check. Check the prediction in the Try it box. ✓
Method. Think about what a fixed pulley does.
- Think about the pulley. A fixed pulley is fixed to a support.fixed to a support
- Write the reason. So it changes only the direction, not the size of the effort.direction
- Write the answer. Accept any sentence that says a fixed pulley only changes the direction of the pull, so the effort is the same as the load.
Check. A fixed pulley is useful for pulling down to lift. ✓
Method. Think about the rope.
- Think about the rope. The rope is in two parts, holding the load.two parts
- Write the reason. So each part carries about half the load, and the effort is about half.about half
- Write the answer. Accept any sentence that says the load is shared between two parts of the rope, so the effort needed is about half.
Check. Sharing the load makes lifting easier. ✓
Gears
Gears are toothed wheels that fit together, so that one turns the other. The gear that is turned by the motor or by the person is the driver. The gear that is turned is the driven gear.
When a small gear turns a large gear, the large gear turns more slowly, but it can turn with a greater effect on a load. When a large gear turns a small gear, the small gear turns faster. Two meshed gears turn in opposite directions.
| Gear | Number of teeth |
|---|---|
| small gear (driver) | 10 |
| large gear (driven) | 20 |
Key ideas
- Gears are toothed wheels that mesh, so that one turns the other.
- A small gear turning a large gear makes the large gear turn more slowly.
- Meshed gears turn in opposite directions.
- A gear with more teeth turns more slowly, with more effect on the load.
Method. Read the table.
- Read the row for the large gear. The large gear has 20 teeth.20 teeth
- Write the number. So the answer is 20.20
- Write the answer. 20
Check. The large gear has twice as many teeth as the small gear. ✓
Method. Divide the large gear’s teeth by the small gear’s teeth.
- Read the teeth. The large gear has 20 teeth, and the small gear has 10.20 and 10
- Divide. The small gear turns 20 divided by 10, which is 2 times.20 divided by 10 is 2
- Write the answer. 2
Check. The small gear turns twice for each turn of the large gear. ✓
Method. Think about how many turns each makes.
- Think about the turns. The large gear makes one turn while the small gear makes two.one turn
- Choose the gear. So the large gear turns more slowly.large gear
- Write the answer. large gear
Check. The large gear has more teeth, so it turns more slowly. ✓
Watch out
Watch out: a gear with more teeth turns more slowly, but it can turn with more effect. A bigger gear does not turn faster.
Try it (with your teacher or a grown-up). Look at two gear wheels, or pictures of a bicycle’s front and back gears, with your teacher. Count the teeth on each wheel. Which one would turn more slowly if the other one turned once? Write one sentence about your answer.
Exam tip
Grown-up tip. Ask: “Which gear has more teeth? Which turns more slowly? Why is that easier when climbing a hill?” Let your child count the teeth.
Exercise 10C Gears
Method. Read section 10.3.
- Read the definition. Gears are toothed wheels that fit together.toothed wheels
- Name them. The word is gears.gears
- Write the answer. gears
Check. Gears can change the speed of turning. ✓
Method. Think about the gear that is turned first.
- Think about the first gear. In the picture, the small gear is turned first, so it is the driver.turned first
- Name the gear. So the driver is the small gear.small gear
- Write the answer. small gear
Check. The driver is the gear that turns the other gear. ✓
| Gear | Number of teeth |
|---|---|
| small gear (driver) | 10 |
| large gear (driven) | 20 |
Method. Read the row for the small gear.
- Read the row. The small gear row shows 10 teeth.10
- Write the number. So the answer is 10.10
- Write the answer. 10
Check. The large gear has 20 teeth. ✓
| Gear | Number of teeth |
|---|---|
| small gear (driver) | 10 |
| large gear (driven) | 20 |
Method. Read the row for the large gear.
- Read the row. The large gear row shows 20 teeth.20
- Write the number. So the answer is 20.20
- Write the answer. 20
Check. The large gear has twice as many teeth. ✓
| Gear | Number of teeth |
|---|---|
| small gear (driver) | 10 |
| large gear (driven) | 20 |
Method. Subtract the small gear teeth from the large gear teeth.
- Read the numbers. The large gear has 20 teeth and the small gear has 10.20 and 10
- Subtract. The difference is 20 minus 10, which is 10 teeth.20 - 10 = 10
- Write the answer. 10
Check. Check: 10 + 10 = 20. ✓
Method. Divide the large gear teeth by the small gear teeth.
- Read the teeth. The large gear has 20 teeth and the small gear has 10.20 and 10
- Divide. 20 divided by 10 is 2, so the small gear turns twice.2
- Write the answer. 2
Check. The large gear turns once while the small gear turns twice. ✓
Method. Think about the number of turns.
- Think about the turns. The large gear turns once while the small gear turns twice.turns once
- Choose the gear. So the large gear turns more slowly.large gear
- Write the answer. large gear
Check. More teeth means a slower turn. ✓
Method. Look at two gears that mesh.
- Look at the gears. When two gears mesh, each tooth pushes the next one round.mesh
- Choose the answer. So the two gears turn in opposite ways.opposite ways
- Write the answer. opposite ways
Check. Meshed gears turn in opposite directions. ✓
Method. Read the key ideas in section 10.3.
- Read the key idea. A gear with more teeth turns more slowly, with more effect on the load.more slowly
- Fill the gap. So the missing word is slowly.slowly
- Write the answer. slowly
Check. The large gear turns slowly but with more effect. ✓
Method. Think about the turns.
- Think about the turns. A large gear turning a small gear makes the small gear turn many times.many times
- Make the prediction. So the small gear is the one that turns faster.small
- Write the answer. small
Check. Check with your teacher, using two gears. ✓
Method. Think about the teeth.
- Think about the teeth. The large gear has more teeth, so each tooth moves it only a little.more teeth
- Write the reason. So it turns more slowly than the small gear.more slowly
- Write the answer. Accept any sentence that says the large gear has more teeth, so one turn of the small gear moves it only part of the way round.
Check. More teeth means each turn covers less distance per tooth. ✓
Method. Think about the effect on the pedals.
- Think about the gear. A small gear at the back makes the wheel turn more slowly.more slowly
- Write the reason. So the pedals are easier to turn, with more effect on the hill.easier to turn
- Write the answer. Accept any sentence that says the small gear makes the wheel turn more slowly, but with more effect, so the pedals are easier to turn.
Check. A slow gear with more effect helps on a hill. ✓
Choosing a mechanism for a job
Every mechanism uses a smaller force to have a greater effect, in a different way:
| Object | Mechanism used | What the smaller force does |
|---|---|---|
| a seesaw | lever | a light child can lift a heavy child on the far end |
| a wheelbarrow | lever | a smaller push lifts a heavier load |
| a bicycle, with a small gear at the front and a large gear at the back | gears | the pedals turn the wheel with more force, though more slowly |
| a crane lifting a heavy load on a rope | pulley | a movable pulley shares the load between two ropes |
Key ideas
- A lever uses a long arm, so that a small effort can lift a load.
- A movable pulley shares the load, so the effort is about half.
- Gears with different numbers of teeth change the speed and effect of turning.
- Choose the mechanism that fits the job, and give a reason.
Method. Read the row for the wheelbarrow.
- Read the row. The wheelbarrow row says lever.lever
- Name the mechanism. So the mechanism is a lever.lever
- Write the answer. lever
Check. A wheelbarrow works like a lever on its wheel. ✓
Method. Read the row for the bicycle.
- Read the row. The bicycle row says gears.gears
- Name the mechanism. So the mechanism is gears.gears
- Write the answer. gears
Check. The bicycle’s gears change the effect of the pedals. ✓
Method. Think about which mechanism shares the load.
- Think about the rope. A movable pulley shares the load between two parts of the rope.shares the load
- Name the mechanism. So the best mechanism is a pulley.pulley
- Write the answer. pulley
Check. A movable pulley halves the effort needed. ✓
Watch out
Watch out: a mechanism does not make the work disappear. A lever, a pulley or gears can make the effort smaller, but it often has to move further.
Try it (with your teacher or a grown-up). Make a lever with a ruler and a pencil, and a second lever with a longer ruler. With your teacher, use the same small weight to lift the same load on each. Which lever did it more easily? Write one sentence giving your reason.
Exam tip
Grown-up tip. Ask: “Which machine did the job more easily? Why do you think so?” Let your child give a reason based on what they saw.
Exercise 10D Choosing a mechanism for a job
| Object | Mechanism used | What the smaller force does |
|---|---|---|
| a seesaw | lever | a light child can lift a heavy child on the far end |
| a wheelbarrow | lever | a smaller push lifts a heavier load |
| a bicycle, with a small gear at the front and a large gear at the back | gears | the pedals turn the wheel with more force, though more slowly |
| a crane lifting a heavy load on a rope | pulley | a movable pulley shares the load between two ropes |
Method. Read the mechanism column.
- Read the column. The seesaw row says lever.lever
- Name the object. So the object is a seesaw, and a wheelbarrow also uses a lever.a seesaw
- Write the answer. a seesaw
Check. A seesaw is a simple lever. ✓
| Object | Mechanism used | What the smaller force does |
|---|---|---|
| a seesaw | lever | a light child can lift a heavy child on the far end |
| a wheelbarrow | lever | a smaller push lifts a heavier load |
| a bicycle, with a small gear at the front and a large gear at the back | gears | the pedals turn the wheel with more force, though more slowly |
| a crane lifting a heavy load on a rope | pulley | a movable pulley shares the load between two ropes |
Method. Read the mechanism column.
- Read the column. The crane row says pulley.pulley
- Name the object. So the object is a crane lifting a load on a rope.a crane
- Write the answer. a crane lifting a heavy load on a rope
Check. A crane uses pulleys to lift loads. ✓
| Object | Mechanism used | What the smaller force does |
|---|---|---|
| a seesaw | lever | a light child can lift a heavy child on the far end |
| a wheelbarrow | lever | a smaller push lifts a heavier load |
| a bicycle, with a small gear at the front and a large gear at the back | gears | the pedals turn the wheel with more force, though more slowly |
| a crane lifting a heavy load on a rope | pulley | a movable pulley shares the load between two ropes |
Method. Read the bicycle row.
- Read the row. The bicycle row says gears.gears
- Name it. So the mechanism is gears.gears
- Write the answer. gears
Check. The bicycle uses gears to make pedalling easier. ✓
| Object | Mechanism used | What the smaller force does |
|---|---|---|
| a seesaw | lever | a light child can lift a heavy child on the far end |
| a wheelbarrow | lever | a smaller push lifts a heavier load |
| a bicycle, with a small gear at the front and a large gear at the back | gears | the pedals turn the wheel with more force, though more slowly |
| a crane lifting a heavy load on a rope | pulley | a movable pulley shares the load between two ropes |
Method. Read the seesaw row.
- Read the row. A light child can lift a heavy child on the far end.light child
- Name the effect. So the smaller force lifts a heavier child.lifts a heavier child
- Write the answer. lifts a heavier child
Check. The longer arm helps the light child. ✓
Method. Think about which mechanism shares the load.
- Think about the rope. A movable pulley shares the load between two ropes.two ropes
- Choose the mechanism. So a pulley is the best choice.pulley
- Write the answer. pulley
Check. A movable pulley halves the effort. ✓
Method. Think about the bicycle.
- Think about the bicycle. A small gear at the back makes the pedals easier to turn.easier to turn
- Choose the mechanism. So gears are the best choice.gears
- Write the answer. gears
Check. Gears change the effect of the pedals. ✓
Method. Think about the handle.
- Think about the handle. A long handle gives a long effort arm, which makes the effort easier.long handle
- Choose the mechanism. So a lever is the best choice.lever
- Write the answer. lever
Check. A longer arm makes the effort easier. ✓
Method. Think about the effort arm.
- Think about the handle. A long handle gives a long effort arm.long handle
- Write the reason. So a smaller effort turns the lid, which is easier.easier
- Write the answer. Accept any sentence that says a longer effort arm, such as a long handle, makes the effort smaller, so the lid is easier to turn.
Check. A longer effort arm makes the effort easier. ✓
Method. Think about the evidence in the table.
- Think about the table. A movable pulley needs 4 weights, and no pulley needs 8.4 and 8
- Write the reason. So the movable pulley needs about half the effort.about half
- Write the answer. Accept any sentence that says the load is shared, so the effort needed is about half.
Check. The evidence from the table supports the choice. ✓
Method. Think about what pulleys do.
- Think about the crane. A crane lifts very heavy loads.heavy loads
- Write the reason. Pulleys make the lift easier, by sharing the load or changing the direction.changing the direction
- Write the answer. Accept any sentence that says pulleys can make lifting easier, by sharing the load or changing the direction of the pull.
Check. Pulleys help lift heavy loads with less effort. ✓
Testing a mechanism fairly
A fair test on a mechanism changes only one thing, such as the set-up of the pulley. The load is kept the same, the number of trials is repeated, and the results are recorded in a table.
A conclusion must match the evidence. It should say what changed, and what happened, without going beyond the results.
| Set-up | Weights needed to lift the same load |
|---|---|
| no pulley, lifting the load straight up | 8 |
| a fixed pulley, fixed to a support | 8 |
| a movable pulley, which moves with the load | 4 |
Key ideas
- A fair test changes only the set-up of the mechanism, and keeps the load the same.
- Record the results in a table with units, and repeat the test to check them.
- A conclusion must match the evidence in the table.
- Say how much you trust the result, and why.
Method. Compare the movable pulley with no pulley.
- Compare the rows. The movable pulley needed 4 weights, but no pulley needed 8.4 and 8
- Write the conclusion. So the movable pulley made the lift easier, needing about half the weights.easier
- Write the answer. the movable pulley made the lift easier
Check. The conclusion matches the table. ✓
Method. Think about what changed.
- Think about the changes. Changing two things at once means the test is not fair.two things
- Decide. So the test is not fair, and the answer is no.no
- Write the answer. no
Check. A fair test changes only one thing. ✓
Method. Think about repeats.
- Think about the repeats. Repeats that agree make the result more reliable.agree
- Decide. So the result can be trusted more, and the answer is yes.yes
- Write the answer. yes
Check. Repeat trials help us trust a result. ✓
Watch out
Watch out: a conclusion must not go further than the evidence. The table shows fewer weights for a movable pulley, but it does not show what happens to the rope length.
Try it (with your teacher or a grown-up). Repeat the pulley test from section 10.2 with your teacher, changing only the set-up each time. Write a short report with three parts: what you did, what you found, and your conclusion. Say whether your test was fair.
Exam tip
Grown-up tip. Ask: “What did you change? What did you keep the same? Does your conclusion match your results?” Let your child read the report aloud.
Exercise 10E Testing a mechanism fairly
Method. Read section 10.5.
- Read the description. A fair test changes only one thing, such as the set-up of the pulley.set-up
- Name the thing. So the thing changed on purpose is the set-up of the pulley.the set-up of the pulley
- Write the answer. the set-up of the pulley
Check. The load is kept the same. ✓
Method. Read the key ideas in section 10.5.
- Read the key idea. The load is kept the same in a fair test.kept the same
- Choose the word. So the load is kept the same.load
- Write the answer. load
Check. Only the set-up changes, so the load must be kept the same. ✓
| Set-up | Weights needed to lift the same load |
|---|---|
| no pulley, lifting the load straight up | 8 |
| a fixed pulley, fixed to a support | 8 |
| a movable pulley, which moves with the load | 4 |
Method. Read the movable pulley row.
- Read the row. The movable pulley row shows 4 weights.4
- Write the number. So the answer is 4.4
- Write the answer. 4
Check. Check the row name before you answer. ✓
| Set-up | Weights needed to lift the same load |
|---|---|
| no pulley, lifting the load straight up | 8 |
| a fixed pulley, fixed to a support | 8 |
| a movable pulley, which moves with the load | 4 |
Method. Subtract the movable pulley number from the no pulley number.
- Read the numbers. No pulley needs 8 weights, and the movable pulley needs 4.8 and 4
- Subtract. The difference is 8 minus 4, which is 4.8 - 4 = 4
- Write the answer. 4
Check. Check: 4 + 4 = 8. ✓
Method. Think about what changed.
- Think about the changes. Changing two things at once means the test is not fair.two things
- Decide. So the test is not fair.no
- Write the answer. no
Check. Only one thing should change in a fair test. ✓
Method. Think about repeats.
- Think about the repeats. Repeats that agree make a result more reliable.agree
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. Repeat trials help us trust a result. ✓
Method. Think about what the test shows.
- Think about the test. The test only compares a movable pulley with no pulley and a fixed pulley.only compares
- Decide. So the conclusion goes beyond the evidence, and the answer is no.no
- Write the answer. no
Check. A conclusion must not go beyond the evidence. ✓
Method. Think about what makes the test fair.
- Think about the test. Only the effort arm should change in a lever test.effort arm
- Write one thing. The load, the pivot and the weights must stay the same.stay the same
- Write the answer. Accept any one of: the load, the pivot position, the same weights.
Check. Keeping one thing the same makes the test fair. ✓
Method. Think about repeats.
- Think about one trial. One trial can be affected by a mistake or by chance.mistake
- Write the reason. Repeats show whether the results agree, which makes the result more trusted.agree
- Write the answer. Accept any sentence that says repeats show whether the results agree, so a single mistake is less likely to affect the result.
Check. Agreement between trials increases trust. ✓
Method. Think about the evidence.
- Think about the table. The table shows only the results of the test.only the results
- Write the reason. So a conclusion must not claim more than the evidence shows.not claim more
- Write the answer. Accept any sentence that says a conclusion must match the evidence, and must not claim more than the table shows.
Check. A conclusion should match its evidence. ✓
Chapter summary
| Skill | Method |
|---|---|
| Levers | A lever turns about a pivot. A longer effort arm makes the effort smaller, so a smaller force has a greater effect. |
| Pulleys | A fixed pulley only changes the direction of the pull. A movable pulley shares the load, so the effort is about half. |
| Gears | Meshed gears turn in opposite directions. A large gear turns more slowly than a small gear that turns it. |
| Choosing | Choose the mechanism that fits the job, and give a reason based on evidence from a fair test. |
| Fair tests | Change only one thing, such as the arm length or the pulley set-up, and keep the load the same. |
Can you…?
- I can name the pivot, the effort and the load in a lever, and explain why a longer effort arm helps
- I can explain how a movable pulley makes lifting easier, and what a fixed pulley does
- I can explain how a small gear turning a large gear changes speed, and find the number of turns
- I can choose a mechanism for a job and give a reason, based on evidence
- I can plan and report a fair test of a mechanism, with a conclusion that matches the evidence
Key terms
- lever
- a rigid bar that turns about a pivot, so that a small force can have a greater effect
- pivot
- the fixed point that a lever turns about; also called the fulcrum
- effort
- the force you apply to a lever or a pulley
- load
- the force that is lifted or moved by a mechanism
- effort arm
- the distance from the pivot to the place where the effort is applied
- pulley
- a wheel with a groove, which a rope runs over to lift or move a load
- movable pulley
- a pulley that moves with the load, so that the load is shared between two parts of the rope
- gear
- a toothed wheel that fits with another gear, so that one turns the other
- driver
- the gear that is turned first, and turns the other gear
- mechanism
- a device, such as a lever, pulley or gears, that makes a force easier to use
Exercise 10Z Check-up
Method. Read section 10.1.
- Read the definition. A lever turns about a fixed point, the pivot.fixed point
- Name it. The word is pivot.pivot
- Write the answer. pivot
Check. The pivot is also called the fulcrum. ✓
| Effort arm (cm) | Weights needed to lift the load |
|---|---|
| 10 | 6 |
| 20 | 3 |
| 30 | 2 |
Method. Read the row for 20 cm.
- Read the row. The 20 cm row needs 3 weights.3 weights
- Write the number. So the answer is 3.3
- Write the answer. 3
Check. A longer effort arm needs fewer weights. ✓
| Set-up | Weights needed to lift the same load |
|---|---|
| no pulley, lifting the load straight up | 8 |
| a fixed pulley, fixed to a support | 8 |
| a movable pulley, which moves with the load | 4 |
Method. Read the movable pulley row.
- Read the row. The movable pulley row shows 4 weights.4
- Write the number. So the answer is 4.4
- Write the answer. 4
Check. The movable pulley halves the load. ✓
| Set-up | Weights needed to lift the same load |
|---|---|
| no pulley, lifting the load straight up | 8 |
| a fixed pulley, fixed to a support | 8 |
| a movable pulley, which moves with the load | 4 |
Method. Compare the rows with no pulley.
- Compare the rows. The fixed pulley row shows 8 weights, the same as no pulley.8
- Name it. So the fixed pulley is the same as no pulley.a fixed pulley
- Write the answer. a fixed pulley
Check. A fixed pulley only changes the direction. ✓
Method. Read section 10.2.
- Read the bullet. The effort needed is about half the load.half
- Fill the gap. So the missing word is half.half
- Write the answer. half
Check. The load is shared between two parts of the rope. ✓
| Gear | Number of teeth |
|---|---|
| small gear (driver) | 10 |
| large gear (driven) | 20 |
Method. Read the large gear row.
- Read the row. The large gear has 20 teeth.20
- Write the number. So the answer is 20.20
- Write the answer. 20
Check. The large gear has twice the teeth of the small gear. ✓
Method. Divide the large gear teeth by the small gear teeth.
- Read the teeth. The large gear has 20 teeth, and the small gear has 10.20 and 10
- Divide. The small gear turns 20 divided by 10, which is 2 times.2
- Write the answer. 2
Check. The large gear turns once while the small gear turns twice. ✓
Method. Think about how many turns each one makes.
- Think about the turns. The large gear makes one turn while the small gear makes two.one turn
- Choose the gear. So the large gear turns more slowly.large gear
- Write the answer. large gear
Check. More teeth means a slower turn. ✓
Method. Read the table in section 10.4.
- Read the row. The wheelbarrow row says lever.lever
- Name the mechanism. So the mechanism is a lever.lever
- Write the answer. lever
Check. A wheelbarrow works like a lever. ✓
Method. Think about which mechanism shares the load.
- Think about the rope. A movable pulley shares the load between two parts of the rope.two parts
- Name the mechanism. So the mechanism is a pulley.pulley
- Write the answer. pulley
Check. A movable pulley halves the effort. ✓
Total 15 marks. Allow about 15 minutes.
Exercise 10X Practice test questions
Method. Read section 10.1.
- Read the definition. A lever turns about a fixed point, the pivot.fixed point
- Name it. The word is pivot.pivot
- Write the answer. pivot
Check. The pivot is also called the fulcrum. ✓
Method. Read section 10.2.
- Read the definition. A pulley is a wheel with a groove, which a rope runs over.a wheel
- Name it. The word is pulley.pulley
- Write the answer. pulley
Check. Pulleys are used to lift loads. ✓
Method. Read section 10.2.
- Read the bullet. A fixed pulley changes only the direction of the pull.changes the direction
- Choose the word. So the answer is fixed.fixed
- Write the answer. fixed
Check. A fixed pulley is fixed to a support. ✓
| Gear | Number of teeth |
|---|---|
| small gear (driver) | 10 |
| large gear (driven) | 20 |
Method. Read the small gear row.
- Read the row. The small gear has 10 teeth.10
- Write the number. So the answer is 10.10
- Write the answer. 10
Check. The large gear has 20 teeth. ✓
| Effort arm (cm) | Weights needed to lift the load |
|---|---|
| 10 | 6 |
| 20 | 3 |
| 30 | 2 |
Method. Read the row for 30 cm.
- Read the row. The 30 cm row needs 2 weights.2 weights
- Write the number. So the answer is 2.2
- Write the answer. 2
Check. A longer effort arm needs fewer weights. ✓
| Set-up | Weights needed to lift the same load |
|---|---|
| no pulley, lifting the load straight up | 8 |
| a fixed pulley, fixed to a support | 8 |
| a movable pulley, which moves with the load | 4 |
Method. Subtract the movable pulley number from the no pulley number.
- Read the numbers. No pulley needs 8 weights, and a movable pulley needs 4.8 and 4
- Subtract. The difference is 8 minus 4, which is 4 weights.8 - 4 = 4
- Write the answer. 4
Check. Check: 4 + 4 = 8. ✓
| Gear | Number of teeth |
|---|---|
| small gear (driver) | 10 |
| large gear (driven) | 20 |
Method. Divide the large gear teeth by the small gear teeth.
- Read the teeth. The large gear has 20 teeth, and the small gear has 10.20 and 10
- Divide. 20 divided by 10 is 2, so the small gear turns twice.2
- Write the answer. 2
Check. The large gear turns once while the small gear turns twice. ✓
Method. Think about the rope.
- Think about the rope. The rope is in two parts, holding the load.two parts
- Write the reason. So each part carries about half the load, and the effort is about half.about half
- Write the answer. Accept any sentence that says the load is shared between two parts of the rope, so the effort needed is about half.
Check. Sharing the load halves the effort. ✓
Method. Look at the lever table.
- Look at the table. The longer the effort arm, the fewer weights are needed.fewer weights
- Give the reason. So a longer arm makes the effort smaller, not bigger.smaller
- Write the answer. no. Accept any sentence that says a longer effort arm makes the effort smaller, so a lighter push lifts the load.
Check. The table shows the pattern. ✓
Method. Think about the teeth.
- Think about the teeth. The large gear has 20 teeth, and the small gear has 10.20 and 10
- Write the reason. So each tooth of the small gear moves the large gear only part of a turn.part of a turn
- Write the answer. Accept any sentence that says the large gear has more teeth, so one turn of the small gear moves it only part of the way round.
Check. More teeth means a slower turn. ✓