AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER
Moments, levers and gears
Forces · Lesson 6 of 20
Warm-up
Answer each one, then check.
1. What is a pivot?
The point about which something turns
2. What is the unit of force?
Newton
3. What is clockwise?
The direction the hands of a clock turn
4. What is the unit of distance?
Metre
5. Give an example of a lever.
A seesaw, crowbar or spanner
Learning Objectives
1. Describe examples in which forces cause rotation.
2. Recall and apply M = Fd.
3. Use the principle of moments to find an unknown force or distance.
4. Explain how levers and gears transmit the rotational effects of forces.
Moments
moment of a force = force × perpendicular distance from the pivot M = Fd
The turning effect of a force is called its moment, in newton-metres (Nm). If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment.
A Balanced Beam
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Clockwise moments equal anticlockwise moments when balanced. |
A balanced beam with a 40 N force 0.50 m from the pivot on one side and an unknown force 0.80 m from the pivot on the other.
Calculating a Moment
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A force of 30 N acts 1.5 m from a pivot at right angles. Calculate the moment. |
1. Write the equation
M = Fd
2. Substitute
M = 30 × 1.5
3. Answer
M = 45 Nm
Answer: 45 Nm
A Balanced Seesaw
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A 40 N child sits 0.50 m from the pivot of a balanced beam. Another force F acts on the other side 0.80 m from the pivot. Calculate F. |
1. Anticlockwise moment
40 × 0.50 = 20 Nm
2. Balanced
F × 0.80 = 20
3. Rearrange
F = 20 ÷ 0.80
4. Answer
F = 25 N
Answer: 25 N
Gears
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A gear with 20 teeth drives a gear with 40 teeth. Describe how the second gear turns compared with the first. |
1. Direction
It turns in the opposite direction
2. Speed
The larger gear turns more slowly (half the speed)
3. Turning effect
The larger gear has a greater turning effect (moment)
Answer: It turns the opposite way at half the speed with a larger moment.
Levers and Gears
Transmitting turning effects.
▸ Lever. A rigid bar that turns about a pivot; a longer distance from the pivot means a smaller force is needed.
▸ Examples. A crowbar, spanner, wheelbarrow, scissors.
▸ Gears. Toothed wheels that mesh to transfer rotation.
▸ Gear ratio. A large gear driven by a small gear turns slower but with a greater moment.
Key Terms
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Moment The turning effect of a force, in newton-metres. |
Pivot The point about which an object turns. |
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Clockwise In the direction of a clock's hands. |
Anticlockwise Opposite to the hands of a clock. |
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Lever A bar that can turn about a pivot. |
Gear A toothed wheel that transmits rotation. |
Your Task: Balance the Beam
12 minutes
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A beam is balanced. A 60 N weight is 0.40 m to the left of the pivot. Where must a 30 N weight be placed on the right? 1. Find the anticlockwise moment. 2. Equal the clockwise moment. |
A good answer shows: Clockwise = anticlockwise: 30 × d = 60 × 0.40 = 24, so d = 0.80 m.
Can I...?
☐ Recall M = Fd.
☐ Use perpendicular distance.
☐ State the principle of moments.
☐ Find an unknown force.
☐ Find an unknown distance.
☐ Explain levers.
☐ Explain gears.
☐ Use Nm.
Summary
✓ M = Fd.
✓ Balanced: total clockwise moment = total anticlockwise moment.
✓ Levers: a longer distance from the pivot gives a greater moment.
✓ Gears: change direction, speed and turning effect.
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EXAM FOCUS A force of 30 N acts 1.5 m from a pivot. Calculate the moment. (2 marks) Multiply force by distance and give Nm. |