AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER
Moments, levers and gears
Forces · Lesson 6 of 20
Teacher copy - includes the notes for whoever is teaching from it.
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.
Note: Ask what happens if d is larger.
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. |
Exam Practice: Moments, levers and gears
Answer all questions. Use the mark allocation as a guide to how much to write. · 15 minutes
▸ Question 1 · 2 marks · Calculate. A force of 30 N is applied at a distance of 1.5 m from a pivot. The force acts at right angles. Calculate the moment of the force. Use the…
▸ Question 2 · 3 marks · Calculate. The diagram shows a uniform beam balanced on a pivot. A 40 N force acts 0.50 m to the left of the pivot. Calculate the force F.
▸ Question 3 · 2 marks · Explain. A spanner is used to undo a nut. Explain why a longer spanner makes it easier.
▸ Question 4 · 3 marks · Explain. A gear with 20 teeth is driven by a motor. It meshes with a gear with 40 teeth. Explain how the larger gear turns compared with the smaller…
▸ Question 5 · 4 marks · Calculate. A 60 N weight is 0.40 m to the left of a pivot on a balanced beam. A 30 N weight is on the right. Calculate its distance from the pivot.
Question 1 · 2 marks · Calculate
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“A force of 30 N is applied at a distance of 1.5 m from a pivot. The force acts at right angles. Calculate the moment of the force. Use the equation: moment = force × distance” |
HOW TO ANSWER IT Command word: Calculate. Worth 2 marks, so plan before writing.
Question 1 · mark scheme
2 marks available. Award a mark for each point made.
▸ Correct substitution. 1 mark
▸ 45 Nm. 1 mark
▸ Model answer. 30 × 1.5 = 45 Nm
Question 2 · 3 marks · Calculate
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The diagram shows a uniform beam balanced on a pivot. A 40 N force acts 0.50 m to the left of the pivot. Calculate the force F. (3 marks) |
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Question 2 · mark scheme
3 marks available. Award a mark for each point made.
▸ Anticlockwise moment 20 Nm. 1 mark
▸ Equates moments. 1 mark
▸ 25 N. 1 mark
▸ Model answer. Anticlockwise moment = 40 × 0.50 = 20 Nm. F × 0.80 = 20, so F = 25 N
Question 3 · 2 marks · Explain
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“A spanner is used to undo a nut. Explain why a longer spanner makes it easier.” |
HOW TO ANSWER IT Command word: Explain. Worth 2 marks, so plan before writing.
Question 3 · mark scheme
2 marks available. Award a mark for each point made.
▸ Moment = force × distance. 1 mark
▸ Greater distance means smaller force needed. 1 mark
▸ Model answer. The moment is force × distance, so a longer spanner gives a bigger moment for the same force, or needs a smaller force.
Question 4 · 3 marks · Explain
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“A gear with 20 teeth is driven by a motor. It meshes with a gear with 40 teeth. Explain how the larger gear turns compared with the smaller gear.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 4 · mark scheme
3 marks available. Award a mark for each point made.
▸ Opposite direction. 1 mark
▸ Slower (half the speed). 1 mark
▸ Greater moment. 1 mark
▸ Model answer. The larger gear turns in the opposite direction, at half the speed, and has a larger turning effect (moment).
Question 5 · 4 marks · Calculate
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“A 60 N weight is 0.40 m to the left of a pivot on a balanced beam. A 30 N weight is on the right. Calculate its distance from the pivot.” |
HOW TO ANSWER IT Command word: Calculate. Worth 4 marks, so plan before writing.
Question 5 · mark scheme
4 marks available. Award a mark for each point made.
▸ Anticlockwise moment 24 Nm. 1 mark
▸ Balanced moments. 1 mark
▸ Correct rearrangement. 1 mark
▸ 0.80 m. 1 mark
▸ Model answer. Anticlockwise = 60 × 0.40 = 24 Nm; 30 × d = 24; d = 0.80 m