Moments
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Lesson notes
What is a Moment?
- A **moment** is a **turning force**.
- Moment = **Force** × **Perpendicular distance** to the pivot.
- Force is measured in **Newtons (N)**, distance in **metres (m)**, so moment is in **Newton-metres (Nm)**.
- A spanner is a **force-multiplier**: applying a force at a greater distance reduces the force needed to turn a bolt.
Principle of Moments
- An object is **balanced** when the sum of **clockwise moments** equals the sum of **anticlockwise moments** about a pivot.
- If moments are unequal, the object will **rotate** in the direction of the larger moment.
- This principle applies to seesaws, levers, and any object that can turn about a pivot.
Calculating Moments
- Use the equation: **Moment = Force × Distance**.
- Distance must be the **perpendicular distance** from the line of action of the force to the pivot.
- For a seesaw, calculate clockwise and anticlockwise moments separately.
- If clockwise moment = anticlockwise moment, the seesaw is **balanced**.
Example: Balanced Seesaw
- Clockwise moment: 10 N × 0.6 m = **6 Nm**.
- Anticlockwise moment: 12 N × 0.5 m = **6 Nm**.
- Since both moments are equal, the seesaw is **balanced**.
Example: Unbalanced Seesaw
- If clockwise moment ≠ anticlockwise moment, the seesaw is **unbalanced**.
- The side with the larger moment will **move downwards**.
- For example, 8 N × 0.4 m = 3.2 Nm clockwise, 6 N × 0.5 m = 3.0 Nm anticlockwise → unbalanced, clockwise side drops.
Key Points
- Moments are turning effects; they depend on **force size** and **distance from pivot**.
- Increasing distance from pivot increases the moment for the same force.
- The principle of moments is used to solve problems about balance and rotation.
A simple series circuit (for reference, not directly related to moments).
Slides
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Practice questions
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1.What is the correct equation for calculating a moment?
Easy- AMoment = Force × Distance
- BMoment = Force ÷ Distance
- CMoment = Distance ÷ Force
- DMoment = Force + Distance
2.What are the correct units for a moment?
Easy- ANewton metres (Nm)
- BNewtons (N)
- CMetres (m)
- DJoules (J)
3.What is the principle of moments?
Easy- AAn object is balanced when the sum of clockwise moments equals the sum of anticlockwise moments about a point.
- BAn object is balanced when the sum of clockwise moments is greater than anticlockwise moments.
- CAn object is balanced when the sum of clockwise moments is less than anticlockwise moments.
- DAn object is balanced when the force equals the distance.
4.A force of 20 N is applied at a perpendicular distance of 0.5 m from a pivot. What is the moment?
Medium- A10 Nm
- B40 Nm
- C20.5 Nm
- D0.025 Nm
5.A seesaw has a 10 N weight 0.6 m from the pivot on the left (anticlockwise) and a 12 N weight 0.5 m from the pivot on the right (clockwise). Is the seesaw balanced?
Medium- AYes, because clockwise moment = anticlockwise moment = 6 Nm
- BNo, because clockwise moment = 6 Nm and anticlockwise moment = 6 Nm, but forces are different
- CNo, because clockwise moment = 5 Nm and anticlockwise moment = 6 Nm
- DYes, because the forces are different so it must be balanced
6.A spanner is used to turn a bolt. A force of 30 N is applied at a distance of 0.2 m from the pivot. What force would be needed if the distance is increased to 0.6 m to produce the same moment?
Hard- A10 N
- B90 N
- C15 N
- D60 N
7.What is a turning force called?
Easy- AA moment
- BA newton
- CA lever
- DA pivot
8.In the equation Moment = Force × Distance, what does 'distance' refer to?
Easy- AThe perpendicular distance from the force to the pivot
- BThe length of the lever
- CThe distance from the force to the ground
- DThe total distance moved by the force
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