Course: Separate Physics only | Tier: Foundation + Higher | Linked equation: M = Fd
Notes
A force or a system of forces can cause an object to rotate. The turning effect of a force is called its moment.
✏️ Copy into your book
- Moment = force × distance (M = F × d), where d is the perpendicular distance from the pivot to the line of action of the force. Unit: newton-metre (Nm).
- Principle of moments: if an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.
- Levers are force multipliers: a small force applied far from the pivot gives a large moment.
- Gears transmit the rotational effect of forces. A small gear driving a larger gear gives a bigger moment but a slower rotation.
- Gears that touch turn in opposite directions.
Everyday examples
A spanner with a longer handle makes it easier to undo a nut. A door handle is placed far from the hinges. On a bike, a small front gear driving a large rear gear makes it easier to cycle uphill.
Questions
Q1 (F) What is a moment?
Show answer
The turning effect of a force.
Q2 (F) A 20 N force is applied 0.30 m from a pivot. Calculate the moment.
Show answer
F: M = F d
I: M = 20 × 0.30
A: 6.0 Nm
Q3 (F) Why is it easier to undo a tight nut with a longer spanner?
Show answer
The distance from the pivot is bigger, so the same force produces a bigger moment.
Q4 (F/H) State the principle of moments.
Show answer
For a balanced object, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.
Q5 (F/H) A parent weighing 400 N sits 1.5 m from the pivot of a seesaw. How far from the pivot must a 300 N child sit on the other side to balance it?
Show answer
Clockwise moment = anticlockwise moment
400 × 1.5 = 300 × d
d = 600 ÷ 300 = 2.0 m
Q6 (H) A small gear drives a larger gear. Compare the speed, direction and moment of the larger gear.
Show answer
The larger gear turns more slowly, in the opposite direction, but with a bigger moment.
Q7 (H) A uniform 2.0 m plank is pivoted at its centre. A 50 N weight hangs 0.80 m left of the pivot. What force is needed 0.40 m right of the pivot to balance it?
Show answer
50 × 0.80 = F × 0.40
F = 40 ÷ 0.40 = 100 N
(The plank’s own weight acts at the pivot, so it has no moment.)
