What is the moment of a 20 N force acting at a perpendicular distance of 0.50 m from a pivot?

10 N m. Force × perpendicular distance.

Through which point does the weight of a body act?

its centre of gravity. For a uniform, symmetrical body this is the middle.

A force acts along a line that passes through the pivot. What is its moment about the pivot?

zero. The perpendicular distance is zero.

What are the units of the moment of a force?

N m. A force multiplied by a distance.

Moments and centre of gravity · Cambridge A Level Physics

In plain words

A force can make something turn as well as move. How strongly it turns depends on two things: how big the force is, and how far from the pivot it acts. That is why a long spanner shifts a stiff nut.

Three things to know

Moment of a force = force × perpendicular distance from the pivot to the line of action of the force. Units: N m.
If the force is at an angle to the lever, use the perpendicular distance, or the component of the force at right angles to the lever.
The weight of a body can be treated as acting at a single point, its centre of gravity. For a uniform beam that is the middle.

Worked example

A 40 N force is applied to the end of a spanner 0.25 m long, at 60° to the spanner. Find the moment about the nut.

Only the component at right angles to the spanner turns it: 40 sin 60° = 34.6 N.
Moment = 34.6 × 0.25.
= 8.7 N m.

Tips and tricks

"Perpendicular distance" is the shortest distance from the pivot to the line the force acts along. It isn't always the length of the lever.
A force whose line of action passes through the pivot has no moment about it.

5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Moments and centre of gravity: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

What is the moment of a 20 N force acting at a perpendicular distance of 0.50 m from a pivot?
- 0.025 N m
- 10 N m (the answer)
- 20.5 N m
- 40 N m
Force × perpendicular distance.
Through which point does the weight of a body act?
- always its geometric centre
- its centre of gravity (the answer)
- its base
- its highest point
For a uniform, symmetrical body this is the middle.
A force acts along a line that passes through the pivot. What is its moment about the pivot?
- zero (the answer)
- force × length of the lever
- half the force × length of the lever
- it depends on the angle
The perpendicular distance is zero.
A 10 N force acts on a door 0.80 m from the hinge, at 30° to the door. What is its moment about the hinge?
- 4.0 N m (the answer)
- 6.9 N m
- 8.0 N m
- 12.5 N m
10 sin 30° × 0.80.
What are the units of the moment of a force?
- N m (the answer)
- N/m
- N s
- N m²
A force multiplied by a distance.

5 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

3 questions, 6 marks. Write your answers on paper, then check them.

Moments and centre of gravity

Cambridge International AS & A Level Physics 9702 · 6 marks · papermunch.org

Name ______________________________ Date ______________

Find the moment of a 15 N force acting at a perpendicular distance of 0.40 m from a pivot.[2]
Show answerHide answer
6.0 N m.
I got this one
A uniform metre rule of weight 1.2 N is pivoted at the 20 cm mark. Find the moment of its weight about the pivot.[2]
Show answerHide answer
0.36 N m. The weight acts at the 50 cm mark, which is 0.30 m from the pivot.
I got this one
State what is meant by the centre of gravity of an object.[2]
Show answerHide answer
The single point at which the whole weight of the object may be taken to act.
I got this one

Answers: Moments and centre of gravity

1. 6.0 N m.
2. 0.36 N m. The weight acts at the 50 cm mark, which is 0.30 m from the pivot.
3. The single point at which the whole weight of the object may be taken to act.