Physics - MOMENTS

Momwnts and fulcrums

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  • Created by: sarrogin
  • Created on: 12-02-11 08:23

What is a moment

mass at distance d from pivot (

We define the turning effect with the equation:

Moment = Force × Perpendicular distance from the pivot

Since force is measured in newtons (N) and distances in metres (m) the unit for a moment is the newton-metre (Nm).

Moments can act in two ways: clockwise or anticlockwise.

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4 N force, 0.4m from pivot ( Moment = 4 N × 0·4 m
Moment = 1·6 Nm anticlockwise 4 N force, 0.25 m from pivot ( Moment = 4 N × 0·25 m
Moment = 1·0 Nm anticlockwise 5 N force, 0.50 m from pivot ( Moment = 5 N × 0·50 m
Moment = 2·5 Nm clockwise

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Balancing Moments I

One force on its own isn't much use to us. We normally look at situations where turning effects are balanced (or not!).

Let's look at the example below and find the missing force F:

clockwise moment being balanced by what force? (

If the system is balanced, the anticlockwise turning effect of force F must equal the clockwise turning effect:

clockwise moment = anticlockwise moment

Clockwise moment = 5 N × 0·50 m = 2·50 Nm.

Anticlockwise moment = F × 0·25 m = 2·50 Nm
Force F = 2·50 Nm ÷ 0·25 m = 10 N

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Balancing moments II

We have seen that more than one moment can act in one direction. We may sometimes wish to work out how these could be balanced.


At what distance must the 6 N force act to balance the other forces?

When balanced: sum of clockwise moments = sum of anticlockwise moments

It is easily shown that the clockwise moment = 3.0 Nm. To balance this, the anticlockwise moment must also be 3·0 Nm. So:

6 × d = 3·0
d = 3·0 ÷ 6 = 0·5 m

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thnks man!!! Really Awesome!!

Teresa Clare Middleton


Thanks. really useful and very grateful :)



Hey I just wanted to say that i'm really grateful for this moments revsion page. I was even stuck on preassure and moments but the only thing is i've got a great revsion sheet for moments (your's) but i don't have one for preassure. Do you think that you could do me one for preassure plz.                                I've got a test tommorow i'm gnna ace it!!!

By the way thnks again it really helped.

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