1. KS3 Physics
9L Moments

2. Contents
9L Pressure and Moments
Moments
Summary activities

3. Force and rotation pivot 5N
A force acting on an object can cause it to turn about a pivot.
What happens to the see-saw when a force is applied on the left-hand side?
Does the seesaw turn? If so, clockwise or anti-clockwise?
pivot 5N

4. Force and rotation – a moment
The left-hand side of the see-saw moves downwards when a force is applied to it – this is an anticlockwise turn.
pivot
The turning effect of a force is called a moment.

5. distance from force to pivot
Using moments
A spanner is a lever that can be used to unscrew a nut.
The spanner exerts a moment or turning force on the nut.
pivot
distance from force to pivot
force
If the moment is big enough it will unscrew the nut.
If not, there are two ways of increasing the moment.

6. Using moments – increasing the moment
1. Increase the distance from the force to the pivot
– apply the force at the end or use a longer spanner.
pivot
distance from force to pivot
force
If the same force is applied over a greater distance, a larger moment is produced.

7. Using moments – increasing the moment
2. Increase the force applied
– push/pull harder or get someone stronger to do it!
pivot
distance from force to pivot
force
If a greater force is applied over the same distance, a larger moment is produced.

8. f x d Moment equation The moment of a force is given by the equation:
moment = force (N) x distance from pivot (cm or m)
moment f x d
Moments are measured in Newton centimetres (Ncm) or Newton metres (Nm).

9. Moment calculation pivot
Gina weighs 500 N and stands on one end of a seesaw. She is 0.5 m from the pivot.
What moment does she exert?
moment = 500 x 0.5
= 250 Nm
0.5 m
500 N
pivot

10. Principle of moments pivot The girl on the left exerts
an anti-clockwise moment,
which equals...
The girl on the right exerts a clockwise moment, which equals...
her weight x her distance
from pivot
her weight x her distance
from pivot

11. Principle of moments pivot
If the anticlockwise moment and clockwise moment are equal then the see-saw is balanced. This is known as the principle of moments.
When something is balanced about a pivot:
total clockwise moment = total anticlockwise moment

12. Both moments are equal and so the see-saw is balanced.
Principle of moments
The principle of moments can be investigated using 10g masses with this balance.
moment (left) = 10 x 7
= 70 gcm
moment (right) = (10 x 3) + (10 x 4)
= 70 gcm
Both moments are equal and so the see-saw is balanced.

13. Principle of moments – calculation
Two girls are sitting on opposite sides of on a see-saw. One girl weighs 200 N and is 1.5 m from the pivot. Where must her 150 N friend sit if the seesaw is to balance?
When the see-saw is balanced:
total clockwise moment = total anticlockwise moment
200 N x 1.5 m = 150 N x distance
200 x 1.5 = distance
150
distance of second girl = 2 m

14. Why don’t cranes fall over?
Tower cranes are essential at any major construction site.
load arm
trolley
counterweight
loading platform
tower
Concrete counterweights are fitted to the crane’s short arm. Why are these needed for lifting heavy loads?

15. Why don’t cranes fall over?
Using the principle of moments, when is the crane balanced?
3 m
6 m
10,000 N ?
moment of = moment of
load counterweight
If a 10,000 N counterweight is three metres from the tower, what weight can be lifted when the loading platform is six metres from the tower?

16. Why don’t cranes fall over?
moment of
load =
= ? x 6
load x distance of load from tower
moment of
counterweight
distance of counterweight from tower =
= 10,000 x 3
= 30,000 Nm
counterweight x
moment of load = moment of counterweight
? x 6 = 30,000
? = 3,000 6
? = 5,000 N

17. Crane operator activity
At what distance can the loading platform carry each load safely?

18. counterbalance – A weight used to balance another weight.
Glossary
counterbalance – A weight used to balance another weight.
effort – The force applied to use a lever.
hydraulics – The use of liquid to transmit pressure from
one place to another.
lever – A simple machine that moves about a pivot and makes work easier by increasing the size of a force.
load – The force moved when using a lever.
moment – The turning effect of a force. It equals the force multiplied by the distance from the pivot.
pascal – A unit of pressure (Pa). 1 Pa = 1 newton per square metre (N/m2).
pivot – The point around which a lever turns.
pressure – The force pushing on a certain area. It equals the force divided by area and can be measured in pascals (Pa).

19. Anagrams

20. Multiple-choice quiz