1. TURNING EFFECT OF FORCES
MOMENTS

2. Lesson Objectives
Identify what are the forces that will give rise to clockwise / anticlockwise moments
Identify the axes of rotation and the line of action
Explain what is clockwise / anticlockwise moments
Explain and apply the principle of moments to solve problems.

4. Inquiry approach activity 1
Distance moved VS Loads applied

5. MOMENTS What is moments?
A force can cause many things to move or stop. When a force causes an object to turn, this turning effect is called moments.
Example: A person sitting on a see-saw.

6. MOMENTS What is moments?
A force can cause many things to move or stop. When a force causes an object to turn, this turning effect is called moments.
Example: A person sitting on a see-saw.

7. Types Of Moments There are 2 types of moments: Clockwise moment
Anticlockwise moment

8. Types Of Moments There are 2 types of moments: Clockwise moment
Anticlockwise moment

9. CALCULATING MOMENTS
Moment = force  perpendicular distance between force and pivot
In symbols: Moments = F  d
Unit for moments: Newton-metre (Nm) d
Weight => F

10. Example 1:
A cat of weight 20N stands on one end of a see-saw and the distance between the cat and the pivot is 3m, find the moment.
Solution: In this case the cat is causing a clockwise moment.
Clockwise moment = F x d
= 20 x 3
= 60 Nm
F = 20N
d = 3m

11. Example 2:
A duck stands on one end of a see-saw, 5m away from the pivot. If the weight of the duck is 10N, find the moment.
Solution: The duck’s weight is causing an anticlockwise moment.
Anticlockwise moment = F x d
= 10 x 5
= 50 Nm
d = 5m
F = 10N

12. Group Assignment:
Identify at least 5 objects from the picture, indicate
the distance from the pivot
The direction of of force applied
Clockwise or anti-clockwise?

13. Examples which is the distance? Where is the force & pivot?

14. Principle of Moments
For an object to be in equilibrium(stable/not moving), the total clockwise moment must be equal to the anticlockwise moment about the same pivot point.
Clockwise moment = anti-clockwise moment
weight
weight
pivot
It is the fixed(non moving) point

15. SOLVING PROBLEMS RELATED TO PRINCIPLE OF MOMENTS
Step 1:
Identify what are the forces that will give rise to clockwise / anticlockwise moment
Step 2:
Find the clockwise / anticlockwise moment
Step 3:
Equate the clockwise and anticlockwise moments

16. WORKED EXAMPLE
10N
30N 6m d
Find the value of d.

17. Step 1:
Identify what are the forces that will give rise to clockwise / anticlockwise moment

18. WORKED EXAMPLE Find the value of d. 10N 30N 6m d Anticlockwise moment

19. Step 2:
Find the clockwise / anticlockwise moment

20. WORKED EXAMPLE Find the value of d. Solution:
Anticlockwise moment
Clockwise moment
Find the value of d.
Solution:
Clockwise moment = Force x distance between force and pivot
= 30 x d
= 30d Nm
Anticlockwise moment = Force x distance between force and pivot
= 10 x 6
= Nm

21. Step 3:
Equate the clockwise and anticlockwise moments

22. WORKED EXAMPLE Find the value of d. Solution:
Anticlockwise moment
Clockwise moment
Find the value of d.
Solution:
Clockwise moment = Force x distance between force and pivot
= 30 x d
= 30d Nm
Anticlockwise moment = Force x distance between force and pivot
= 10 x 6
= Nm
Using the principle of moments,
Clockwise moment = Anti-clockwise moment
30d = 60
d = 60  30
d = 2 m

23. Points to note:
The unit for force must be in Newtons, the unit for distance must be in metres.
The distance must measured from the force to the pivot.

24. Next lesson! Turning effect of forces Moments
Definitions of a torque, i.e. Force x perpendicular distance from the pivot
Describe and apply to examples
Principle of moments for a body in equilibrium
Anti-clockwise moments = Clockwise Moments
Centre of gravity (COG)
Weight of body taken to be acting at a single point
Qualitative decription of COG on object’s stability
Next lesson!