1. Moment of a Force

2. Learning Objectives
recall and use the relationship between the moment of a force and its distance from the pivot:
Moment = force x perpendicular distance form pivot
recall that the weight of a body acts through the centre gravity
Define the torque of a couple

3. Moments
Forces can make objects turn if there is a pivot.

4. The see-saw
In order to make the seesaw turn about its pivot, forces have to be applied on either side of the plank

5. The see-saw
What happens when one person moves closer to the fulcrum or pivot?
The turning effect of the force is also dependent on the distance of the force from the pivot.

6. Turning effect
the turning effect is called the moment of force (or simply "moment“)
the distance is called the moment arm (or lever arm) of the force.

7. Moment of a Force To work out a moment, we need to know two things:
the force or weight applied
the distance from the pivot that the force or weight is applied.
Force and distance must be perpendicular to each other

8. Moment of a Force Product of force and the perpendicular distance
From the pivot

9. Moment and Equilibrium
Conditions for Equilibrium
Net force is zero
Net moment is zero at any point

10. Example
Find the force F that will balance the seesaw.

11. Example
Find the force F that will balance the seesaw.

12. Center of gravity
The centre of gravity of an object is the point where the whole weight of the object may be considered to act.

13. Center of gravity
For a regularly-shaped object, the centre of gravity is at its centre and, where supported there, it balances.

14. Example
A uniform plank, 100 cm long and weighing 1.0 N is balanced at its midpoint by a support. A weight of 1.20 N is hanged 5 cm from the left end. It was found out that an unknown weight W would balance the plank if it is positioned 72 cm from the left end.
What is the weight W of the object?
How much force is exerted by the support on the plank?

15. Couple Two parallel forces equal in magnitude
But opposite in direction
Has a turning effect about a pivot located midway between them

16. Torque of a Couple Product of One of the forces
And the distance between them

17. Your turn!
Answer numbers 1 to 4, page 56