Moment of a Force

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

Moments Forces can make

objects turn if there is a pivot.

The see-saw In order to make the seesaw turn

about its pivot, forces have to be applied on either side of the plank

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.

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.

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

Moment of a Force Product of force and the

perpendicular distance From the pivot

Moment and Equilibrium Conditions for Equilibrium

Net force is zero Net moment is zero at any point

Example Find the force F that will balance the

seesaw.

Example Find the force F that will balance the

seesaw.

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.

Center of gravity For a regularly-

shaped object, the centre of gravity is at its centre and, where supported there, it balances.

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?

Couple Two parallel forces

equal in magnitude But opposite in

direction Has a turning

effect about a pivot located midway between them

Torque of a Couple Product of

One of the forces And the distance between them

Your turn!

Answer numbers 1 to 4, page 56