Turning effects of forces (4.1)

What is the centre of gravity?

Centre of gravity - the point at which the weight of the object may be considered to act

For symmetrical objects it’s located at the point of symmetry

When is an object stable?

An object is stable when its centre of gravity lies above its base

Thus the wider the base of an object, the lower its centre of gravity and its more stable

What is a moment?

Moment - the turning effect of a force - the product of the force and perpendicular distance from the pivot

M=Fd

Where M is moment in Nm

Where F is force in N

Where d is perpendicular distance from the pivot in m

When the force isn’t acting perpendicular to the object:

M=Fdcosθ

What is a couple?

Couple - a pair of forces that act to produce rotation only

The forces in a couple must be:

• Equal in magnitude

• Opposite in direction

• Perpendicular to the distance in between them

They produce a resultant force of 0

What is torque?

Torque - the moment of a couple

τ=Fd

Where τ is torque in Nm

Where F is force in N

Where d is the perpendicular distance between the forces