11. Turning Effect of Forces

Question 1

Moment of a force

Definition, examples and formula

Answer 1

Moment of a force (torque) is the product of the force and the perpendicular distance from the pivot to the line of action of the force

Moment of a force can produce a turning effect. (using a spanner, opening/closing a door, etc.)

Moment of force = force x perpendicular distance from the pivot
SI Unit = N m

Question 2

Centre of Gravity

Definition

Answer 2

The centre of gravity is the point at which the weight of the object appears to act.

Question 3

Conditions for Equilibrium

Answer 3

1. The resultant force on the object is zero (vector sum of forces is 0)
2. The resultant moment about any point on the object is zero

Taking moments about the (pivot), sum of clockwise moments = sum of anti-clockwise moments.

Question 4

Principle of Moments

Definition

Answer 4

For an object to be in equilibrium,

1. The resultant force on the object is zero (vector sum of forces is zero).
2. The resultant moment about any point on the object is zero (the sum of clockwise moments about any point is equal to the sum of anti-clockwise moments about the same point).

Question 5

Stability

Definition

Answer 5

Stability of an object is a measure of its ability to return to its original position after it is slightly displaced.

Question 6

How can the stability of an object be increased?

Answer 6

1. Lower its centre of gravity
2. Increase its base area

This ensures the line of action of weight falls within its base

Question 7

Resting position of a pivoted body

Answer 7

Centre of gravity is directly below the pivot.
Weight of the object does not produce a moment about the pivot when balanced.
Perpendicular distance from line of action of weight to pivot is zero