Moment of force:

Moment of force: the rotational effect of a force on a body about an axis of rotation is described in terms of moment of force.

Moment of a force about an axis of rotation is measured as the product of magnitude of force and the perpendicular distance of direction of force from the axis of rotation.

I.e. Moment of force = Force x moment arm

·         It is a vector quantity.

·         Its SI unit is Newton metre (Nm)

Centre of Gravity: the centre of gravity of a body is that point through which the entire weight of body acts. The centre of gravity of a body does not change with the change in orientation of body in space.

The weight of body acts through centre of gravity in the downward direction. Hence a body can be brought to equilibrium by applying a force equal to its weight in the vertically upward direction through centre of gravity.

Equilibrium: if the resultant of all the forces acting on a body is zero then the body is said to be in equilibrium.

If a body is in equilibrium, it will be either at rest or in uniform motion if it is at rest, the equilibrium is called static, otherwise dynamic.

Static equilibrium is of the following three types:

(i)                  Stable Equilibrium: If on slight displacement from equilibrium position, a body has tendency to regain its original position, it is said to be in stable equilibrium.

(ii)                Unstable equilibrium: if on slight displacement from equilibrium position, a body moves in the direction of displacement and does not regain its original position, the equilibrium is said to unstable equilibrium. In this equilibrium, the centre of gravity of the body is at the highest position.

(iii)               Neutral equilibrium: if on slight displacement from equilibrium position a body has no tendency to come back to its original position or to move in the direction of displacement, it is said to be in neutral equilibrium. In neutral equilibrium, the centre of gravity always remains at the same height.

Conditions for stable Equilibrium: For stable equilibrium of a body, the following two conditions should be fulfilled.

(i)                  The centre of gravity of the body should be at the minimum height.

The vertical line passing through the centre of gravity of the body should pass through the base of the body.