Question

What is meant by the term ‘moment of force’?

Answer 1

Hint : The force acting on a body not only affects the linear part of the motion of the body, it can also give rise to a turning motion in the body about an axis of rotation. The moment of force plays a role in this turning motion.
Formula used:
$\tau ={{r}_{\bot }}F$

Complete step by step solution :
The force on a body not only has an effect on its linear motion but can also produce a turning effect on the body that can produce a rotational motion in it about an axis of rotation. This turning effect of force is termed as the ‘moment of force’.
The moment of force tries to rotate the body about an axis of rotation. This moment of force is also termed as torque acting on the body.
The torque acting on the body rotating about a fixed point or fixed axis of rotation is related to the magnitude of the force and the radial distance of the point of application of the force to the point or axis of rotation, perpendicular to the line of action of the force.
Mathematically, the magnitude of torque $\tau$ or moment of force on a body is given by
$\tau ={{r}_{\bot }}.F$
Where $F$ is the magnitude of the force acting on the body and ${{r}_{\bot }}$ is the radial distance of the point of application of the force to the point or axis of rotation, perpendicular to the line of action of the force.
The direction of the torque is perpendicular to the plane containing the force vector and the radial distance vector.

Note : Since the moment of force is dependent on the length of the radial arm, this concept can be exploited to produce many machines which a greater force with a smaller radial arm on a load upon application of a small force with a longer radial arm on the effort side. This happens to get the same balance of torque on both sides. This concept is used in many lever mechanisms and machines to get a greater force upon application of a smaller force.