The turning effect of a force is called moment of force or torque. It is measured by the product of the force and the perpendicular distance of the line of action of force and axis of rotation. Thus, torque = force × perpendicular distance from the axis of rotation.
Torque τ = F × r = mar = mr2α
∵ α = r α
For the entire body τ = (Σ mr2) α = I α.
∵ Σ mr2 = I, the moment of inertia and α is angular acceleration.
