The measure of a force that can cause an object to rotate about an axis is called:

Torque, also called the moment of force, is the turning or rotating effect of a force about an axis or a fixed point (pivot). It is the rotational equivalent of force: just as a force causes linear motion, torque causes rotational motion.

Torque is calculated as:

Torque (τ) = Force (F) × perpendicular distance (r) from the axis of rotation to the line of action of the force.

In vector form, τ = r × F (cross product). Torque is a vector quantity. Its SI unit is newton-metre (N m). Its dimensional formula is [M L² T⁻²], which is the same as that of work and energy, although torque and work are different physical quantities.

The turning effect of a force depends on two things: the magnitude of the force and the distance from the axis (called the moment arm or lever arm). A larger distance produces a larger torque with the same force. This explains many everyday observations:

A door handle is placed far from the hinges, so that a small force can open the door easily. Pushing near the hinges is much harder.

A long spanner (wrench) makes it easier to loosen a tight nut.

A mechanic uses a long handle to turn a car's wheel nuts.

The handle of a hand pump or a long crowbar helps to lift heavy objects (levers).

The steering wheel of a large vehicle is big, to increase the torque applied by the driver.

A pair of equal and opposite forces acting at different points on a body form a couple, which produces only rotation. Turning a tap, a key in a lock or a steering wheel with two hands applies a couple.

A body is in rotational equilibrium when the total clockwise torque equals the total anticlockwise torque about any point. This is the principle of moments, used in a beam balance and a see-saw.

Why not the other options?

• ✗Impulse is force multiplied by time.
• ✗Work is force multiplied by displacement in the direction of the force.
• ✗Pressure is force per unit area.