Moment of Force (Torque)

The turning effect produced in a body about a fixed point due to an applied force is called the moment of force or torque. It is a vector quantity with the SI unit Nm (Newton meter). Moment of force plays the same role in rotational motion as force in translational motion.

Mathematically, τ = F x d, where τ is the moment of force, F is the force, and d is the perpendicular distance from the axis of rotation to the line of action of the force (moment arm). Increasing the magnitude of force or moment arm increases the moment of force and vice versa.

Axis of Rotation

Rotational motion is the turning or spinning motion of an object about an axis that passes through it. The axis of rotation is a line about which rotation takes place and remains fixed during rotational motion. The axis of rotation can be a pivot, hinges, or any other support.

Clockwise and Anticlockwise Moments

If the force is capable of rotating the body in a clockwise direction, the moment is known as a clockwise moment. If the force is capable of producing rotation in the anticlockwise direction, the moment is known as an anticlockwise moment. Conventionally, clockwise moment is taken as negative, while anticlockwise moment is taken as positive.

Example 4.1

The teacher applied a force of 60 N to open a door. The moment arms were:

• d₁ = 0.40 m: The torque produced was T = 24.0 Nm (anticlockwise)
• d₂ = 0.20 m: The torque produced was T = 12.0 Nm (anticlockwise)
• d₃ = 0.0 m: The torque produced was T = 0 Nm (since the line of action of force passes through the axis of rotation)

Key Concepts

• Moment of force (or torque) is the vector product of force and moment arm.
• Moment of force plays a crucial role in rotational motion, similar to how force affects translational motion.
• The axis of rotation is the fixed line about which an object rotates.
• Clockwise and anticlockwise moments are distinguished and have different signs.