Advanced · 12 min
Torque and equilibrium
Calculate turning effects and balance forces and torques in static situations.
A force can turn an object
Torque, also called moment of a force, is the turning effect about a chosen pivot. Its magnitude is force multiplied by the perpendicular distance from the pivot to the force's line of action. This distance is not always the length of the lever. Torque is measured in N·m. Although joules have the same dimensions, use N·m for torque to distinguish it from energy.
Worked example
A 15 N force acts perpendicular to a lever 0.4 m from its pivot. Torque magnitude = 15 × 0.4 = 6 N·m
Balance both kinds of motion
For static equilibrium, the net force must be zero and the net torque about any pivot must be zero. In a simple balanced lever, clockwise and anticlockwise torques are equal. Include the lever's own weight when it matters. A support force at the pivot has zero torque about that pivot but still belongs in the force balance.
Worked example
A weightless horizontal lever has 20 N downward at 0.3 m to the left of its pivot. A downward force at 0.6 m to the right must be 20 × 0.3 ÷ 0.6 = 10 N.
Balance a lever on paper
- Draw a weightless horizontal lever with a pivot. Place a 40 N downward force 0.2 m to the left and calculate the balancing force 0.8 m to the right.
- Find the upward support force needed at the pivot so the total vertical force is also zero.