Physics
Torque and Rotation
Quick fact
Torque is a vector quantity; its direction is perpendicular to the plane of rotation, given by the right-hand rule.
Why this is interesting
Why is it easier to turn a door knob at the edge than near the hinge? The answer lies in torque, the rotational counterpart of force.
Read the full explanation
Understanding Torque and Rotation
Torque measures how effectively a force causes an object to rotate around a pivot. Imagine opening a door: pushing near the hinge requires more effort because the lever arm—the perpendicular distance from the pivot to the line of force—is short. Pushing at the handle gives a longer lever arm, so the same force produces a larger torque. Mathematically, torque (τ) = force (F) × lever arm (r) × sin(θ), where θ is the angle between the force vector and the lever arm. The effect of torque is angular acceleration, similar to how force causes linear acceleration. In rotational motion, torque plays the role that force does in linear motion.
A deeper explanation
The underlying principle is Newton's second law for rotation: net torque (τnet) equals moment of inertia (I) times angular acceleration (α), τ = Iα. The moment of inertia is the rotational analog of mass, depending on how an object's mass is distributed relative to the pivot. Torque matters because it governs rotational motion everywhere—from a child on a seesaw to the engine of a car. Understanding torque explains why longer wrenches make bolts easier to turn, why bicycle pedals are placed at the end of cranks, and why a tightrope walker uses a long pole to maintain balance. Without torque, there would be no rotation—only linear motion.