KinexiaLEARN PHYSICS

Torque — Tightening a Bolt (Worked Example)

Easy
Problem

A mechanic pulls perpendicularly on a wrench 0.25 m from the bolt centre with a force of 80 N. What torque does she apply to the bolt?

Given

  • r = 0.25 m

  • F = 80 N (perpendicular)

Find

The torque τ.

Principle

Torque is the rotational equivalent of force: τ = rF for a perpendicular pull, tending to rotate the bolt.

Formula

Solution

  1. Write the formula: τ=rF\tau = rF

  2. Substitute: τ=(0.25)(80)\tau = (0.25)(80)

  3. Multiply: τ=20 N⋅m\tau = 20\ \mathrm{N·m}

20 N·m clockwise

Unit check

m × N = N·m — torque units, correct.

What this means physically

20 N·m — a typical firm tightening torque for a small bolt. Longer wrenches multiply the same hand force into bigger torque, which is why cheater bars work.

Common mistake

Reporting the answer in joules. N·m for torque and J for energy share dimensions but torque is never called joules — keep the concepts separate.