KinexiaLEARN PHYSICS

Torque — Seesaw Balance (Worked Example)

Medium
Problem

A child weighing 300 N sits on a seesaw 1.5 m from the pivot. What torque does her weight exert about the pivot?

Given

  • r = 1.5 m

  • F = 300 N (weight, perpendicular to seesaw)

Find

The torque τ.

Principle

Weight acts vertically at the child's seat; with the seesaw horizontal the lever arm is the full 1.5 m, so τ = rF.

Formula

Solution

  1. Write the formula: τ=rF\tau = rF

  2. Substitute: τ=(1.5)(300)\tau = (1.5)(300)

  3. Multiply: τ=450 N⋅m\tau = 450\ \mathrm{N·m}

450 N·m

Unit check

m × N = N·m — correct.

What this means physically

450 N·m trying to rotate her side down. To balance, an adult must supply an equal opposite torque — e.g. 600 N at 0.75 m. Seesaws are torque-balance machines.

Common mistake

Using the child's mass (30.6 kg) as F. Torque needs force — weight in newtons, not mass in kilograms.