KinexiaLEARN PHYSICS

Hooke's Law — Kitchen Scale (Worked Example)

Easy
Problem

The spring inside a kitchen scale has k = 250 N/m. Hanging a bag of flour stretches it by 0.20 m. What is the weight of the flour?

Given

  • k = 250 N/m

  • x = 0.20 m

Find

The spring force F (= the flour's weight).

Principle

At equilibrium the spring force balances the weight, so F = kx directly reads the weight.

Formula

Solution

  1. Write the law: F=kxF = kx

  2. Substitute: F=(250)(0.20)F = (250)(0.20)

  3. Multiply: F=50 NF = 50\ \mathrm{N}

50 N

Unit check

N/m × m = N — a force, correct.

What this means physically

50 N corresponds to about 5.1 kg of flour — a big bag. Old-fashioned scales are just calibrated springs with a dial.

Common mistake

Confusing mass and weight. The spring measures force (newtons); divide by g to get the mass in kilograms.