Mass Stays, Weight Changes
- Physics
- Ages 11–14 · Grades 6–8
- Hands-on experiment
What you’ll find out
- Mass is the amount of matter, in kilograms; weight is a force, in newtons.
- Weight equals mass times the gravitational field strength of the world you are on.
- A balance compares masses anywhere; a spring scale measures weight, which varies.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Two different quantities
In everyday speech, weight and mass mean the same thing, but in science they do not. Mass is the amount of matter in an object, measured in kilograms, and it stays the same wherever the object goes. Weight is the force of gravity pulling on that mass, and because it is a force it is measured in newtons. On Earth, gravity pulls each kilogram with a force of about 9.8 newtons, so a student with a mass of 60 kg weighs about 590 newtons. A bathroom scale really measures that force.
The same mass, a different weight
How strongly gravity pulls depends on the world you are standing on. The Moon has far less mass than Earth, so it pulls each kilogram with only about 1.6 newtons, roughly a sixth as hard. Our 60 kg student would still have a mass of 60 kg on the Moon, but would weigh only about 96 newtons, which is why astronauts could bounce along in heavy suits. On Mars, gravity pulls with about 3.7 newtons per kilogram, so the same student would weigh about 222 newtons.
Curriculum links: NGSS MS-PS2-4