Mass Is Not Weight
An astronaut stands on a bathroom scale at home, and it says 60 kg. She takes the same scale to the Moon, stands on it again, and it says 10 kg. She didn't eat less or lose any part of her body on the way. So what changed?
The pull
In the What Is Mass? lesson we saw that mass is how much stuff a thing is made of. Her mass didn't change: she is made of exactly the same stuff on the Moon.
What changed is the pull. The Earth pulls everything towards it: that is why a ball falls when you let go, and why you stay on the ground. This pull is called gravity. How hard gravity pulls on you is your weight.
The Moon pulls too, but much less hard than the Earth: about one sixth as hard. So on the Moon her weight is about one sixth of her weight on the Earth.
What a bathroom scale really measures
A bathroom scale doesn't see your stuff. It feels how hard you press down on it, and that is your weight. Then it shows a number in kilograms, because it was made for the Earth, where the pull is always the same.
On the Moon she presses down only one sixth as hard, so the scale shows one sixth of the number: 10 kg instead of 60. The scale is fooled. Her mass is still 60 kg.
A balance is not fooled. On the Moon, both pans are pulled less, by the same amount, so an apple still balances the same pieces as on the Earth. A balance compares masses; it gives the right answer anywhere.
"Weigh" in everyday words
In everyday life people say "I weigh 60 kilograms" or "weigh the flour". On the Earth that is fine: the pull is the same everywhere you are likely to go, so a scale's number is your mass. Scientists keep the two words apart, because once you leave the Earth, they are not the same any more.
Try it at home
- Stand on a bathroom scale and note the number. Now press down on a table with your hands as you stand. What happens to the number?
- You didn't change your mass. You changed how hard you pressed - and that is what the scale measures.
Words to know
- gravity - the pull of the Earth (or the Moon, or any planet) on everything
- weight - how hard gravity pulls on a thing
Big idea
Mass is how much stuff you are made of; weight is how hard gravity pulls on that stuff. On the Moon your weight is about one sixth of what it is on the Earth, but your mass doesn't change.