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Mass and weight

Gravity and orbits · level 2, lesson 2 of 6

A question to start

An astronaut takes a 10 kg bag of rice to Mars. On Mars it feels much lighter. Is there less rice in the bag?

The idea

There are two different things here, and they have two names.

  • Mass is how much stuff something is made of. The bag holds the same grains of rice on Mars as on Earth, so its mass is the same: 10 kg, anywhere in the universe. Mass is measured in kilograms (kg).
  • Weight is how hard a planet pulls on that stuff. Mars pulls less hard than Earth, so the bag weighs less on Mars. Weight is measured in newtons (N).
The 10 kg bag of rice Mars Earth
Mass (how much rice) 10 kg 10 kg
Weight (how hard it is pulled) about 37 N about 98 N

There is a second clue that mass stays the same. Put the bag on a smooth floor and give it a push sideways. Getting it moving takes the same effort on Mars as on Earth, because it is just as much rice to move. A heavy-looking cart on Mars still needs a good push to start and a good pull to stop, even though it is easy to lift.

In everyday talk on Earth, we say "I weigh 30 kg", and that is fine at home. But a scientist would say "my mass is 30 kg, and my weight is about 300 N". On Mars the mass would still be 30 kg, but the weight would be about 110 N.

Worked examples

1. Lifting and pushing on Mars. An astronaut can lift a 50 kg box over their head on Mars, because it weighs as much as a 19 kg box at home. But if the box is sliding towards them on ice, it is still a full 50 kg to stop.

2. A balance and a spring. A seesaw balance compares two masses: if they balance on Earth, they balance on Mars too, because gravity pulls both sides less by the same share. A spring scale measures weight, so on Mars it reads lower.

Try it

  1. Which stays the same on Mars: mass or weight?
  2. A 2 kg melon goes to Mars. What is its mass there?
  3. Why is it still hard to stop a heavy cart rolling towards you on Mars?
  4. Would a seesaw balance work on Mars? Would a spring scale give the same reading as on Earth?
Answers
  1. Mass.
  2. Still 2 kg.
  3. Its mass is the same, and stopping a moving thing depends on its mass, not its weight.
  4. The balance works and gives the same answer. The spring scale reads less, because it measures the pull.

Common mistake

"In space, things have no mass." In space things can float and feel weightless, but they keep all their mass. An astronaut in the space station still has to push hard to move a big crate, and it is just as hard to stop.

Where it leads