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Moving things on Mars

Forces and motion · level 1, lesson 6 of 6

A question to start

On Mars, gravity pulls much less than on Earth. A box that feels heavy here would feel light there. So would it be easy to push a big box across the floor of a Mars home?

The idea

There are two different jobs:

  • Lifting a box means pulling it up against gravity. Gravity on Mars pulls only a little more than a third as hard as on Earth. So lifting is much easier on Mars.
  • Getting it moving sideways, or stopping it, depends on how much stuff is in the box: its mass. The box has exactly the same mass on Mars as on Earth. So on smooth wheels, it is just as hard to get going, and just as hard to stop.
on Earthon Mars a big pull downa small pull down
Mars Earth
How hard gravity pulls a bit more than a third all of it
A box that weighs 30 kg on Earth feels like about 11 kg 30 kg
Its mass, the stuff in it 30 kg 30 kg
How hard to get it rolling on wheels the same the same

Sliding a box along the floor, without wheels, is a bit different. Then the floor rubs on the box, and a lighter box presses down less, so it rubs less. That is easier on Mars.

Worked examples

1. A cart full of rocks. An astronaut pulls a cart of rock samples. Getting it rolling takes the same pull as on Earth, and once it is rolling it is just as hard to stop. Running into a full cart would hurt just as much.

2. Lifting a toolbox. A toolbox that is hard to lift on Earth could be lifted with one hand on Mars.

Try it

  1. Is lifting a box easier on Mars or on Earth?
  2. Does a box have more mass, less, or the same on Mars?
  3. Is it easier to get a box on wheels rolling on Mars?
  4. A runaway cart rolls towards you on Mars. Is it easier to stop than on Earth?
Answers
  1. On Mars: gravity pulls less.
  2. The same.
  3. No, just as hard: that depends on its mass.
  4. No, just as hard (if it is going as fast).

Common mistake

"On Mars everything is lighter, so everything is easier." Only the jobs that fight gravity, like lifting and climbing, are easier. Starting and stopping things depends on mass, which does not change.

Where it leads