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Floating on Mars

Air and fluids · level 2, lesson 6 of 6

A question to start

One day, people living on Mars might have a pool inside their base. Would a toy boat float there? Would it sit higher, because everything is lighter on Mars?

The idea

On Mars, gravity pulls about two fifths as hard as on Earth. A toy boat weighs less. But the water in the pool weighs less too, so it pushes up less.

Both change by the same amount. So the toy boat floats at exactly the same depth on Mars as on Earth. Things that float on Earth float on Mars, and things that sink on Earth sink on Mars.

EarthMarsthe same depth on both

Balloons are a different story. Mars's air is very thin, so it gives a very weak push. A helium party balloon would not rise there at all. To float in Mars air, a balloon would need to be enormous.

Floating Mars Earth
A 10 kg bag feels like about 4 kg 10 kg
How deep a toy boat sits in water the same the same
Can a helium party balloon float? no: the air is too thin yes

Worked examples

1. Lighter, not higher. On Mars, a toy boat weighs about two fifths of its Earth weight. The water's push up is also two fifths as big. So it sits just as deep.

2. Ice in a drink floats with the same share below the surface on Mars as on Earth.

3. Swimming. A swimmer on Mars would float as high in the water as on Earth. They would just feel lighter getting in and out.

Try it

  1. Is gravity on Mars stronger or weaker than on Earth?
  2. Would a toy boat sit higher in the water on Mars?
  3. A stone sinks on Earth. What does it do on Mars?
  4. Why would a party balloon not float in Mars's air?
Answers
  1. Weaker: about two fifths as strong.
  2. No. It sits at the same depth, because the water pushes up less as well.
  3. It sinks.
  4. Mars's air is so thin that it gives a very weak push.

Common mistake

"Things float better on Mars." Weaker gravity makes the boat lighter, but it makes the water's push weaker too. The two changes cancel out.

Where it leads