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A world at 0.38 g

Gravity and orbits · level 3, lesson 5 of 6

A question to start

Pilots and astronauts talk about "g". "We pulled 3 g in that turn." "Mars is 0.38 g." What does that mean, and what would it be like to live your whole life at 0.38 g?

The idea

One g is the pull of gravity at the Earth's surface: what makes things fall 9.8 m/s faster every second. Every other pull is measured against it. Mars's surface pull is 3.71 m/s² (things gain 3.71 m/s every second), and \(3.71 \div 9.81 \approx 0.38\): so Mars is 0.38 g.

Mars Earth Moon
Surface pull 3.71 m/s² 9.81 m/s² 1.62 m/s²
In g 0.38 g 1 g 0.17 g

What would change at 0.38 g?

  • Walking. Each step lifts you more and you come down more slowly. On the Moon, Apollo astronauts found that hopping was easier than walking. At 0.38 g people would probably walk with a long, bouncy stride, and could run in big loping bounds.
  • Pouring and splashing. Water pours more slowly and splashes higher. A dropped cup takes about 1.6 times as long to hit the floor, so you have more time to catch it.
  • Building. A wall or roof has to hold up only 38% of the weight it would on Earth. Buildings can have thinner columns and wider spans. But a pressurised building on Mars is pushed outwards by the air inside it, far harder than gravity pushes it down, so it must be strong in a different way.
  • Dust. Dust settles more slowly, and the thin air means it can hang high in storms for weeks.
  • The body. Muscles and bones grow strong by working against gravity. In 0 g on the space station, astronauts lose bone and muscle unless they exercise for about two hours a day. Nobody yet knows whether 0.38 g is enough to keep bones healthy over years, or what it would mean for children growing up on Mars.

Worked examples

1. Carrying water. A 20-litre water can has a mass of 20 kg. On Mars it weighs as much as \(20 \times 0.38 = 7.6\) kg does at home. A child could carry it.

2. A fall from a ladder. A fall from 2 m on Mars lands you at the same speed as a fall from \(2 \times 0.38 = 0.76\) m on Earth. (You will see why in level 5.)

Try it

  1. What does "1 g" mean?
  2. How many g is the Moon? (Use the table.)
  3. Name two things that would be easier on Mars than on Earth, and one that would be harder.
  4. Why might children growing up on Mars need special exercise?
Answers
  1. The strength of the pull of gravity at the Earth's surface, 9.8 m/s².
  2. 0.17 g, about a sixth.
  3. Easier: lifting, jumping, carrying, building tall. Harder: keeping bones and muscles strong; stopping a heavy moving thing is not easier at all (mass is the same).
  4. Their bones and muscles would work less against gravity and might not grow as strong as on Earth.

Common mistake

"Everything is 0.38 times as hard on Mars." Only things that fight gravity get easier: lifting, climbing, holding up. Starting, stopping and turning a heavy thing take just as much effort as at home, because the mass is the same.

Where it leads