Star wars and others: The unbearable lightness of being

Everybody probably remembers the image of North American spacemen stepping over the Moon. Their steps over our satellite were light, because we weigh less on the Moon than we do on the Earth. How can we slim so suddenly? It’s because in our ordinary lives we give the same meaning to two different physical concepts: mass and weight. Mass is an inner property of an object, while weight is the strength with which it is attracted by a greater object (our planet in this case). Therefore, a person having a mass of 70 Kg will always have the same mass in the outer space, but he or she should no longer say ‘I weigh 70 Kg’ out of the Earth.

The force of gravity on a planet or satellite depends on its mass and its radius. The Moon is quite lighter than the Earth, that’s why a man with a mass of 60 kg (60 kg weight on the Earth) weighs only 10 Kg on the Moon, even though he still has the same 60 Kg mass. This has an effect on many things: you can jump higher, raise objects that would be too heavy on the Earth, etc. It has its disadvantages, though: the frictional force against the ground gets tiny too. This force is important for walking, as we would slide over the ground without it. With such little gravity, the Moon resembles a skating rink .... Nevertheless, gravity is twice or three times bigger on Jupiter than it is on Earth: our 60 Kg man would weight over 150 Kg there. This would cause problems such as broken joints.

All these considerations are rarely taken into account in the movies: the Star Wars series is one of the most remarkable examples. Humans there are able to walk on the surface of several planets, never feeling lighter, nor heavier. Gravity has an influence on the appearance of extraterrestrials as well: Moon inhabitants would be tall and slim, while Jupiter men would be stocky in order to adapt themselves to their environment.