Last modified on 3 August 2013, at 20:52

Wikijunior:Solar System/About weight and gravity

You may've read or heard people say that your weight would be different on the Moon, or on other planets and their moons. Does this mean that when you leave the Earth you somehow become fatter or thinner, without gorging on food or going on a diet?  Hopefully you'll be glad to hear the answer is no!  Here's why:


Have you ever wondered why everything on the Earth doesn't just float up into the sky and off into space?  Or perhaps why you land back on the ground whenever you jump up?

So far as we know, everything in the universe tries to pull on everything else. This pull is what's known as gravity. It's very weak, but it becomes stronger as things get closer together and when any of them are very bulky. The more bulky something is, the more it pulls on other things around it.

We're all living on the Earth, which is very bulky compared to each of us, so it pulls on each of us very strongly. That's why we don't all float away from the Earth and why we come back down to the ground whenever we jump up. We each pull on the Earth, but, as each of us is tiny compared to the Earth, our pull on the Earth is tiny too.

Other places in our Solar System and the rest of the universe are more or less bulky than the Earth, so their gravity is more or less than that here on Earth. So, if we leave the Earth and visit them, their pull on us will be more or less than that we know here on Earth. The Moon, for example, is less bulky than the Earth, so if you weighed yourself there with a bathroom scale, it would say you weighed less than when you weighed yourself with it on the Earth. This is why people say your weight is different at different places in the Solar System or the rest of the universe! You haven't changed – what's changed is the strength of the gravity keeping you on the surface of whatever place you're visiting.

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Orbits and weightlessnessEdit

You don't even have to travel that far from the Earth to experience this change. If you travel fast enough around it, the Earth's surface will keep curving away from you as fast as Earth's gravity pulls you toward it. You'll keep going round and round the Earth, never getting any closer to it. This is called orbiting the Earth and the path you follow around the Earth is called an orbit. Since you don't get any closer to the Earth, it's as if the Earth is no longer pulling on you and so you have no weight — you're weightless.

The same is true if you travel fast enough around anything else in the universe. The speed you'll need depends on how bulky it is and how far away you are from it.

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