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:
Gravity
[edit | edit source]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 actually very weak, but the more bulky things are and the nearer they are to each other, the stronger the pull becomes. The more bulky something is, the more it pulls on other things around it.
Compared to [each of us], the Earth is very bulky, so its pull on [each one of us] is very strong. That's why we don't all float away from the Earth – and why, whenever we jump up, we're pulled back down to the ground. We each pull on the Earth, just as it pulls on [each of us]. [Each of us] is tiny compared to the Earth, though, so our pull on the Earth is tiny too.
Other places in our Solar System [– and in the rest of the Universe –] are much more bulky than the Earth, while others are much less bulky than the Earth, so
{Still too complicated / sentences too long:} 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. 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.
{Mention planet and comet formation?} {Perhaps even some simple(!) imagery to suggest curved space (without using that phrase) in place of "pull/ing"?}
Orbits and weightlessness
[edit | edit source]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.
{Include (parts of) How does gravity change what happens elsewhere? ...?}
{Do not include any material from the So how much would I weigh on the Moon? and Astronomerese sections as it is inappropriate and/or too complicated for a Wikijunior book ...?}
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