Home
Assignments
Bookmarks
Mass and gravity
Why does the moon orbit the Earth?
Oh, I feel so heavy. I must have eaten at least ten kilos of cookies. I wish I were on the moon. Everything is lighter there so then I would only have eaten like one and a half kilos of cookies. Ten kilos of cookies are ten kilos of cookies wherever you are. What do you mean? Isn't stuff lighter in space? Lighter yes, but just as many kilos.
You're confusing mass with weight. Cookies consist of matter, just like you and me. And mass, that's the amount of matter. Mass is measured in kilograms, but often we just say kilos. In everyday language, we use kilos to describe the weight of something.
That is, how much force one needs to lift something off the ground. Weight doesn't just depend on the amount of matter - the amount of kilos. in other words, There has to be something pulling that matter too, and the force that pulls something down to earth is called gravity. The larger the mass, the stronger the gravitational pull. That's why we experience something as heavier if it has more mass. It's not just earth that has gravity.
The fact is, all things that have mass have gravity and pull on each other. And the greater an object's mass, the more gravitational force it exerts. Take the moon for instance. The moon is pulled towards the earth with the help of earth's gravity. But the moon helps out too, and pulls the earth with the help of its weaker gravity.
It's the combined gravity of the earth and the moon that keeps the moon in orbit. In the same way, our own bodies are pulling the earth, just as the earth is pulling the bodies downward. We have mass, therefore we have gravity, but it's so weak that we don't notice it. Oh, those cookies really pulled me in. But that was sugar, not gravity. Gravity is a force that pulls on all things. Just like a magnet? No. Magnetism is something completely different. A magnet attracts only some things.
And magnets can both attract and repel. Just like you. Me? Look here. If we were on the moon-- Hello. If we were on the moon. Thanks. Wow, I feel lighter here.
That's just what I thought. Exactly. The mass of the moon is less than that of the earth, so gravity isn't as strong - about a sixth compared to earth. Awesome. Check this out, I do weigh only 10 kilos, you were wrong. No, it does say kilo on the scale, but that isn't right. That scale doesn't measure mass. It measures force. Force? Yeah, force - how strongly you're pulled to the ground.
How heavy you are. Force isn't measured in kilos, but in newtons. Isaac Newton, that's the guy who got an apple in his head. Yep. The unit for force is Newton, named after him. So how many newtons is gravity then? Gravity on earth is 9.8 Newtons per kilograms of mass. The earth's gravity is pulling each kilogram matter downward with a force of almost 10 Newtons. What about the moon? 1.6 Newtons per kilo. Okay. My mass is 60 kilos both on the moon and on the earth, but my weight is different? Right.
On the earth, the gravitational force of your body is 600 Newtons. On the moon, it's just 100 Newtons. Out in empty space where there is almost no gravity at all, you are weightless. But not massless? No, exactly. The amount of matter is still 60 kilograms. I see. But I've still eaten too many cookies. Maybe you should have eaten apples instead.