An Apple Falls, But Not the Moon. Why?
Now this brings us to one of our last questions, which is, very fundamental – if you ask me. The earth attracts the moon; the moon attracts the earth…What must happen? Shouldn’t they both just meet each other and collide? Why isn’t that happening? What keeps the moon going around the earth all the time and why isn’t’ the moon falling? Ya. We will phrase this question this way. Why isn’t the moon falling? If earth attracts the moon, moon attracts the earth, the moon must be falling. Why isn’t it falling?
So in order to answer this question, let’s take a very simple case of throwing a stone. If I would climb upon to a very very very tall building on the earth…extremely tall…may be the tallest structure ever built. Go stand on top of it and just drop a stone. What’s going to happen? The stone is going to fall down and reach the surface of the earth. But now, if I want to drop it with some amount of horizontal velocity – in another word -throw it instead of dropping it with some horizontal velocity, it’s going to go in and fall little further away. I throw it harder… it’s going to go and fall even more further away. I can keep doing this. Right? But the fact is, if the earth was flat and if I went on for ever and ever and ever, this probably wouldn’t make much of a difference how fast I throw. Because it keeps landing further and further and further and further away. But the fact that earth happens to be spherical means that when you throw it hard enough, after a point, is that stone going to fall? Yes. But is it going to hit? No, it’s going to miss. Then it is going to go to the other side. And there it’s going start falling all over again in the opposite direction and start coming back over here. And this will keep happening for ever and ever and ever. There is nothing to slow down that stone. If thrown, it’s going to come back. It’s going to keep doing this. But what if I try and throw it extremely hard. Right? If I throw it extremely fast. What’s going to happen? It’s going to so far away that it is going to escape out of earth’s gravitation. So it means that I can throw it too slow, it falls on the surface of the earth or I can throw it very very fast, it goes and leaves the earth forever. It means that somewhere in between there exists one velocity exactly at which I can throw such that it is going to keep coming around and coming back to the same place over and over again. And because there is no friction to stop or slow it down, it’s going to keep doing this. So in another words, is that stone falling? Yes it is. But it’s just that it’s not hitting the surface because it’s falling and coming back because it wants to go to the other side, the gravity starts pulling it back in its direction. It could go and hit in the bottom part but if you throw exactly and precisely in a way that’s very well constructed, then it going to come back to the same place.