Originally Posted by e1618978
Well - all movement is relative. In order to say that something is moving you have to say "moving with respect to X" where X is something else.
Saying that the earth is stationary is a perfectly valid thing to do - you just define all other movement of objects in the universe as being wrt the earth. It does not lead to the least complex mathematical model, though.
Ok, prepare to be confused. I'm not yet very good at explaining physics (I think I learn that when I do my doctoral thesis) but here goes:
That is not a really correct statement. It is accurate to say that LINEAR motion is relative, and that it is dependent on your inertial frame of reference. However, rotational motion is not relative. To get a grasp on the reason that linear motion is relative, think of an object floating in space with no forces being exerted on it (neglecting gravitational forces exerted by distant stellar objects). The object will continue with the same velocity (speed and direction) forever until something causes it to change direction and/or speed. It keeps going as it's going until something makes it go differently. In other words, it experiences zero LINEAR ACCELERATION, because no forces are acting on it. Thus, any linear movement that it has requires a reference point, or you can't put a number on it. Now picture, and keep this picture in your head for a bit, an astronaut floating in the void. In fact let's make it a perfect vacuum: There is nothing in existence but the astronaut and whatever he's carrying. Now, lets tie to our astronaut a tennis ball, using a .5 meter length of cold-proof string (yeah, right. Stay with me here, it gets better.). Now, keep that image in mind, of the astronaut, and his tennis ball (no racket, unfortunately for him). Note that the tennis ball will be going the same speed and direction as the astronaut, and so will be floating limply connected by the string.
Now, rotational motion is different. Whenever an object, lets say a beach ball, is spinning, and lets say it is doing so at some fixed rate, ? revolutions per second, then any given point on the surface of our beach ball will also be rotating about the axis of revolution (the center of the ball) at ? rpm. Now, sit back for a second, and imagine the beach ball frozen in time. use your Sharpie, and put a dot somewhere on the surface of the ball. Good. Now, while looking at that dot, lets think again about the linear motion we discussed a second ago. Do we agree that if we don't exert any force on an object, or in this case a particle which is our dot, then our dot will not experience any acceleration, and will continue in whatever straight path it was traveling in before we stopped time? In other words, if there is no force exerted on the dot, then that means that there is no linear acceleration, and it will keep going as it was going before, right? Think about that for a second. Good. Now, lets agree that it's also fair to say that if the dot DOES NOT continue going as it was going, that if it changes direction from the straight line it was traveling in, then that means that it has experienced an acceleration due to some force. Think about that for a second. That the same thing we just said, but backwards. It still holds true. So, if we are looking at the dot, and it does not travel in a straight line, then it must be experiencing some acceleration, right? Ok, now lets start time and spin our beach ball (don't make the axis of rotation go through the dot! That just makes it harder to visualize!). Now look at the dot as it spins around the center of the beach ball. Look at it from overhead. Is it going in a straight line, or is its path changing? It's path is changing. Therefore, it must be experiencing an acceleration of some sort. The acceleration that changes the path of the dot, and makes it go in a circle, rather than a straight line, is called Centripetal acceleration.
"Uh, ok, so what's your point? If I'm the dot, then obviously the rest of the universe is spinning around me." Almost. Now that we know how the dot is traveling, lets think about our astronaut again. Lets give him a spin. What happens to the tennis ball that's strapped to him? Well, logic and our own ever day experience tells us that the string will go taut, and the ball will appear, from the astronauts perspective, to be pulled straight out from him. Think about swinging a rope around your head. Now, lets say the astronaut pulls the tennis ball to his chest, then lets go of it (he's still spinning). What will happen? The ball will appear to shoot back out away from the astronaut. Make sense? What is happening is that the ball is not actually shooting out, or being forced away from the astronaut, the astronaut is causing the ball's linear path to change, making it go in a circle, rather than a straight line. Same as with the beach ball.
So what's the point?
Well, if we think about pure linear motion, such as an astronaut just floating in the void, then we see that when he has no forces acting on him, and therefore no acceleration, we have to have some point of reference if we are to say that he is in motion. Even if he has a tennis ball hanging from him, we have no way to tell if he and the ball are actually in motion. However, if he has rotational motion, then we can tell without any other reference point, because the ball will appear to shoot out from him. The faster it shoots out, the faster he is spinning. We can actually put a number to his rate of rotation just by looking at the ball. What if he doesn't have the ball? Well, that doesn't really matter, though it's natural to think that it would. Remember that, as with the beach ball, every little bit of the astronaut's body is experiencing the same linear acceleration as the the ball was, and so his hair, blood, snot, whatever (maybe arms are a better example) will want to 'shoot out' just like the ball did. Rotational motion is NOT relative the way linear motion is because it always comes along with an acceleration, which is measurable. Because this rotational motion is measurable without the benefit of an external frame of reference, it is an actual, absolute property, and is not relative.
It IS important to specify what it it is that something is revolving around. You CAN build a model where the Earth is actually stationary, and where the universe revolves around it. However, doing so requires that you make an 'Exception to Policy' of sorts, and apply the laws (well, there really all just theories, anyway) with a double standard, depending on what and where an object is. Such a model is absurdly complex, as was mentioned above, and is therefore useless, and does not reflect reality. Linear motion is relative. Rotational motion is absolute.
OK! now that I've rambled forever and a half, someone go through this thing, pull the sensible nuggets out and repost it without the babbling!