Relative Motion (A Thought Experiment)

T650 If we are on the surface of earth aboard a train travelling at 60 mph we can use the train as a fixed reference point and state that the earth is moving relative to our reference point on board. It is not indecisive but relative to the reference frame. Well the problem here is that you will not have a reference frame to establish that you are moving however if in doing experiments aboard the spacecraft you had determined that in all your trials that an object moves in reaction to an opposite force then you could tentitively consider motion on your part also due to the thrust of the engine.You could not verify that though.It would go a long way to explaining also why you experience the need to react to the acceleration by bracing your legs against it. They would each orbit about the barycenter of their gravitaional system. One would orbit the other in reference to the other one.This message has been edited by sidelined, 07-24-2004 03:46 PM

RingoKidA point of reference must be seen to be evaluated as relative to your motion.

HD13 No this means that space has no existence independant of reference to bodies within it. There is no absolute space. In other words it is meaningless to assign a motion to a single body in an otherwise empty void.You are going to be learning to fly jets and if you do it will be necessary for you to develop a keen sense of your surroundings which you can only do by maintaining a reference frame by which to relate to "other bodies around you" and by that know your own motion relative to the space defined by those other bodies.

Tony650 This was my mistake and you are of course correct.I erred with this statement. It would not matter if they are tidally locked or not since without a background reference you could not say they orbit one another. You may be able to say that the body other than the one you are on was rotating as the surface view you had of it changed.

MrJackIf there are no background stars to reference the accelerated motion by then how do you make a measurement?

Mr.Jack But we are talking here about a void where we have no background by which to judge two bodies as gravitationally in orbit about one another.

Mr.Jack How do you determine that you are not both stationary since in this thought experiment it is implied {I hope} that we are confined to one of the planets? Also what experiment do you perform on your planet to determine that the gravity on your planet is also on the other?

NosyNedTouche' Ned. Now how do we determine that this gravity acts between planets in such a way as to cause them to orbit one another?