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Jun17-09, 08:47 AM
DaveC426913's Avatar
P: 15,319
Quote Quote by ray b View Post
in your mind
anyone really think you can draw an X in space
or that there is any need to
nobodys going to meassure back to it anyway
no you just set the X Y & Z at zero and see how far away
the earth travels in a year

I really doNOT see whats so hard to grasp here
Ray, how do you ensure that your X is not moving? If we did this for real, we'd be in orbit (around the sun, if not around the Earth) and when we dropped the X it too would be in orbit. We would have to apply a counter-force to "bring it to rest". But we don't know what "rest" is. No matter what thought experiment you devise to drop the X and bring it to rest, there is no way even in principle of making it stop without us knowing what "stop" is, and then applying forces to bring it there.

Thus is the nature of Newton's first law. As we sail through space, we have no way of determining that we are moving except in relation to some chosen reference point.