Marko S. wrote:Source of the post And also: Does the Moon looks like it travels from west to east during the eclipse because it travels slower than Sun on our sky? Because a lot of people have trouble answering this question. i think I got that right. Well, that's what I would answer if someone asked me.
That's correct. Both the Moon and Sun move across the sky east to west, but when you add in the orbital motion of the Moon around the Earth (which is counterclockwise viewed from above the north pole, or west to east), it crosses the sky more slowly. West to east relative to the Sun and the stars.
Starlight Glimmer wrote:Source of the post Lets say we had a piece of wood, about the width of a 2x4, a lightyear long somehow. Lets say it was in space, and you were in a spacesuit with some form of rocketpack, you turn it on and push the piece of would. Would the entire thing move as you push 1 end? Thatd break the speed of light though.
FFT has is right. The block does not move all at once. The far end of the block is at rest until the information that you pushed it can reach it, which in a solid or 'rigid' body is always slower than the speed of light in vacuum.
FastFourierTransform wrote:Source of the post But I think there is an interesting issue here appart from this. Would you say that the rod has been deformed? or is acting as a deformed body? or is totally rigid but in a very special geometry (space-time)?. With this I mean for example that if you consider the fact that simultaneity can be broken could you consider from the frame of reference of the traveling perturbation that the rod is moving as a rigid body? Wouldn't from that frame of reference all the parts of the rod moved at the same time?
Yes, the rod has been very slightly deformed, and the force of the push propagates down its length as a sound wave (a distortion of the structure.) So the concept of 'rigidity' is always slightly wrong -- there is no such thing as a perfectly rigid body in nature.
You could consider the frame of reference of the wave propagating down the rod. If the wave propagates at the speed of light, then it is correct that the whole rod moves all at once according to you. But then the rod's length is also infinitely short, because of the relativistic length contraction! You will also find that clocks fixed to the ends of the rod read very different times -- a synchronized network of clocks in the rod's frame are not in sync according to you!
When you account for all this, you'll discover the pulse actually is travelling through the rod at the same speed in all frames of reference.