Physics Emergency: Two Trains Moving Away at 51% Light Speed

[Rico Bamchug]

Okay, I made a statement in another forum completely unrelated to physics and I believe it to be wrong even though I made the statement in good faith. (Which is to say, I THOUGHT I knew what I was talking about.)

So please, if you can, bail me out.

Two trains are moving away from each other, both traveling at 51% the speed of light. In one train, viewing the other, would it appear to be traveling at 102% the speed of light?

Okay, that's it! Any help will be greatly appreciated!

 

[JesseM]

Okay, I made a statement in another forum completely unrelated to physics and I believe it to be wrong even though I made the statement in good faith. (Which is to say, I THOUGHT I knew what I was talking about.)

So please, if you can, bail me out.

Two trains are moving away from each other, both traveling at 51% the speed of light. In one train, viewing the other, would it appear to be traveling at 102% the speed of light?

Nope, you have to use the relativistic velocity addition formula here--in one train's frame, the other train will be moving at (0.51c + 0.51c)/(1 + 0.51^2) = 0.80946c, i.e. 80.946% the speed of light.

 

[Rico Bamchug]

Awesome! And thank you Jesse!

 

[TVP45]

Whenever you talk about this kind of problem, it helps to always add "as measured ..." When you say a train is moving at 0.51c, state relative to what or measured relative to what. That helps keep things straight.