Special relativity, relative velocities

[C.E]

1. Hi, could somebody please help me to answer the following Relativity question?
Rocket A is traveling at 0.5c relative to an Earth based observer and Rocket B travels at 0.8c Relative to rocket A. At what speed does rocket B appear to catch rocket A to an Earth based observer?


2. We are given that for 1 dimensional motion a particles velocity vx in the frame S is related to its velocity vx' in the frame S' (which is moving with velocity u relative to frame S) by
vx=vx'+u/(1+uvx'/c^2).

3.Can somebody please explain how to tackle this kind of question? I keep getting really confused (particularly about what is in which reference frame) and don't even know how to start. (By the way this is not assesed work merely revision so feel free to give as much help as you deem appropriate).

 

[tiny-tim]

Rocket A is traveling at 0.5c relative to an Earth based observer and Rocket B travels at 0.8c Relative to rocket A. At what speed does rocket B appear to catch rocket A to an Earth based observer?

Hi C.E!

You just use the equation they give you …

We are given that for 1 dimensional motion a particles velocity vx in the frame S is related to its velocity vx' in the frame S' (which is moving with velocity u relative to frame S) by
vx=vx'+u/(1+uvx'/c^2).

but you must use the same frame for the whole equation (!), so in this case, that would be A's frame (because A appears in both measurements) …

so that will give you B's velocity relative to Earth