# Homework Help: Relative velocities in special relativity

1. Dec 29, 2017

### Ray Vickson

Well... what do YOU think? You are given some initial data in the rest-frame of the space station, and are then asked about what would be seen from (the rest-frame of) B.

2. Dec 29, 2017

### Kennedy

So, yes... That becomes obvious because they’re asking about more than one reference frame in the problem. Okay, so to solve this without considering the aspect of special relativity, I need to look at both the reference frame of A and B, because if B were travelling at the same speed as A, than in the reference frame of A, the relative velocity of B would be zero, and likewise for in the reference frame of B.

3. Dec 29, 2017

### PeroK

You need to find a way to visualise what is happening - and back up that visualisation with the appropriate calculations.

Since your homework was SR, here's my solution to the non-relativistic case:

Let the speed of B be $v$ in the space-station frame. The speed of A is $8m/s$ in this frame. Now, in B's reference frame:

The speed of A is $8m/s -v$ and the speed of the spacestation is $v$. We need these to be equal, hence:

$8m/s - v = v$ and $v = 4m/s$

(Note that, in any case, I would have a diagram showing A, B and the spacestation and the known and unknown velocities in the original frame. And then another diagram for B's frame.)

It is important to realise that your difficulties in this case were not with SR, per se, but with reference frames. I would really work on this, otherwise SR is going to be very tricky - or, at leasty, even trickier than it need be!