# Realtivity question

1. Sep 5, 2007

### strangequark

I have a problem from my modern physic class I'm hoping to get some insight on... I got an answer, but it seems, well, odd...

1. The problem statement, all variables and given/known data

Inertial frame S' moves with speed v=$$\frac{3c}{5}$$ in the +xdirextion past inertial frameS. Event A is a synchronizing event. Event B occurs at t=0 in Frame S and at position x'=1 meter in frame S'. Given:

For Event A:
$$x_{A}=0$$
$$x_{A}'=0$$
$$t_{A}=0$$
$$t_{A}'=0$$

For Event B:
$$x_{B}'=1 meter$$
$$t_{B}=0$$

Find,
$$x_{B}$$ and $$t_{B}$$'

2. Relevant equations

The relevent equations are the two lorentz transforms:

$$x'=\gamma(x-vt)$$

and

$$t=\gamma(t'+\frac{vx'}{c/^{2}}$$

3. The attempt at a solution

For $$x_{B}$$:
Applying equation 1 I get,

$$\frac{x'}{\gamma}+vt=x$$
Then $$x=\frac{4}{5} meters$$

and for $$t_{B}$$' I get (applying equation 2),

$$t'=\frac{t}{\gamma}-\frac{vx'}{c^{2}}=-2.001 x 10^{-9} seconds$$

I'm confused about the second answer. Did I do something wrong? or does this just mean that in frame S' the event happens $$-2.001 x 10^{-9}$$ seconds prior to t=0 in frame S?

Last edited: Sep 5, 2007
2. Sep 5, 2007

### learningphysics

You didn't do anything wrong. The event happens $$2*10^{-9}$$ seconds before t'=0 (that is in the S' frame). It just means that in the S' frame, this event happens before the synchronization event.

I wouldn't put it that way. You can't really say that an event happens in frame S' before that same event happens in frame S... before and after can only be used when comparing events in the same frame...

3. Sep 5, 2007

### strangequark

OK, that makes sense... thanks alot, i really appreciate it.