# Basic Special Relativity

1. Oct 14, 2013

### PsychonautQQ

1. The problem statement, all variables and given/known data
A star is 27c*y away from the rest reference of earth. A rocket reaches star A from earth and according to clocks on it 12 hours have passed. How fast is the rocket traveling in earth's reference frame?

2. Relevant equations
tpγ=t
Lp/γ=L

3. The attempt at a solution
The problem is under the category of length contraction and time dilation so I assume these are the two equations I must use. However when trying to do the problem I only was given t and Lp, and yet there are three variables (tp, L, and v). Help ;-(

2. Oct 14, 2013

### Enigman

Can you calculate what time has passed wrt IRearth?
(I am assuming Earth is assumed to be an IR and the ship doesn't accelerate.)

3. Oct 14, 2013

### PsychonautQQ

yes, no acceleration.
I don't think I can calculate the time wrt IRearth..? I would need to know the velocity to do that so I knew the gamma factor correct?

4. Oct 14, 2013

### Enigman

vearth=dearth/tearth
tearth=dearth/vearth
plug and chug.

5. Oct 14, 2013

### PsychonautQQ

I don't know t_earth though? I only know it took 12 years from the spaceships frame..? oh... is Vearth 27c*y / 12 years...? i'm a noob X_X

6. Oct 14, 2013

### PsychonautQQ

but that's not REALLY the speed earth see's it moving is it? Because earth couldn't see the rocket traveling faster than C could it?

7. Oct 14, 2013

### Enigman

tearthγ=t
or, tearth=t/γ
equate the tearth