# Speed of light questions

1. Oct 5, 2008

### vysis

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

A signal (traveling at the speed of light) is sent from -4 ls (light seconds) towards a moving spaceship traveling at 0.866c. How long does it take for the signal to reach the ship?

2. Relevant equations
I'm pretty sure its t=(delta)D/c-v

But I continue to get the wrong answer with it.

3. The attempt at a solution

using the equation above [rearranged from d(light) = (delta)d + vt]. I get 29 seconds. I do it by multiplying 4 ls by 3e8 and use that as my distance.

according to a spacetime diagram I have, the answer should be around 12s (my diagram might be wrong, but it doesn't seem so). But I can't seem to get it with this equation.

2. Oct 5, 2008

### Staff: Mentor

Which way is the ship moving? Where is it at t = 0?

3. Oct 5, 2008

### vysis

the ship is moving in the positive direct and is at 0 ls when t = 0

4. Oct 5, 2008

### vysis

btw, this is all from signal sender's frame of reference.

5. Oct 5, 2008