# Homework Help: Physics Relativity Question?

1. Feb 7, 2012

### justinh8

Physics Relativity Question?????

1. The problem statement, all variables and given/known data
A rocket ship is moving directly toward Earth with a constant speed of 0.50 c relative to Earth. It turns on its headlight, and the light beam travels toward Earth with speed c relative to the ship. According to the rocketship astronauts, Earth is 6.00 x 10^12 m away when the headlight is turned on. In the frame of the astronauts, how long does it take for the light to reach Earth? (Hint: in the frame of the astronauts, Earth is approaching the ship with a speed of 0.5c)

2. Relevant equations
c = 3x10^8 m/s

3. The attempt at a solution
I just got 20000 seconds but i know its wrong

2. Feb 7, 2012

Re: Physics Relativity Question?????

3. Feb 7, 2012

### DaveC426913

Re: Physics Relativity Question?????

He divided 6x1012 by 3x108. I think even justinh8 knows that doesn't qualify as "an attempt at a solution".

justinh8, you haven't been following along in class. Read the appropriate chapter in your textbook on length contraction and Lorentzian Transform.

4. Feb 7, 2012

Re: Physics Relativity Question?????

You need to take into account time dilation.

5. Feb 7, 2012

### justinh8

Re: Physics Relativity Question?????

There was no lesson for this, it was just a do you remember from grade 11 physics but i do not recall any lorentzian transformations in grade 11. Plus we havent gotten our textbooks yet so i have no clue. However, i think i know one equation that might be relevant. Its ct^2 = vt^2 + ct^2. Its from grade 11 but not sure if it would work...

Last edited: Feb 7, 2012
6. Feb 7, 2012

Re: Physics Relativity Question?????

Hmm that's odd then. Well the equation to use is $$t=\frac{t'}{\sqrt{1-\frac{v^2}{c^2}}}, \text{Where } t'= \text{dilated time and } t = \text{stationary time}$$

7. Feb 7, 2012

### SammyS

Staff Emeritus
Re: Physics Relativity Question?????

I'm pretty sure he doesn't need to be concerned about time dilation.

Everything is given and asked in the frame of the rocket ship.

8. Feb 8, 2012

Re: Physics Relativity Question?????

I think you're right, my mistake.

9. Feb 8, 2012

### Thundagere

Re: Physics Relativity Question?????

In which case, I think all he has to take into account is relative motions.

10. Feb 8, 2012

### justinh8

Re: Physics Relativity Question?????

I just talked to my teacher today and he said there is nothing to do with time dilation, he just said think simple so i think it might be using just the v= d/t and just rearrange it to find time??