# Homework Help: Relativistic Kinematics

1. Sep 24, 2006

### busted

A rocket approaches a mirror on the ground at a speed of 0.90c. The distance D between the front of the rocket and the mirror is 2.4 * 10^12 m, as measured by ground observers, when a light signal is sent from the front of teh rocket. When is the reflected signal recieved by the rocket as measured by
a) the observers on the ground
b) the observers on the rocket?

I tried for a) working out the time interval using the formula and substituting the values known of speed of rocket and distance of rocket from mirror but I ended up with a huge value. The actual answer is given as 8.42n * 10^3 s and I am unaware how to get to this.
Can anybody give me some help?

2. Sep 24, 2006

### busted

any ideas anyone???

3. Sep 24, 2006

### Mindscrape

Which formula is "the formula"? Are you talking about a time dilation formula or a Lorentz transformation? I assume that the light signal is actually traveling at the speed of light. The problem is much like any kinematics problem if you can find all the necessary information in each frame. Basically, you have the rocket's velocity, and you want to know how fast it is going according to the ground. When working part b, watch your distance.

4. Sep 24, 2006

### busted

i was talking about the time dilation formulca, is this the right one to use?

5. Sep 24, 2006

### busted

i was talking about the time dilation formulca, is this the right one to use?

6. Sep 24, 2006

### Mindscrape

The first part you actually don't need to do any transformations, if I am reading it right, since all the information given to you is with reference to the ground. Is this the part you got wrong?

Think of it like a regular kinematics problem because the only difference is that the numbers are bigger, but the algebra is the same. It is almost the same as some guy skateboarding, throwing a ball at a wall, and then catching it. Just remember that the light out of the spaceship can't exceed the speed of light!

The second part you will need more than just time dilation.