# Homework Help: Two Relativistic Rockets Pass eachother

1. Aug 28, 2008

### brett812718

1. The problem statement, all variables and given/known data
Two relativistic rockets move toward each other. as seen by an observer on earth rocket A of proper length 500m travels at a speed of .8c and rocket b of proper length travels at speed .6c. What is the speed of the rockets relative to each other? the earthbound observer sets her clock to t=0 when the noses of the rockets pass each other. what will the observers clock read when the tails pass eachother?

2. Relevant equations
(u-u')/(1-uu'/c^2)=v
x'=$$\gamma$$(x-vt)
t=$$\gamma$$to
3. The attempt at a solution
I got the first part which was .95c. the second part I started by calculating the length contraction x'a sqrt(1-(.8c/c)^2)=xa=300m
x'b(1-(.6c/c)^2)=xb=800m
but then I got totally lost.

2. Aug 28, 2008

### brett812718

1) do the rockets pass each other at a relative speed of .95c as viewed from earth?
2) you should not have to do anythin with time dialation since it is viewed from earth and not one of the spaceships??

3. Aug 28, 2008

### Phrak

Is there anything in relativity that says that the relative velocity of two objects can't exceed c?

4. Aug 28, 2008

### granpa

that IS relativity.

I think you meant, 'Is there anything in relativity that says that the relative velocity of 2 objects cant exceed c from the point of view of a third observer'.

5. Aug 28, 2008

### brett812718

so i guess the answer is no?

6. Aug 28, 2008

### brett812718

Xa+Va t =Xb-Vb t
t= (Xb-Xa)/(Va+Vb)
would I use this to get the time?

7. Aug 28, 2008

### granpa

maybe you should explain what everything means. what are you solving for? how does the problem break down?

8. Aug 28, 2008

### granpa

no. I dont think your solution is right. t=d/v

9. Sep 3, 2008

### Phrak

I should have said exactly that. I stand corrected... in the context of special relativity that relates to this thread.

10. Sep 7, 2008