Two Relativistic Rockets Pass eachother

[brett812718]

Homework Statement

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 each other?

Homework Equations

(u-u')/(1-uu'/c^2)=v
x'=\gamma(x-vt)
t=\gammato

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.

 

[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 dilation since it is viewed from Earth and not one of the spaceships??

 

[Phrak]

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

 

[granpa]

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

that IS relativity.

I think you meant, 'Is there anything in relativity that says that the relative velocity of 2 objects can't exceed c from the point of view of a third observer'.

 

[brett812718]

so i guess the answer is no?

 

[brett812718]

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

 

[granpa]

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

 

[granpa]

no. I don't think your solution is right. t=d/v

 

[Phrak]

that IS relativity.

I think you meant, 'Is there anything in relativity that says that the relative velocity of 2 objects can't exceed c from the point of view of a third observer'.

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

 

[vega_1992]

who want to know answer look at ~answer~ post