# Special relativity spacecraft speed question

alexmahone
A pursuit spacecraft from the planet Tatooine is attempting to catch up with a Trade Federation cruiser. As measured by an observer on Tatooine, the cruiser is traveling away from the planet with a speed of 0.6c. The pursuit ship is traveling at a speed of 0.8c relative to Tatooine, in the same direction as the cruiser. What is the speed of the pursuit ship relative to the cruiser?

## Answers and Replies

Homework Helper
Is the problem physics, that you do not know the formula for combining (relativistic) speeds or mathematics, that you cannot do the arithmetic? Or is the difficulty reading- that you were not able to read the forum rules that you said you had when you registered for this forum?

If you are asked to do a problem for a class, someone clearly expects that you know and can use a formula for this problem. And we have a right to expect that you have read the forum rules and know you must make an attempt to solve the problem yourself and post that attempt here. We cannot give hints if we do not know exactly where you had a problem.

alexmahone
Is the problem physics, that you do not know the formula for combining (relativistic) speeds or mathematics, that you cannot do the arithmetic? Or is the difficulty reading- that you were not able to read the forum rules that you said you had when you registered for this forum?

If you are asked to do a problem for a class, someone clearly expects that you know and can use a formula for this problem. And we have a right to expect that you have read the forum rules and know you must make an attempt to solve the problem yourself and post that attempt here. We cannot give hints if we do not know exactly where you had a problem.

I get the answer 0.135c whereas my textbook says 0.385c. I was just hoping that someone could confirm that my answer is right. I didn't post my solution because I didn't want others to be biased by it.

alexmahone
Let S be Tatooine and S' be the pursuit ship. Let P be the cruiser.

$$v=0.6c$$ $$u=0.8c$$

$$v'=\frac{v-u}{1-uv/c^2}$$

$$=\frac{0.6c-0.8c}{1-0.8c*0.6c/c^2}$$

$$=-0.385c$$

Is that correct?

Last edited:
Mentor
$$=\frac{0.6c-0.8c}{1+0.8c*0.6c/c^2}$$
You made a sign error in the denominator.

alexmahone
You made a sign error in the denominator.

Oops; thanks.