# Homework Help: Relativity help

1. Feb 22, 2005

### Twigs

An imperial spaceship, moving at high speed relative to the planet Arrakis, fires a rocket toward the planet with a speed of 0.930{\it c} relative to the spaceship. An observer on Arrakis measures that the rocket is approaching with a speed of 0.330{\it c}.

What is the speed of the spaceship relative to Arrakis?
i keep getting .964, can anyone help?

2. Feb 22, 2005

### Twigs

i use this: (v_1+u)/(1+v_1*u)

3. Feb 22, 2005

### learningphysics

Show exactly how you used the above equation... show the substitutions.

4. Feb 22, 2005

### Davorak

I am going to bed after this, I can give you a little advise before that though. You need to use the relativistic velocity-addition formulas. They are derived from the Lorentz transformation. I do not think that is the equation you showed in your second post. I am sure you can find the equation online or in you book.

good luck sorry I will not be awake to help more

5. Feb 22, 2005

### learningphysics

The equation is right, as long as you remember that you're using the ratio of speed to c. In other words if u=0.930c, you plug in 0.930 into the equation.

6. Feb 22, 2005

### learningphysics

You're doing a wrong substitution... draw a picture, and be careful about what you use for v and u. Are you using v to refer to the speed of the rocket relative to Arrakis, or the speed of the spaceship relative Arrakis. You can use both approaches, but you need to be careful with how you use u and v. Be careful of signs!

Last edited: Feb 22, 2005