# Homework Help: Relativity_velocity addition and doppler effect

1. Sep 26, 2009

### kkkeleven

1. The problem statement, all variables and given/known data
The Enterorise NCC1701 is cruising 0.6c above the moon of Eridani when it spots a Klingon warbird sunwards.
i. It fires a proton torpedo at speed 0.75c relative to the ship at the Klingon warbird. Assuming the proton torpedo does not accelerate, at what velocity must the Klingon warbird move to evade the proton torpedo I. relative to Enterprise II. relative to the moon of Eridani.
ii. Seeing that the warbird has moved just enough to evade the proton torpedo, the Enteprise proceeds to “open hailing frequencies”. The ship’s computer scans the library and determines that Klingon warbireds communicate at frequencies of 2.00 THZ, which frequency must the Enterprise use?
iii. Just as the Enterprise is about to send the message, colonists on the moon of Eridani also wants to hail the Klingon warbird. What frequencies should they use?

Thank you very much!

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Sep 26, 2009

### willem2

This is a bad problem

the direction of movement of the ship vs. the moon of Eridiani isn't given so you can't answer the questions about this moon.

How can the torpedo not accelerate and stil hit the Klingon ship if it tries to evade? It seems the first question is only answerable if the warbird and the enterprise and the torpedo all move only on a single line. Then you can use the equation for relativistic velocity addition.

3. Sep 27, 2009

### Redbelly98

Staff Emeritus
Welcome to PF
We don't provide solutions to students, but we give hints to point you in the right direction.

What equations, dealing with relativistic velocities, are you aware of?
Any equations dealing with how frequency shifts with velocity?