Astro-Physics question.

[Xenon14]

Ok, I'm not sure about this question, so if anyone can critique my answers...thanks.

Two spaceships (A and B) approach your spacecraft at 9/10 the speed of light (0.9c) from opposite directions. They send out radio messages. What is the speed you measure for the radio waves from A and B? What is the speed of the radio waves from A as measured by B. What speed does B measure for your motion? How fast does spaceship B observe spaceship A to be moving?

Correct me if i'm wrong:

1) Speed of radio waves that I measure = the speed of light, C.
2) Speed of radio waves as measured from A by B = speed of light, C.
3) B measures your motion to be 0.9C.
4) Spaceship B observes spaceship A to be moving at 2(0.9C) - This one I'm pretty sure is wrong, as it is not possible to observe something go faster than the speed of light, since light is used (ie. radar) to determine speed.

LMK.

 

[cquark]

Your first two answers are correct, and while the wording of the problem is a bit vague, if we assume that the 0.9c velocity of A and B is measured in your intertial reference frame, then #3 is correct too. However, you're using Galilean velocity addition in #4 so you are wrong as you expect. You need to use the special relativity velocity formula, which you can find in the Usenet Physics FAQ.