# Homework Help: Basic relativity.

1. Feb 1, 2006

### fasterthanjoao

Right.

Some simple questions, I want to make sure I'm not making stupid mistakes before moving on. I have the questions and my answers, If you tell me that I got them incorrect, i'll run through my working or something (if there could be any..).

1/ How fast must you travel towards a red light (650nm) for it to appear green (525nm)?

Can this just be treated as length contraction?

2/ How great must the relative speed of two observers be for their time-interval measurements to differ by 1%?

3/ A distant galaxy is moving away from the earth such that each wavelength is shifted by a factor of two; what is the speed of the galaxy relative to us?

can this just be treated via length contracion?

4/ A particle moves north at 0.8c, what is this particles speed as measured by an observed travelling south relative to the earth at 0.5c?

5/Two spaceships of proper length 100m are travelling towards each other, each at a speed of 0.8c, measured on earth.
(i) how long is each ship as measured by the terrestial observer?
(ii)how long is earch ship when measured from the rest-frame of the other?

Thats all the questions I have just now. My idea is that, since I haven't done relativity for ages I should try to brush up on this simple stuff before my new course starts. tomorrow i'm hitting the library for a textbook, sigh.

2. Feb 1, 2006

### Staff: Mentor

No. This, and #3, are examples of the relativistic Doppler effect.

3. Feb 1, 2006

### fasterthanjoao

I'm remembering something about the redshift parameter (z) being the change in wavelength over the 'rest'-wavelength?

I think I've followed on from that:

v/c = ((z+1)^2 - 1)/((z+1)^2 + 1)?

is that what I'm looking for? thanks for the response by the way.

(+sorry for the lack of `tex, I'm on the verge of not thinking straight enough to use it.)

4. Feb 1, 2006