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1. Nov 1, 2015

### mfreeman

1. The problem statement, all variables and given/known data
Two asteroids are approaching one another moving with the same speed speed as measured from a stationary observer on Europa. Their relative speed is 0.5c. Find the speed of one of asteroids relative to Europa.

I understand how relativistic velocity addition works but am not able to solve this question. Is it possible there is not enough information given in the problem? Any help would be much appreciated.

2. Relevant equations
u = (u' + v) / (1 + (u'v)/c^2)

3. The attempt at a solution
The ambiguity of the problem statement has led me down several different paths each resulting in a quadratic that in most cases results in a speed faster than c.

2. Nov 1, 2015

### Staff: Mentor

You might find the following form of the equation easier to understand.

$$V_{a/c} = \frac{V_{a/b} + V_{b/c}}{1 + (V_{a/b} V_{b/c})/c^2}$$

Hint: Let "b" be Europa. ("a/c" means the velocity of "a" as measured in the frame of "c".)

3. Nov 1, 2015

### mfreeman

Assuming "b" is Europa I would let V(a/c) be the speed of Ship A relative to Ship B which we know to be 0.50c. By the problem statement we have V(a/b) = V(b/c) = V(c/b) = V'. Thus we have 0.50c = 2V' / (1 + (V'^2/c^2)). This leads to a quadratic which I was told is the incorrect approach. Was that incorrect advice? Thank you very much for your help.

4. Nov 1, 2015

### Staff: Mentor

To keep your sanity, let "a" stand for asteroid #1 and "c" stand for asteroid #2. Furthermore, let asteroid "a" move to the right and "c" move to the left. (As seen from Europa.)

Careful! Signs matter. Let "to the right" be positive.

5. Nov 1, 2015

### Staff: Mentor

Actually, this equation looks fine to me.

6. Nov 1, 2015

### mfreeman

Gosh I thought so! Thanks.

7. Nov 1, 2015

### Staff: Mentor

Good!

Just for the record, if V(a/b) = V', then V(b/a) = - V'.

8. Nov 1, 2015

### Ray Vickson

There is no ambiguity. Relative to Europa, asteroid A moves at velocity +v and asteroid B at velocity -v. You are given that the velocity of B in the rest-frame of A is -c/2.