# Homework Help: H.W. Question about Relativity

1. Jul 29, 2015

### Electrophy6

1. The problem statement, all variables and given/known data
Rocket "A" velocity is 0.8c to the right ,reltaive to earth
Rocket "B" velocity is 0.6c to the left , relative to rocket "A"
(This one I succedeed)

My problem was in this question:

Repeat the same problem ,but bow rocket "A" moves 0.8c * upwards *(positive y axis)
For some reason , I didn't make well

2. Relevant equations

3. The attempt at a solution

Tried using these equation while:

V=0.8,vx'=-0.6c,vy'=-0.8

Now , I know I got something wrong but I couldn't figure it out by myself(although I tried).....

Thank you!

Last edited by a moderator: Jul 29, 2015
2. Jul 29, 2015

### TSny

Hello.

3. Jul 29, 2015

### Electrophy6

Oh sorry ,forgot the th most important part
The question is :

"What is the velocity of Rocket B , relative to earth ?"

4. Jul 29, 2015

### TSny

OK

Just to be totally clear, what frame are you taking to be the "unprimed" frame? Is it the earth frame, the A-rocket frame, or the B-rocket frame?
Same question for the "primed frame".

5. Jul 29, 2015

### Electrophy6

Ok ,let's use the following notations

Earth- S
Rocket A- S'
Rocket B- S''(not necessary)

6. Jul 29, 2015

### TSny

For your list of relevant equations, what basic assumption is made about the direction of the velocity of the primed frame relative to the unprimed frame?

7. Jul 29, 2015

### Electrophy6

let's say we define the "right" to be the x positive direction and assume rocket A (O') moves in this direction realtive to O (originally S ,S' but it doesn't matter)

8. Jul 29, 2015

### Electrophy6

Oh sorry again ,wrong diagram
S' goes in the y positive direction(upwards) relative to S

9. Jul 29, 2015

### TSny

OK. Back to my earlier question. The equations that you wrote down as the Relevant equations are only valid if the primed frame moves in what direction relative to the unprimed frame?

10. Jul 29, 2015

### Electrophy6

Sorry , I misunderstood the question
However ,I will try answering it: I guess these equations are true If S' goes to the right relative to S
(So ,does it mean ,they can't fit the directions I just presentd ?)

11. Jul 29, 2015

### TSny

The formulas you listed assume that the primed frame is moving in the x direction relative to the unprimed frame.
Since rocket A (frame S') moves in the y direction relative to the earth (frame S), you will not be able to use the equations as written.

Can you see how to modify the equations so that they are applicable?

12. Jul 29, 2015

### Electrophy6

Maybe ....
in the eqaution of vx should I use Vx
-----------------v
x-------------------Vy
-----------------v
z------------------Vz

?

13. Jul 29, 2015

### TSny

I'm not sure I'm following you here.

In the equations that you listed, the relative motion of the two frames is assumed to be along the x direction. In your problem the relative motion is along the y direction. So, you can just replace all the x and x' subscripts in the equations by y and y' subscripts. Likewise, replace all y and y' subscripts in the equations by x and x' subscripts.

14. Jul 29, 2015

### Electrophy6

Ohhh you are right