# Homework Help: Angle of a rod (relativity)

1. Feb 14, 2009

### kron

I don't know how to edit the title but the problem is solved.

One needs two simultaneous events in frame S not in S'.
(Just in case someone tried to figure it out)

1. The problem statement, all variables and given/known data

Hi,

this is not a homework, but an exercise of the book Introduction to Special Relativity by Wolfgang Rindler
(Oxford Science Publications):

In S' a straight rod parallel to the x'-axis moves in the y'-direction with constant velocity u.
Show that in S the rod is inclined to the x-axis at an angle

correction:

$$\theta=-arctan( \frac{\gamma u v }{c^2})$$

2. Relevant equations

LT:

$$(1)~ x=\gamma (x' + v t')$$
$$(2)~ t = \gamma(t' + \frac{v x'}{c^2})$$

3. The attempt at a solution

The x and x' axis of S and S' are parallel. The beginning point of the rod may be placed
in x'=y'=0 at t'=0.
For t'=0 I get two events in S for the beginning and end point of the rod, which are [with (1) and (2)]

$$P_1 : x=0, t=0$$

$$P_2 : x=\gamma L, t=\gamma \frac{v L}{c^2}$$

And u_y' = u transforms like

$$\frac{u}{\gamma (1 + \frac{v u_x'}{c^2})}$$

with u_x' = 0 this is

$$u_y = \frac{u}{\gamma}$$

so with

$$\Delta y = u_y t = \frac{u v L}{c^2}$$

$$\tan(\theta) = \frac{\Delta y}{\Delta x} = \frac{u v}{\gamma c^2}$$

which is not the same as the solution given above.

Thanks.

Last edited: Feb 14, 2009