- #1
71GA
- 208
- 0
Homework Statement
Two sticks (both with the proper length ##\scriptsize 1m##) travel one toward another along their lengths. In the proper system of one stick they measure time ##\scriptsize 12.5ns## between two events:
- right ends are aligned (this happens first)
- left ends are aligned (this happens last)
Question:
What is the relative speed ##\scriptsize u## between the sticks?
Homework Equations
- Lorentz transformations
- Lenght contraction
The Attempt at a Solution
I first draw the picture:
If i use the Lorentz transformation and try to calculate the relative speed ##\scriptsize u## i get speed that is supposed to be greater than the speed of light:
\begin{aligned}
\Delta x' &= \gamma (\Delta x - u \Delta t) \xleftarrow{\text{Is this ok? I don't think my system is in the standard configuration}}\\
\frac{1}{\gamma}\Delta x &= \gamma (\Delta x - u \Delta t)\xleftarrow{\text{$\Delta x$ is equal to the proper length (in $xy$ the botom stick is standing still)}}\\
\tfrac{1}{\gamma^2}\Delta x &= \Delta x - u \Delta t\\
\left( 1 - \tfrac{u^2}{c^2}\right)\Delta x &= \Delta x - u \Delta t\\
\Delta x - \tfrac{u^2}{c^2}\Delta x &= \Delta x - u \Delta t\\
\tfrac{u^2}{c^2}\Delta x &= u \Delta t\\
\tfrac{u}{c^2}\Delta x &= \Delta t\\
u &= \tfrac{\Delta t}{\Delta x} c^2\\
u &= \tfrac{12.5\cdot 10^{-9}s}{1m} 2.99\cdot 10^{8}\tfrac{m}{s} c\\
u &= 3.74 c
\end{aligned}
In the solutions it says i should get ##\scriptsize u=0.5 c##, but i get ##\scriptsize u=3.74c##. I used the Lorentz transformation found on Wikipedia. These are for the standard configuration, but in my case is it the standard configuration?