(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

This is actually an exam problem, not a homework problem. The solution have been posted, but I don't understand the solution. Here is the question, as posted:

A very long rod travels parallel and very close to a flat surface in our laboratory. The

surface contains chalk guns that are spaced apart every meter. All the chalk guns fire

simultaneously in the lab frame at t = 0 and leave marks on the rod at the point just

above them at the time they fired.

The chalk marks on the rod are measured by an observer on the rod and are

found to be 2 meters apart. How fast is the rod moving with respect to the lab?

2. Relevant equations

Lorentz Transformations

3. The attempt at a solution

When I took the exam, I arrived at the same answer as in the solutions. I was skeptical though, because I thought the chalk guns should appear closer together in the frame of the rod, not spaced at 2 meters. Anyway, the solutions say v=sqrt(3)*c/2. After the exam though, I tried using Mathematica to solve for v explicitly, and it only returned a complex solution. If I take the length of the chalk guns to be 1/2 meters apart in the frame of the rod, then I get the solution posted. Am I just confused, or should the distance between the chalk guns in the rod's frame be spaced closed together? Isn't that why it's called a length contraction, not dilation?

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# Homework Help: Lorentz Contraction and Chalk Guns

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