Anna is on a railroad flatcar moving at 0.6c relative to Bob. (Their clocks read zero as Anna's center of mass passes Bob's) Anna's arm is outstretched in the direction the flatcar moves, and in her hand a flashbulb. When the flashbulb goes off, the reading on Anna's clock is 100ns. The reading on Bob's differs by 27 ns. How long is Anna's arm (i.e., from her hand to her center of mass)?
The Attempt at a Solution
Okay I know the answer is d=0.6c*100E-9 but I don't understand the reasoning behind that. My train of thought on this problem (...no pun intended) was as follows: Anna sees the light go off at 100ns, which means the light took that long to get to her so that she could see it, and she doesn't see herself as moving so the distance the light traveled to her eyes was d=ct=c*100E-9---> why is this incorrect?