Special Relativity, Length Contraction

[eprparadox]

Homework Statement

As a rocket ship moves by at 0.95c a mark is made on a stationary axis at the front end of the rocket and 9 × 10^−8 s later a mark is made on the axis at the back end. The marks are found to be 100 m apart. The rest length of the rocket is:
A. 31m
B. 78m
C. 100m
D. 240m
E. 320m

Homework Equations

L = Lp / gamma, where Lp is the proper length.

The Attempt at a Solution

The answer given is 240 m, but I get about 400 meters for my answer and the discrepancy lies in whether you use 100+25 m or 100 - 25 m.

My explanation goes something like this: In the time before the second person marks the end of the rocket, the back of the train has moved an additional 25 meters, so we have the total length measured in the stationary axis frame as 100 + 25 (that is my L value). Plugging into the above equation gives about 400 meters.

Is this a correct argument or am I going wrong somewhere? Thanks a lot ahead of time.

 

[Doc Al]

Your argument is correct. It looks like the answer key got the front and back of the rocket mixed up.