1. The problem statement, all variables and given/known data Spaceship 1 passes spaceship 2 with a relative speed v. An observer in spaceship 1 measures a time interval ∆t for spaceship 2 to pass by. Find the length of spaceship 2 as measured in its own rest frame, i.e., find the proper length of spaceship 2 in terms of ∆t. 2. Relevant equations Time dilation and length contraction formulas: L=vΔt L'=vΔt' Δt'=γΔt 3. The attempt at a solution Since I'm looking for L' in terms of Δt, I just plugged the equation for Δt' into the L' equation. It doesn't seem right to me though. I'm unsure if I need to do a Lorentz transformation for this problem, and I don't really know how to do one. Though looking at the velocity addition rule in my book there is also a Δx involved... I'm lost.