1. The problem statement, all variables and given/known data http://www.aapt.org/physicsteam/2010/upload/2010_FmaSolutions.pdf See question 23 A' = 1/2A 2. Relevant equations Av = A'v' 3. The attempt at a solution I thought this was just a simple Av = A'v' problem which would lead to v' = 2v. But there is apparently more to it, as the answer is √2v. It could have to do with the momentum because F_net = 0 so it is conserved. In that case, mv = mv but I'm confused about how to use that to solve the problem.