2008 AP Physics B Free Response MISTAKE??? 1. The problem statement, all variables and given/known data Question number two can be found here... http://apcentral.collegeboard.com/apc/public/repository/ap08_physics_b_frq.pdf 2. Relevant equations 3. The attempt at a solution Ok I was working out question number two and was doing (e). How come when they solved it in the rubric http://www.collegeboard.com/prod_downloads/ap/students/physics/ap08_physics_b_sgs_rev.pdf they solved it this way .5 (mass of block b) v^2 = .5 kx^2 how come they only included the mass of block b and not the mass of the whole system??? The wording of the question states specifically In a new situation, the blocks and spring are moving together... this would allow one to conclude that the whole system was moving which would mean block A and block B but yet they only include the mass of block b in their calculation... Can you please explain why they chose only the mass of B in this calculation and didn't include the other block thanks!