1. The problem statement, all variables and given/known data Three blocks of identical mass are placed on a frictionless table as shown. The center block is at rest, whereas the other two blocks are moving directly towards it at identical speeds v. The center block is initially closer to the left block than the right one. All motion takes place along a single horizontal line. Problem #2 on this website: https://www.aapt.org/physicsteam/2010/upload/2009_F-ma.pdf Suppose that all collisions are instantaneous and perfectly elastic. After a long time, which of the following is true? (A) The center block is moving to the left. (B) The center block is moving to the right. (C) The center block is at rest somewhere to the left of its initial position. (D) The center block is at rest at its initial position. (E) The center block is at rest somewhere to the right of its initial position. (Correct Answer: D) 2. Relevant equations p = mv mv1 + mv2 = mv3 + mv4 KE = (0.5)m(v^2) (0.5)m(v1^2) + (0.5)m(v2^2) = (0.5)m(v3^2) + (0.5)m(v4^2) 3. The attempt at a solution I observed that there was no friction, meaning that the velocity of each mass should, in theory, remain constant until it experienced a force from another mass after colliding with it. This, in my mind, would mean that the blocks should continue colliding forever due to Newton's first law of inertia... but that thinking is obviously flawed. I tried writing equations using conservation of kinetic energy and conservation of linear momentum, but I seemed to be getting nowhere in terms of finding the position after a long period of time. Honestly, I don't even know how I'm supposed to attempt such a problem. Should I be considering velocity of the center of mass or change in position of the center of mass?