1. The problem statement, all variables and given/known data The cylinder and piston are made of a thermal insulator. An atom of a gas collides with the piston at an angle and bounces off at an angle. State with a reason whether momentum of the atom is conserved in this collision. 2. Relevant equations 3. The attempt at a solution I think that momentum of atom is not conserved as it is a vector quantity and hence change in momentum when there is a change in direction of the atom velocity. However, the answer says that it is how you consider what is the "system". If atom is the "system", momentum is changed. If "piston+cylinder+atom"=system, momentum is conserved. Im really confused! Which one is right?