I was watching MIT Classical Mechanics Lecture16. On completely elastic collision, (at around 24min), professor Walter Lewin shown that when a tennis ball with mass m and speed v hitting the wall, it bouced back with speed v. (Cut from the lecture) << ...Kinetic energy is conserved. All the kinetic energy is in the tennis ball; nothing is in the wall. The wall has an infinitely large mass, but the momentum of this tennis ball has changed by an amount 2 mv. That momentum must be in the wall--it's nonnegotiable, because momentum must be conserved. So now here you see in front of your eyes a case that the wall has momentum, but it has no kinetic energy. That the wall has momentum 2 mv, it's nonnegotiable. It must have momentum, and yet it has no kinetic energy...>> How can that be? The wall is NOT moving, how can it has momentum? I velocity for wall is zero, there is NO kinetic energy nor momentum, correct? But then why did he say that? Was it a trick question?