TQ 9.4

Momentum is a vector quantity so the total momentum of the system being zero does not mean that all particles of the system are at rest (imagine two particles moving with the same speed but in opposite directions). Since the particles are not necessarily at rest, the total kinetic energy of the system may be non-zero. Of course it is also quite possible that the particles are indeed at rest, in which case the kinetic energy of the system is zero.