The diagram shows a particle P, travelling at speed v, about to collide with a stationary particle Q of the same mass. The collision is perfectly elastic.
Which statement describes the motion of P and of Q immediately after the collision?
P comes to rest and Q acquires speed 
.
P rebounds with speed and Q remains stationary.
P and Q both travel in the same direction with speed 
.
P rebounds with speed and Q acquires speed .
