consider the collision of two billiard balls: A stationary 8-ball is hit by a cue ball moving with uniform motion. the 8-ball quickly accelerates to some velocity. by newton 2, we can explain that the cue ball exerted an unbalanced external force on the 8-ball and the 8-ball accelerated in accordance with the relation F=ma. we might then part ourselves on the back for phyzjob well done and call it quits for the rest of the day. but wait. what happened to the cue ball? Did the cue ball maintain uniform motion?