A bowling ball is initially at rest. A Ping-Pong ball moving in the direction hits the bowling ball and bounces off it, traveling back in the direction. Consider a time interval extending from...

A bowling ball is initially at rest. A Ping-Pong ball moving in the direction hits the bowling ball and bounces off it,

traveling back in the direction. Consider a time interval extending from slightly before to slightly after the collision.

(a) In this time interval, what is the sign of for the system consisting of both balls?