Ball a 1.55kg moving right at 8.76 m/s makes a head-on collision with ball b (0.752 kg) moving left at 11.4 m/s. after, ball b moves right at 9.03 m/s. what is the final velocity of ball a?

1.15 m/s to the left (3 sig. fig.).

Momentum is conserved between the two balls if they are not in contact with any other object. In other words,

, where

Let the velocity of objects moving to the right be positive.

Before the two balls collide:

After the two balls collide:

Again,

,

.

.

is negative? Don't panic. Recall that velocities to the right is considered positive. Accordingly, negative velocities are directed to the left.

Hence, ball A will be travelling to the left at 1.15 m/s (3 sig. fig. as in the question) after the collision.