Question

A bat hits a moving baseball. If the bat delivers a net eastward impulse of 0.7 N-s and the ball starts with an initial horizontal velocity of 3.8 m/s to the west and leaves with a 5.2 m/s velocity to the east, what is the mass of the ball (in grams)?

Answer 1

mass * (velocity change) = impulse
The velocity changes from +3.8 to -5.2 so the velocity change is 9.0 m/s. (The direction change is important)
Mass = 0.7 N-s/9.0 m/s = 0.0778 kg = 77.8 g

(The official mass of a Major League baseball is 145 g, so they did a poor job dreaming up this question)