A 0.15 kg. baseball flies rightward at 15m/s to hit a tennis ball flying at it from the opposite direction of 22 m/s. The baseball and the tennis ball have a

final speed of 1.2 m/s and 13 m/s respectively, both to the right.

What is the mass of the tennis ball?

(w/solution)

Let’s use *queue dramatic voice* THE GREAT LAW OF THE CONSERVATION OF MOMENTUM

m1v1i + m2v2i = m1v1f + m2v2f

m1v1i - m1v1f = m2(v2f - v2i)

m2 = (m1v1i - m1v1f) / (v2f - v2i)

m2 = [(0.15)(15) - (0.15)(1.2)] / (13 - -22)

Henceforth, my good sire, the mass indeed works out to a fine approximate value of:
m = 0.06 kg

That’s a pretty small tennis ball!