a girl is skipping stones across a lake. one of the stones accidentally ricochets off a toy boat that is initially at rest in the water (see the drawing). the 0.072-kg stone strikes the boat at a velocity of 13 m/s, 15° below due east, and ricochets off at a velocity of 11 m/s, 12° above due east. after being struck by the stone, the boat's velocity is 2.1 m/s, due east. what is the mass of the boat? assume the water offers no resistance to the boat's motion.