Answer
a. They strike at the same time.
b. The water-filled tennis ball strikes first.
Work Step by Step
a. With no air resistance, the balls have the same acceleration g, directed downward. Their motion is identical.
b. With air resistance, as seen in equation 5.12 and explained on page 149, the heavier ball will fall faster. The two balls have the same size, so they encounter the same air drag at the same speed, but the heavier ball falls faster, and reaches a higher terminal speed. At that higher speed, the upward air drag is able to counter the water-filled ball's larger weight.
