But although a twiceasheavy ball strikes first, it falls only a little faster, and not twice as fast, which is what followers of Aristotle believed. The heavier tennis ball will strike the ground first for the same reason the heavier parachutist in Figure 4.16 strikes the ground first. Note that although the air resistance on the heavier ball is smaller relative to the weight of the ball, it is actually greater than the air resistance that acts on the other ball. Because the heavier ball falls faster and air resistance is greater for greater speed. The velocity of the rock is momentarily zero, but the rate of change of velocity is still there.
Air resistance decreases the velocity of a moving object.
Hence the ball has less than its initial speed when it returns to the level from which it was thrown. The effect is easy to see for a feather projected upward by a sling shot. No way will it return to its starting point with its initial speed. The ball rises in less time than it falls. By exaggerating the circumstance and considering the feather example in the preceding answer, the time for the feather to flutter from its maximum altitude is clearly longer than the time it took to attain that altitude.