The Ping Pong Throw

QUESTION:

When a Ping Pong ball that has been dropped from the top of a high building reaches terminal speed, its acceleration is zero. Now suppose the same ball is projected upward with an initial speed greater than this terminal speed. At the instant its speed equals this terminal speed on the way up, the magnitude of its acceleration is

1. zero.
2. more than zero, but less than g.
3. g.
4. more than g.

ANSWER: D

The answer is d; for the ball's acceleration is then –2g, as seen from the formul a = Fnet/m. Going up, air resistance R and weight W are in the same direction.

Letting up be + and down –, we see:

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