AS Daily A Level Physics question
A small ball is released from rest in a tall tube of fluid. Assume the only significant forces are its weight (downward) and a drag force that increases with speed (upward). Before the ball reaches terminal speed, which statement must be true?
Answer
The correct answer is C.
Correct: C — The acceleration decreases with time but is still downward. A is wrong because drag equals weight only at terminal speed; before that, weight is larger than drag. B is wrong because an upward resultant would require drag to exceed weight, which it does not just after release. C is right: as speed increases, drag grows, reducing the net downward force, so the downward acceleration diminishes until it eventually becomes zero at terminal speed. D is wrong because the speed is not constant while the forces are unbalanced; it increases until the forces balance.