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AS Daily A Level Physics question

2026-01-15 OCR A Forces and Motion (M3) 3.2.2(d)(i) 3.2.2(b)

Two spheres of the same diameter, one steel and one plastic, are dropped from a tall stairwell into still air. After falling long enough for each to reach terminal speed, which statement must be true about their terminal speeds?

  1. A The plastic sphere has a higher terminal speed because it is less affected by air resistance.
  2. B The steel sphere has a higher terminal speed because a larger drag is needed to balance its greater weight. (correct)
  3. C They have the same terminal speed because they have the same size and thus experience the same drag.
  4. D Neither reaches terminal speed because drag cannot grow large enough to balance weight in free fall.

Answer

The correct answer is B.

Correct: B — The steel sphere has a higher terminal speed because a larger drag is needed to balance its greater weight. A The lighter (plastic) sphere needs a smaller drag to balance its smaller weight, so it reaches balance at a lower speed, not a higher one. B This is correct because at terminal speed drag equals weight; with the same size (same area), the heavier steel sphere must go faster to generate the larger drag required. C Same size means same drag at the same speed, but terminal speed depends on when drag equals weight, and the weights differ. D Drag increases with speed and can match weight; both can reach terminal speed given sufficient fall distance.

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