AIR RESISTANCE AND TERMINAL VELOCITY
5 marks · HardA student drops two objects of equal mass but different shapes (a smooth metal ball and a large feather) at the same time from the same height in a classroom. Assume air resistance is significant for the feather.
State which object will reach the floor first and explain your answer.
Describe, in terms of forces, what happens to the feather as it falls until it reaches its terminal velocity.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) The smooth metal ball reaches the floor first because it experiences much less air resistance than the feather. The large feather's shape creates greater air resistance, slowing its fall.
(b) At first, only the feather's weight acts downwards and air resistance is low, so it accelerates downwards. As speed increases, air resistance increases upwards. Eventually, air resistance becomes equal to the feather's weight, and the feather stops accelerating and falls at constant (terminal) velocity.
Common mistakes: Saying both objects reach at the same time; Not describing force changes on the feather
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies that the metal ball reaches the floor first.
B2 (a) · 1 mark · Gives a correct reason: states it is due to less air resistance (drag) or links shape to smaller drag.
M1 (b) · 1 mark · States downward force (weight) is greater than upward force (air resistance) at first, so feather accelerates.
M2 (b) · 1 mark · Mentions that air resistance increases as speed increases.
M3 (b) · 1 mark · Explains that terminal velocity is reached when air resistance equals weight and feather moves at constant speed.
Worked solution
Part (a): The smooth metal ball reaches the floor first. This is because air resistance (drag) is much smaller for the ball due to its shape, while the feather experiences much more drag, slowing it down.
Part (b): At the start, the only forces on the feather are its weight (down) and a small air resistance (up). The feather accelerates downwards as weight is greater. As it speeds up, air resistance increases. Eventually, air resistance equals the weight; at this point, forces are balanced, and the feather stops accelerating. It then falls at constant, terminal velocity.