AIR RESISTANCE AND TERMINAL VELOCITY
3 marks · NormalA basketball is dropped from a tall stand. Explain, with reasons, why the basketball accelerates at first but later falls at a constant speed. Refer to the forces acting on the ball at each stage.
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At first, weight acts downward and air resistance is small, so the resultant force is downwards and the ball accelerates. As it speeds up, air resistance increases. Eventually, air resistance balances weight, so the resultant force is zero and the ball falls at terminal velocity, a constant speed.
Must show: initial acceleration due to greater weight than air resistance, increase in air resistance with speed, forces balanced at terminal velocity (no further acceleration)
Also accept: Mentions acceleration stops when air resistance equals weight; constant speed after that
Common mistakes: Does not mention forces changing; Says the ball stops moving instead of moves at constant speed
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Marking points
B1 · 1 mark · States that at first, weight is greater than air resistance so ball accelerates
B2 · 1 mark · States that as speed increases, air resistance increases
B3 · 1 mark · States that eventually air resistance equals weight so resultant force is zero, and the ball moves at constant (terminal) velocity
Worked solution
At first, the only significant force is weight, acting downwards; air resistance is very small so the resultant force is downward and the ball accelerates.
As the ball speeds up, air resistance increases.
Eventually, air resistance becomes equal to the weight so the resultant force is zero. The ball then falls at constant speed—its terminal velocity.