AIR RESISTANCE AND TERMINAL VELOCITY
4 marks · HardA helicopter drops a small probe of mass 0.60 kg from a height above the ground. The gravitational field strength is 9.8 N/kg. The probe quickly reaches terminal velocity as it falls through the air.
Calculate the weight of the probe.
At terminal velocity, what is the size of the air resistance force acting on the probe? Explain your answer.
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) 5.88 N
(b) At terminal velocity, the air resistance is equal in size to the weight of the probe (5.88 N), but acts upward. This is because the forces are balanced and the probe falls at constant speed.
Common mistakes: Giving air resistance less or greater than weight at terminal velocity
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Marking points
M1 (a) · 1 mark · Uses W = m × g correctly: (0.60 kg × 9.8 N/kg)
B1 (a) · 1 mark · Correct answer: 5.88 N (accept rounding to 5.9 N)
B2 (b) · 1 mark · States air resistance is equal in size to the weight at terminal velocity.
B3 (b) · 1 mark · Gives correct reason (forces are balanced, no further acceleration, constant speed).
Worked solution
Part (a): Weight = mass × gravitational field strength = 0.60 kg × 9.8 N/kg = 5.88 N
Part (b): At terminal velocity, the downward force (weight) is balanced by the upward force (air resistance). Therefore, air resistance is also 5.88 N, and the probe moves at constant velocity.