1. (a) What is the acceleration of two falling sky divers (mass 115.0 kg includi

Question

1. (a) What is the acceleration of two falling sky divers (mass 115.0 kg including parachute) when the upward force of air resistance is equal to one-fourth of their weight?
m/s2 (downward)
(b) After popping open the parachute, the divers descend leisurely to the ground at constant speed. What now is the force of air resistance on the sky divers and their parachute? See the figure below.
N (upward)

2. A person stands on a bathroom scale in a motionless elevator. When the elevator begins to move, the scale briefly reads only 0.62 of the person's regular weight. Calculate the acceleration of the elevator, and find the direction of acceleration.
( )m/s2

not enough information to decide

downwards

upwards

Explanation / Answer

1.(a) Net force downward, F = mg - mg/4 = 3mg/4

Net acceleration downward, a = 3g/4 = 3 * 9.81 / 4 = 7.36 m/s2

(b) Since the divers descend at constant speed, net force on them is zero. So, the air resistance is equal to their weight.

Air resistance = mg = 115 * 9.81 = 1128 N (upward)

2) The scale read the normal reaction applied by the elevator floor on the person. Since normal reaction (N) is less than weight (mg), the elevator is accelerating downward.

Force balance: mg - N = ma

=> mg - 0.62mg = ma

=> a = 0.38g = 0.38 * 9.81 = 3.73 m/s2 (downward)

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