SAT Subject Physics Practice Question 269: Answer and Explanation
40. A sky diver jumps from an airplane. After “free falling” for a while, she opens her parachute and her descent speed begins to decrease. While her descent speed decreases, let F denote the magnitude of the gravitational force on the sky diver and let D denote the magnitude of the upward force of air resistance (drag). Which of the following is then true?
A. F > D
B. F < D
C. F + D < weight of the sky diver
D. F – D > weight of the sky diver
E. F – D > 0
Correct Answer: B
A and E are identical, so both may be eliminated. Since F is equal to the weight of the sky diver, C and D don’t make sense. If the sky diver’s downward velocity is decreasing, she must be experiencing an upward acceleration. Therefore, the net force on the sky diver must be upward. Since D is an upward force and F is a downward force, D must be greater than F to give a net force, D – F, that’s upward.