SAT Subject Physics Practice Question 11: Answer and Explanation
2. A person who weighs 800 N steps onto a scale that is on the floor of an elevator car. If the elevator accelerates upward at a rate of 5 m/s2, what will the scale read?
A. 400 N
B. 800 N
C. 1000 N
D. 1200 N
E. 1600 N
Correct Answer: D
First draw a free-body diagram.
The person exerts a downward force on the scale, and the scale pushes up on the person with an equal (but opposite) force, FN. Thus, the scale reading is FN, the magnitude of the normal force. Since FN – Fw = ma, we have FN = Fw + ma = (800 N) + [800 N/(10 )](5 m/s2) = 1200 N.