A student stands on a bathroom scale in an elevator at rest on the 64th floor of a building. The scale reads 836 N.
a) As the elevator moves up, the scale reading increases to 935 N, then decreases back to 836 N. Find the acceleration of the elevator. (Answer: 1.2 m/s/s)
b) As the elevator approaches the 74th floor, the scale reading drops as low as 782 N. What is the acceleration of the elevator? (Answer given: -0.63 m/s/s)

Question

A student stands on a bathroom scale in an elevator at rest on the 64th floor of a building. The scale reads 836 N.
a) As the elevator moves up, the scale reading increases to 935 N, then decreases back to 836 N. Find the acceleration of the elevator. (Answer: 1.2 m/s/s)
b) As the elevator approaches the 74th floor, the scale reading drops as low as 782 N. What is the acceleration of the elevator? (Answer given: -0.63 m/s/s)

kittiwitti1 · Answer

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$$\Sigma F _{y} = N - W - 99$$$$W = mg = ...?$$I got stuck after that. This is copied from a classmate who didn't have time to explain the answer and solving process, which HE got from another classmate.