An object of mass m is hanging by a string from the ceiling of an elevator. The elevator is moving upward, but slowing down.

What is the tension in the string?

a) zero

b) greater than mg

c) exactly mg

d) less than mg

###### Solution.

According to Newton’s second law of motion, the net force equals mass times acceleration. We are going to use a free body diagram (force diagram) to show that the equation of the motion is given by

T – mg = – ma

Thereby,

T = mg – ma

and the answer is: (d)

the tension less than mg.

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