Question

A construction crane lowers a beam into place at constant speed. Consider the work Wg done by gravity on the beam and the work WT done by the tension in the cable on the beam. Which is true?

A) Wi is positive and Wr is positive.
B) We is positive and W'r is negative.
C) Wg is negative and W'r is negative.
D) Wi and W'r are both zero.
E) None of the above

Answer 1

Answer:

Wg is positive and WT negative.

(Letters in options are all wrongly written).

Explanation:

Remember that the work of a force is the internal product between the force and the displacement $W=F.d$.

Since the displacement is downwards like the weight, the work done by gravity is positive, while the work done by the tension is negative since it points upwards.