Acrane lowers a steel girder into place at a construction site. the girder moves with constant speed. consider the work wg done by gravity and the work wt done by the tension in the cable. which of the following is correct? 1. wg is positive and wt is positive. 2. wg is negative and wt is negative. 3. wg is positive and wt is negative. 4. wg and wt are both zero. 5. wg is negative and wt is positive.

3. Wg is positive and WT is negative.

Explanation:

The work done by a force is given by:

where

F is the force

d is the displacement of the object

is the angle between the directions of F and d

This means that:

- When force and displacement are parallel, , so the work done is positive

- When force and displacement are anti-parallel, , so the work done is negative

In this case, the crane is moving downward. The force of gravity is also downward, while the tension in the cable is upward. so we have:

- Wg is positive, because gravity is parallel to the displacement

- Wt is negative, because the tension is opposite to the displacement

increases it

no. dot product   =0 when vecs at right angles