Question

energy changes: recognizing external work
when is the person doing external work?
a) person lifts box off of the ground
b) person carries the box in a straight line at constant speed
c) person gets on a sled and slides down a hill
d) a & c
e) a & b

Explanation:

Work is done when a force is applied over a distance in the direction of the force. In option A, the person applies an upward force to lift the box against gravity over a distance, so work is done. In option B, the force applied by the person to carry the box is vertical while the displacement is horizontal, so no work is done in the sense of the physics - work formula $W = F\cdot d\cdot\cos\theta$ ($\theta = 90^{\circ}$, $\cos\theta=0$). In option C, gravity is doing work on the person - sled system as the person slides down the hill, and the person is not doing external work on the system in the context of this question.

Answer:

A. Person lifts box off of the ground