## College Physics (4th Edition)

Since the string's tension acts at a $90^{\circ}$ to the direction of motion, the string does no work on the plane.
Let $T$ be the tension in the string. We can find the work done by the string on the plane: $Work = T~d~cos~\theta$ $Work = T~d~cos~90^{\circ}$ $Work = 0$ Since the string's tension acts at a $90^{\circ}$ angle to the direction of motion, the string does no work on the plane.