A wooden block is pulled along a horizontal laboratory bench by a student who exerts…
PHYSICS · Multi-part · A Level · Physics · Mechanics and materials · Mechanics and materials
Problem
A wooden block is pulled along a horizontal laboratory bench by a student who exerts a steadily increasing horizontal force with a cord. At first the block is at rest, but after traveling a distance of 1.5 m it is moving at 1.2 m/s.
The bench surface and the cord are shown to be rough; the magnitude of the kinetic friction force on the block is 2.5 N in the direction opposite the motion throughout the interval described.
Neglect air resistance.
Assume the block travels in the +x direction.
(a) Identify each force that does non-zero work on the block during the 1.5 m displacement and state whether the work done by that force is positive or negative.
(b) Without performing detailed calculations, explain why the net work done on the block must be positive even though one of the forces listed in part (a) does negative work.
(c) Describe qualitatively how the kinetic energy of the block would change if an identical experiment were repeated with a smoother bench that exerts a kinetic friction force of only 1.0 N. Justify your answer in terms of the works done by each force.