When a crate of mass m is dragged a distance d along a surface with coefficient of kinetic friction μ_k, then dragged back along the same path to its original position, the work done by friction is A) 0. B) −μ_kmgd. C) +μ_kmgd. D) −2μ_kmgd. E) +2μ_kmgd.

Question

Quizplus · Accepted Answer

The work done by friction over the entire round trip is negative because friction always opposes the motion, and it is doubled because the crate is dragged the same distance in both directions. The work done by friction in one direction is $-\mu_k mgd$, so for the round trip, it is $-2\mu_k mgd$.

When a Crate of Mass M Is Dragged a Distance