Its More Difficult to Start Moving a Heavy Carton from Rest

Multiple Choice

Its more difficult to start moving a heavy carton from rest than it is to keep pushing it with constant velocity, because

A) The normal force is greater when the carton is at rest.
B) μ_s < μ_k.
C) Initially, the normal force is not perpendicular to the applied force.
D) μ_k < μ_s.
E) μ_s = μ_k.