試題詳解

13. A boy pulls a wooden box along a rough horizontal floor at constant speed by means of a force as shown. In the diagram f is the magnitude of the force of friction, N is the magnitude of the normal force, and Fg is the magnitude of the force of gravity. Which of the following must be true?
(A) P = f and N = Fg
(B) P = f and N ＞ Fg
(C) P ＞ f and N ＜ Fg
(D) P ＞ f and N = Fg
(E) None of the above.