Physics midterm question: work done on diagonally-pulled crate

Physics 205A Midterm 2, fall semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 6.1

A rope that makes an angle of 35° above the horizontal drags a crate to the right across a frictionless floor. The __________ force does positive work on the crate.
(A) weight.
(B) rope tension.
(C) normal.
(D) (More than one of the above choices.)
(E) (None of the above choices.)

Correct answer (highlight to unhide): (B)

The definition of work done is:

W = F·s·cosθ,

and since the angle between the displacement s (along the +x direction) is 90° for both the normal force and the weight force, these forces do zero work on the crate. Since the angle between the tension force of the rope and the displacement s is between 0° and 90°, then the rope does positive work on the crate.

Sections 70854, 70855
Exam code: midterm02fR3q
(A) : 1 student
(B) : 30 students
(C) : 0 students
(D) : 19 students
(E) : 3 students

Success level: 57%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.22