Physics quiz question: box under box, with pulley

Physics 8A Quiz 4, Spring Semester 2007
Cuesta College, San Luis Obispo, CA

Physics 8A learning goal Q4.5

[3.0 points.] A 5.00 kg box and a 1.75 kg box are attached by an ideally stretchless, massless cable that passes around a frictionless, massless pulley. The coefficient of kinetic friction between all surfaces is 0.250. A force is applied to the 1.75 kg box, as shown, and as a result, the 1.75 kg box moves to the left at a constant speed.

(Cf. Young and Freeman, University Physics, 11/e, Problem 5.81.)

[Version 1]
Which one of the following choices best corresponds to the force that has the smallest magnitude?
(A) F_applied.
(B) NetF_{on 1.75 kg box}.
(C) T_{5.00 kg box on 1.75 kg box}.
(D) f_{k, 5.00 kg box on 1.75 kg box}.

Correct answer: (B)
The net force on the 1.75 kg box must be zero, according to Newton's first law, because it is moving at constant speed.

Student responses:
(A) : 2 students
(B) : 10 students
(C) : 3 students
(D) : 5 students

[Version 2]
Which one of the following choices best corresponds to the force that has the largest magnitude?
(A) F_applied.
(B) NetF_{on 1.75 kg box}.
(C) T_{5.00 kg box on 1.75 kg box}.
(D) f_{k, 5.00 kg box on 1.75 kg box}.

Correct answer: (A)
The applied force (exerted to the left), according to Newton's first law, must be equal to both the kinetic frictional force of the 5.00 kg box on the 1.75 kg box and the tension force of the 5.00 kg box on the 1.75 kg box added together (which are both exerted to the right).

Student responses:
(A) : 7 students
(B) : 4 students
(C) : 0 students
(D) : 3 students