Physics quiz question: coupled freight car and locomotive

Physics 205A Quiz 4, fall semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 7.41

(Ignore friction, drag, and other external forces during this brief collision.) An empty freight car rolls along a straight level track, and collides with an initially stationary diesel locomotive, and couple (attach) together. Based solely on the information given, __________ must be conserved.
(A) momentum.
(B) translational kinetic energy.
(C) (Both of the above choices.)
(D) (None of the above choices.)

Correct answer (highlight to unhide): (A)

Negligible net external force and brief time duration for this collision makes the external impulse on this system zero, such that momentum is conserved. Since the freight car and locomotive "couple together," this is a completely inelastic collision, such that kinetic energy is not conserved.

Sections 70854, 70855
Exam code: quiz04iMpL
(A) : 41 students
(B) : 1 student
(C) : 6 students
(D) : 3 students

Success level: 80%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.64