Physics quiz question: pellet bouncing off of block

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

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 7.49

(Ignore friction, drag, and other external forces during this brief collision.) A pellet moving to the right hits a motionless block, and the pellet bounces off to the left, while the block moves to the right. 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 pellet bounces off of, instead of embedding itself into the block, this is not a completely inelastic collision, leaving two possibilities: a (partially) inelastic collision (such that translational kinetic energy is not conserved), or an elastic collision (such that translational kinetic energy is conserved). However, without additional information regarding the resulting permanent deformation (if any) of the pellet and/or block after the collision, only momentum can be said to be conserved.

Sections 70854, 70855
Exam code: quiz04iMpL
(A) : 33 students
(B) : 3 students
(C) : 14 students
(D) : 1 student

Success level: 65%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.36