Physics quiz question: bullets fused in mid-air

Physics 205A Quiz 4, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 7.43

(Consider that the net external force is negligible compared with the forces the objects exert on each other during these brief collisions.) Two bullets of slightly different masses travel towards each other with different speeds, and collide horizontally in mid-air, fusing together as a result(*). Based solely on the information given, what must be conserved?
(A) Momentum.
(B) Kinetic energy.
(C) (Both of the above choices.)
(D) (None of the above choices.)

(*) Source: "French and Russian Bullets Collided in Flight," http://paradoxoff.com/french-and-russian-bullets-collided-in-flight.html.

Correct answer: (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 two bullets fuse together, this is a perfectly inelastic collision, such that kinetic energy is not conserved.

Sections 70854, 70855
Exam code: quiz04Bmw3
(A) : 37 students
(B) : 2 students
(C) : 8 students
(D) : 5 students

Success level: 72%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.63