Physics quiz question: Mission Impossible: II mid-air collision, conservation laws

Physics 205A Quiz 4, fall semester 2014
Cuesta College, San Luis Obispo, CA

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

In a scene from Mission: Impossible II (Paramount Pictures, 2000), Dougray Scott (mass 90 kg) and Tom Cruise (mass 80 kg) are shown jumping off motorcycles headed towards each other with different speeds until their bodies collide and stick in midair, completely stopping their horizontal motion[*]. Ignore friction, drag, and external forces, and treat this as a horizontal collision.

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.)

[*] Adam Weiner, "Hollywood Physics—Collision: Impossible," popsci.com/entertainment-gaming/gallery/2007-09/hollywood-physics.

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 Dougray Scott and Tom Cruise stick together in midair, this is a completely inelastic collision, such that translational kinetic energy is not conserved.

Sections 70854, 70855, 73320
Exam code: quiz04w3Rc
(A) : 30 students
(B) : 14 students
(C) : 11 students
(D) : 10 students

Success level: 46%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.29

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