Physics quiz question: bullet into block

Physics 5A Quiz 4, Fall Semester 2007
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 7.43

[3.0 points.] A block of wood of 1.20 kg is initially at rest. A bullet of mass 0.030 kg traveling at 180 m/s in the +x direction strikes the block and becomes embedded in it. Consider the frictional force between the block and the ground to be negligible. What is conserved for this collision?
(A) Momentum only.
(B) Kinetic energy only.
(C) Both momentum and kinetic energy.
(D) (Neither momentum nor kinetic energy are conserved for this collision.)

Correct answer: (A)

Since there are no external horizontal impulses, momentum is conserved. However, since there is deformation with no rebound and separation in this perfectly inelastic collision, kinetic energy is not conserved.

Student responses
Sections 0906, 0907
(A) : 26 students
(B) : 6 students
(C) : 7 students
(D) : 1 student