Physics quiz question: kinetic energy of SHM object at equilibrium

Physics 205A Quiz 6, Spring Semester 2009
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Multiple-Choice Questions 10.11-10.20

[Version 1]
[3.0 points.] A graph of kinetic energy versus time for an object in SHM is shown at right. What is the earliest time that the object will be located at equilibrium?
(A) 0 s.
(B) 1 s.
(C) 2 s.
(D) 3 s.

Correct answer: (A)

An object in simple harmonic motion will be momentarily stationary (and thus have zero kinetic energy) at its x = +/- A amplitude endpoints, and have its fastest speeds as it passes through equilibrium (x = 0). Since the kinetic energy graph already has it maximum value at t = 0 s, that is the earliest time that the object is at equilibrium.

Student responses
Sections 30880, 30881
(A) : 14 students
(B) : 0 students
(C) : 7 students
(D) : 0 students

[Version 2]
[3.0 points.] A graph of kinetic energy versus time for an object in SHM is shown at right. What is the earliest time that the object will be located at equilibrium?
(A) 0 s.
(B) 1 s.
(C) 2 s.
(D) 3 s.

Correct answer: (C)

An object in simple harmonic motion will be momentarily stationary (and thus have zero kinetic energy) at its x = +/- A amplitude endpoints, and have its fastest speeds as it passes through equilibrium (x = 0). Since the kinetic energy graph does not have its maximum value until t = 2 s, that is the earliest time that the object is at equilibrium.

Student responses
Sections 30880, 30881
(A) : 4 students
(B) : 1 student
(C) : 14 students
(D) : 0 students

"Difficulty level": 70%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.69