20151122

Physics quiz question: Home Alone paint can pendulum

Physics 205A Quiz 6, fall semester 2015
Cuesta College, San Luis Obispo, CA

Home Alone
Twentieth Century Fox (1990)

A full paint can (mass[*] 5.6 kg) tied to a string, starting at one side from rest, swings for 3.0 s to reach its fastest speed when the string is vertically downwards. Approximate this as a simple pendulum, and ignore friction/drag. The oscillation period of the paint can is:
(A) 1.5 s.
(B) 1.9 s.
(C) 4.7 s.
(D) 12.0 s.

[*] cockeyed.com/science/weight/paint-gallon.html.

Correct answer (highlight to unhide): (D)

The period of a pendulum is the time for it to complete one cycle of motion, returning to its starting point with the same velocity that it started with. Since it takes 3.0 s for the paint can to start from rest and swing to hang straight down, it will take an additional 3.0 s for the paint can to swing to the other side, and an additional 6.0 s for it to swing back to its original position, for a total of 12.0 s seconds.

(The amount of time that the paint can takes to swing downwards in this movie is exaggerated for dramatic effect.)

(Response (A) is one-half of the time to swing downwards; response (B) is the mass m divided by the time to swing downwards; response (C) is 2·π·√(m/g).)

Sections 70854, 70855, 73320
Exam code: quiz06m45S
(A) : 9 students
(B) : 6 students
(C) : 8 students
(D) : 50 students

Success level: 68%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.61