Physics clicker questions: kinetic energy-momentum sequence

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

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 7.7

Students were asked the following clicker questions (Classroom Performance System, einstruction.com) near the beginning of their learning cycle:

Mass of (bowling?) ball = 5.0 kg

[0.6 participation points.] The ball is initially at rest, and then you push it such that it has a final speed of 3.0 m/s in the +x direction. What was the change in kinetic energy of the ball?
(A) –23 J.
(B) –15 J.
(C) 0 J.
(D) +15 J.
(E) +23 J.
(F) (I'm lost, and don't know how to answer this.)

Sections 0906, 0907
(A) : 1 student
(B) : 1 student
(C) : 1 student
(D) : 9 students
(E) : 22 students

Correct answer: (E)

[0.6 participation points.] What was the change in momentum of the ball?
(A) –23 kg*m/s.
(B) –15 kg*m/s.
(C) 0 kg*m/s.
(D) +15 kg*m/s.
(E) +23 kg*m/s.
(F) (I'm lost, and don't know how to answer this.)

Sections 0906, 0907
(A) : 0 students
(B) : 0 students
(C) : 0 students
(D) : 32 students
(E) : 2 students
(F) : 0 students

Correct answer: (D)

So far, so good--students seem to understand how to calculate changes in kinetic energy, and changes in momentum, especially when the initial velocity is zero.

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Mass of (bowling) ball = 5.0 kg

[0.6 participation points.] The ball is moving with a speed of 3.0 m/s in the +x direction, and then hits a wall and bounces back with the same speed in the –x direction. What was the change in kinetic energy of the ball?
(A) –23 J.
(B) –15 J.
(C) 0 J.
(D) +15 J.
(E) +23 J.
(F) (I'm lost, and don't know how to answer this.)

Sections 0906, 0907
(A) : 1 student
(B) : 2 students
(C) : 25 students
(D) : 0 students
(E) : 5 students
(F) : 0 students

Correct answer: (C)

The final and initial speed is the same, so there is no change in kinetic energy.

[0.6 participation points.] What was the change in momentum of the ball?
(A) –23 kg*m/s.
(B) –15 kg*m/s.
(C) 0 kg*m/s.
(D) +15 kg*m/s.
(E) +23 kg*m/s.
(F) (I'm lost, and don't know how to answer this.)

Sections 0906, 0907
(A) : 27 student
(B) : 2 students
(C) : 5 students
(D) : 0 students
(E) : 0 students
(F) : 0 students

Correct answer: (A)

The final velocity is negative, and the initial velocity is positive, such that the change in momentum (the impulse) points in the -x direction. (Note that a negative change in kinetic energy corresponds to a decrease in speed; a negative change in momentum corresponds to the -x direction.)