Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 7.7 (extended)
Students were asked the following clicker questions (Classroom Performance System, einstruction.com) in the beginning of their learning cycle:
A 5.0 kg 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 is the sign of the impulse exerted by the wall on the ball?
(A) Negative.
(B) Positive.
(C) (Zero.)
(D) (I'm lost, and don't know how to answer this.)
Sections 30880, 30881
(A) : 22 students
(B) : 9 students
(C) : 8 students
(D) : 0 students
This question was asked again after displaying the tallied results with the lack of consensus, with the following results. No comments were made by the instructor, in order to see if students were going to be able to discuss and determine the correct answer among themselves.
Sections 30880, 30881
(A) : 27 students
(B) : 6 students
(C) : 5 students
(D) : 0 students
Correct answer: (A)
The impulse exerted by the wall on the ball is the force exerted on the ball multiplied by the time elapsed during wall-ball contact, which will result in a vector pointing in the negative direction. Also the impulse can be calculated from the initial-to-final change in momentum m*v_f - m*v_i = (5.0 kg)*((-3.0 m/s) - (+3/0 m/s) = -30 N*s, which also points in the negative direction.
Pre- to post- peer-interaction gains:
pre-interaction correct = 56%
post-interaction correct = 71%
Hake, or normalized gain
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