Physics clicker question: dropped and horizontally thrown balls

Physics 205A, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Multiple-Choice Question 3.10

Students were asked the following clicker question (Classroom Performance System, einstruction.com) at the end of their learning cycle.

A ball is released from rest, at a certain height above a floor. At the same time, a ball is thrown from the same height above the floor, with an initial horizontal velocity. Which ball will hit the floor with the fastest speed?
(A) The ball released from rest.
(B) The ball with an initial horizontal velocity.
(C) (There is a tie.)
(D) (Not enough information is given.)
(E) (I'm lost, and I don't know how to answer this.)

Sections 70854, 70855
(A) : 2 students
(B) : 25 students
(C) : 15 students
(D) : 2 students
(E) : 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 70854, 70855
(A) : 0 students
(B) : 35 students
(C) : 9 students
(D) : 0 students
(E) : 8 students

Correct answer: (B)

Both balls will reach the ground at the same time, as discussed in a previous clicker question. During the whole-class discussion, students offered several explanations as to why the initial horizontal velocity ball would have a faster speed as it hits the ground: (i) its trajectory would carry it over a longer distance traveled, and thus must have a longer (average) speed over the time interval; (ii) its final velocity vector would have its original horizontal component in addition to its downwards vertical component.

Pre- to post- peer-interaction gains:
pre-interaction correct = 57%
post-interaction correct = 80%
Hake, or normalized gain <g> = 53%