Cuesta College, San Luis Obispo, CA
Students were asked near the end of their learning cycle the following think-pair-share question, to be answered using flashcards.
The ball thrown __________ would have more Ktr just before hitting the floor. (A) upwards.
(B) downwards.
(C) to the right.
(D) (There is two-way tie.)
(E) (There is three-way tie.)
(F) (Unsure/guessing/lost/help!)
Sections 70854, 70855
(A) : 2 students (estimated)
(B) : 2 students (estimated)
(C) : 5 students (estimated)
(D) : 5 students (estimated)
(E) : 20 students (estimated)
(F) : 20 students (estimated)
Correct answer: (E)
From the energy balance equation:
Wnc = ∆Ktr + ∆Ugrav + ∆Uelas,
where Wnc = 0 (no external gains/losses of mechanical energy), and ∆Uelas = 0 (no springs involved), such that for all three balls,
0 = ∆Ktr + ∆Ugrav.
Each ball experiences the same initial-to-final loss of Ugrav, so they must experience the same increase in Ktr, and since they all have the same initial Ktr, then they will have the same final Ktr.
2 comments:
Mea culpa to the fall semester 2011 students, where the wrong explanation was given in class.
...and a case where the simplest explanation is the best explanation.
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