Physics flashcard question: three thrown balls

Physics 205A, fall semester 2011
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.


Patrick M. Len said...

Mea culpa to the fall semester 2011 students, where the wrong explanation was given in class.

Patrick M. Len said...

...and a case where the simplest explanation is the best explanation.