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 K_tr 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:

W_nc = ∆K_tr + ∆U_grav + ∆U_elas,

where W_nc = 0 (no external gains/losses of mechanical energy), and ∆U_elas = 0 (no springs involved), such that for all three balls,

0 = ∆K_tr + ∆U_grav.

Each ball experiences the same initial-to-final loss of U_grav, so they must experience the same increase in K_tr, and since they all have the same initial K_tr, then they will have the same final K_tr.