Physics quiz question: energy changes of block sliding up wall

Physics 205A Quiz 5, fall semester 2016
Cuesta College, San Luis Obispo, CA

A sanding block is pressed against the wall, and slides up at constant speed as a diagonal force is applied to it. The wall is not frictionless. The __________ of the block remains constant as it slides up the wall.
(A) gravitational potential energy.
(B) translational kinetic energy.
(C) (Both of the above choices.)
(D) (Neither of the above choices.)

Correct answer (highlight to unhide): (B)

The energy transfer-balance equation is given by:

W_nc = ∆KE_tr + ∆PE_grav + ∆PE_elas,

where ∆PE_elas = 0, as there is no spring involved in this process.

Just looking at the two remaining terms on the right-hand side of the energy transfer-balance equation, for the change in translational kinetic energy:

∆KE_tr = (1/2)·m·(v_f² – v₀²),

and since the speed is constant, v₀ and v_f have the same magnitude, then KE_tr is constant (∆KE_tr = 0).

Also for the change in gravitational potential energy:

∆PE_grav = m·g·(y_f – y₀),

and since y_f is greater than y₀, then PE_grav increases (∆PE_grav is positive).

Sections 70854, 70855, 73320
Exam code: quiz04th1R
(A) : 6 students
(B) : 37 students
(C) : 5 students
(D) : 3 students

Success level: 73%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.64