Physics quiz question: energy changes of pulley-raised box

Physics 205A Quiz 5, fall semester 2018
Cuesta College, San Luis Obispo, CA

A Physics 205A student uses an ideal rope and pulley to lift a 5.0 kg box at a constant speed. The box moves up a vertical distance of 1.0 m. Ignore friction and drag. For this process, the __________ of the box increases.
(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): (A)

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).

(In order for the equality to hold for the energy transfer-balance equation, the student must then be doing positive work on the box in order to increase its gravitational potential energy.)

Sections 70854, 70855
Exam code: quiz04W3rK
(A) : 39 students
(B) : 3 students
(C) : 5 students
(D) : 4 students

Success level: 76%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.48