Physics quiz question: Hafþór Björnsson world record one-handed vertical toss

Physics 205A Quiz 2, fall semester 2017
Cuesta College, San Luis Obispo, CA

"2017 Rogue Record Breaker - Hafthor Bjornsson's 19' 7" Weight For Height"
RogueFitness
youtu.be/k8cWd933WoM

In 2017, Game of Thrones actor Hafþór Björnsson tossed one-handed a 56 lb weight upwards to a world record maximum height. Video analysis shows that from the moment it left his hand (with an initial upwards speed), the weight took 0.92 s to reach its highest height[*]. Neglect air resistance. Choose up to be the +y direction. At the moment it left his hand, the weight had an initial speed of:
(A) 4.1 m/s.
(B) 9.0 m/s.
(C) 11 m/s.
(D) 18 m/s.

[*] instagram.com/p/BRUStRIgPfX/.

Correct answer (highlight to unhide): (B)

The following quantities are given (or assumed to be known):

(t₀ = 0 s),
(y₀ = 0 m),
v_y = 0 m/s (momentarily stationary at its highest point),
t = 0.92 s (time when at its highest point),
a_y = –9.80 m/s².

So in the equations for constant acceleration motion in the vertical direction, the following quantities are unknown, or are to be explicitly solved for:

v_y = v_0y + a_y·t,

y = (1/2)·(v_y + v_0y)·t,

y = v_0y·t + (1/2)·a_y·(t)²,

v_y² = v_0y² + 2·a_y·y.

With the unknown quantity v_0y to be solved for appearing as the only unknown in just the first equation above, with all other quantities given (or assumed to be known), then:

v_y = v_0y + a_y·t,

0 m/s = v_0y + (–9.80 m/s²)·(0.92 s),

v_0y = (9.80 m/s²)·(0.92 s) = +9.016 m/s,

or to two significant figures, the initial speed of the weight was 9.0 m/s (in the upwards direction).

(Response (A) is (1/2)·a_y/t²; response (C) is a_y/t; response (D) is 2a_y·t.)

Sections 70854, 70855
Exam code: quiz02BjRn
(A) : 5 students
(B) : 39 students
(C) : 11 students
(D) : 0 students

Success level: 71%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.42