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"

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),
vy = 0 m/s (momentarily stationary at its highest point),
t = 0.92 s (time when at its highest point),
ay = –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:

vy = v0y + ay·t,

y = (1/2)·(vy + v0yt,

y = v0y·t + (1/2)·ay·(t)2,

vy2 = v0y2 + 2·ay·y.

With the unknown quantity v0y 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:

vy = v0y + ay·t,

0 m/s = v0y + (–9.80 m/s2)·(0.92 s),

v0y = (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)·ay/t2; response (C) is ay/t; response (D) is 2ay·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

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