Physics quiz question: trajectory highest point speed

Physics 205A Quiz 3, fall semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 3.49(b)

A ball is thrown with an initial velocity vector that has x- and y-components:

v_0x = +4.0 m/s,
v_0y = +7.0 m/s.

At the highest point in its trajectory, the speed of the ball is:
(A) 4.0 m/s.
(B) 5.7 m/s.
(C) 7.0 m/s.
(D) 8.1 m/s.

Correct answer (highlight to unhide): (A)

The horizontal component of the ball's velocity never changes, while at the highest point in its trajectory, the vertical component of the ball's velocity is zero. Thus the speed of the ball at the highest point in its trajectory is equal to the magnitude of the initial horizontal velocity vector.

(Response (C) is the magnitude of the ball's initial vertical velocity. Response (D) is the magnitude of the ball's initial speed, sqrt(v_0x² + v_0y²); and response (B) is a garbled calculation of the initial speed, sqrt(–v_0x² + v_0y²).)

Sections 70854, 70855
Exam code: quiz03Fn37
(A) : 35 students
(B) : 6 students
(C) : 5 students
(D) : 9 students

Success level: 64%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.81