Physics quiz question: dropped golf ball

Physics 5A Quiz 2, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 2.45(b)

[3.0 points.] A golf ball is dropped from a building 180 m tall. Take upwards to be the positive vertical direction. Neglect air resistance. How far would the golf ball fall in twice the time it takes to fall 6.00 m downwards, if released from rest?
(A) –144 m.
(B) –36.0 m.
(C) –24.0 m.
(D) –12.0 m.

Correct answer: (C)

The displacement delta(y) is given by:

delta(y) = y_0 + v_y0*delta(t) + (1/2)*a_y*delta(t)^2.

Both y_0 and v_y0 are set to 0, and a_y = -9.80 m/s^2. With delta(t) twice as much as before, this term gets squared, resulting in a new delta(y) that is four times more than before.

Student responses
Sections 4987, 4988
(A) : 4 students
(B) : 10 students
(C) : 25 students
(D) : 7 students