Physics quiz question: crate on floor, suspending a hanging book

Physics 205A Quiz 3, fall semester 2016
Cuesta College, San Luis Obispo, CA

A 2.0 kg book is suspended by an ideal pulley and rope, which is attached on the other end to a 10.0 kg crate that rests on a horizontal frictionless floor. Newton's __________ law tells you that these two forces are equal in magnitude and opposite in direction:

Weight force of Earth on the crate.
Normal force of floor on the crate.

(A) first.
(B) second.
(C) third.
(D) (These forces are not equal in magnitude and/or opposite in direction.)

Correct answer (highlight to unhide): (D)

The crate has three vertical forces acting on it:

Weight force of Earth on crate (downwards, magnitude w = m·g = 98.0 N).
Normal force of floor on crate (upwards, magnitude N).
Tension force of book on crate (upwards, magnitude T).

Because the crate is stationary in the vertical direction, from Newton's first law all of the up and down forces must sum to zero. This means that the two upwards forces (normal and tension) are together equal to the one downwards force (weight), and the upwards normal force of floor on the crate is less than the magnitude of the (opposite direction) weight force of Earth on the crate.

Sections 70854, 70855, 73320
Exam code: quiz03sHO7
(A) : 33 students
(B) : 12 students
(C) : 6 students
(D) : 6 students

Success level: 11%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.42