Physics quiz question: hanging book, suspended by crate on floor

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 book.
Tension force of crate on the book.

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

Correct answer (highlight to unhide): (A)

The book has two vertical forces acting on it:

Weight force of Earth on book (downwards, magnitude w = m·g = 19.6 N).
Tension force of crate on book (upwards, magnitude T = ?).

Because the book 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 upwards tension force must be equal in magnitude to the downwards weight force.

Sections 70854, 70855, 73320
Exam code: quiz03sHO7
(A) : 26 students
(B) : 11 students
(C) : 12 students
(D) : 8 students

Success level: 46%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.71