Physics clicker questions: contact force sequence

Physics 5A, Fall Semester 2007
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 4.51

Students were asked the following clicker questions (Classroom Performance System, einstruction.com) near the beginning of their learning cycle:

Mass of book = 0.50 kg.
Coefficient of static friction mu_s = 0.45.
Coefficient of kinetic friction mu_k = 0.40.


[0.6 participation points.] You push down on the book with a force of 1.0 N. The normal force of the table on the book has a magnitude of:
(A) 4.9 N.
(B) 1.0 N.
(C) 4.9 N – 1.0 N.
(D) 4.9 N + 1.0 N.
(E) (I'm lost, and don't know how to answer this.)

Sections 0906, 0907
(A) : 2 students
(B) : 22 students
(C) : 2 students
(D) : 9 students
(E) : 0 students

This question was asked again after displaying the tallied results with the lack of consensus, with the following results. No comments were made by the instructor, in order to see if students were going to be able to discuss and determine the correct answer among themselves.

Sections 0906, 0907
(A) : 1 student
(B) : 18 students
(C) : 2 students
(D) : 15 students
(E) : 0 students

Correct answer: (D)

The book has the weight force of the Earth pulling down on it (4.9 N), the applied force (1.0 N) pushing down on it, and since Newton's first law applies, the normal force of the table pushing up on the book is 5.9 N in magnitude.

---

[0.6 participation points.] You push to the right on the book with a force of 1.0 N. The static friction force of the table on the book has a magnitude of:
(A) 4.9 N.
(B) 1.0 N.
(C) 2.0 N.
(D) 2.2 N.
(E) (I'm lost, and don't know how to answer this.)

Sections 0906, 0907
(A) : 10 students
(B) : 8 students
(C) : 7 students
(D) : 12 students
(E) : 0 students

Correct answer: (B)

The maximum static friction force is mu_s*N = 2.2 N. Since the applied force is less than this, the book remains stuck, Newton's first law applies, and the static friction force is equal in magnitude and opposite in direction to the applied force.

---

[0.6 participation points.] You pull to the right on the book with a force of 6.0 N. The kinetic friction force of the table on the book has a magnitude of:
(A) 4.9 N.
(B) 6.0 N.
(C) 2.0 N.
(D) 2.2 N.
(E) (I'm lost, and don't know how to answer this.)

Sections 0906, 0907
(A) : 1 student
(B) : 4 students
(C) : 28 students
(D) : 4 students
(E) : 0 students

Correct answer: (C)

Since the applied force is more than the maximum static friction force magnitude mu_s*N = 2.2 N, the book is effectively unstuck, and begins to slide, thus kinetic friction begins to act on the book, which always has the magnitude mu_k*N = 2.0 N.