It has been noted that the static friction force poses challenges for students and, at times, even their instructors. Unlike the gravitational force, which has a precise and unambiguous magnitude (FG = mg), the magnitude and direction of the static friction force depend on other forces at play. Friction can be understood rather well in terms of complicated atomic-scale interactions between surfaces. Ringlein and Robbins survey aspects of the atomic origins of friction, and Folkerts explores factors that affect the value of static friction. However, what students typically encounter in an introductory course ignores the atomic origins of friction (beyond perhaps a brief overview of the atomic model). The rules of dry friction (i.e., non-lubricated surfaces in contact) taught in introductory physics were originally published in 1699 by Guillaume Amontons. Amontons’s first law states that the force of friction is directly proportional to the applied load, i.e., f = μFN, where FN is the normal force and μ is the coefficient of friction. His second law states that the force of friction is independent of the macroscopic area of contact. These laws were verified by Coulomb in 1781.

1.
James
Ringlein
and
Mark O.
Robbins
, “
Understanding and illustrating the atomic origins of friction
,”
Am. J. Phys.
72
,
884
890
(
July
2004
).
2.
William M.
Wehrbein
, “
Frictional forces on an inclined plane
,”
Am. J. Phys
.
60
,
57
58
(
Jan
.
1992
).
3.
P.
Onorato
,
D.
Mascoli
, and
A.
DeAmbrosis
, “
Damped oscillations and equilibrium in a mass-spring system subject to sliding forces: Integrating experimental and theoretical analyses
,”
Am. J. Phys
.
78
,
1120
1126
(
Nov.
2010
).
4.
Daniel
Ludwigsen
and
Kathryn
Svinarich
, “
Choose wisely: Static or kinetic friction – The power of dimensionless plots
,”
Phys. Teach
.
47
,
158
161
(
March
2009
).
5.
Randall D.
Knight
,
Physics for Scientists and Engineers: A Strategic Approach
, 3rd ed. (
Pearson, Upper Saddle River
,
NJ
,
2013
), p.
148
.
6.
See Ref. 1.
7.
Timothy J.
Folkerts
, “
Exploring factors affecting measurements of the static friction coefficient of friction: An application of fractional factorial experiment design
,”
Am. J. Phys
.
72
,
1335
1341
(
Oct.
2004
).
8.
Guillaume
Amontons
, “Moyen de substituer commodement l’action du feu, a la force des hommes et des cheveaux pour mouvoir les machines” (“Method of substituting the force of fire for horse and man power to move machines”), Mémoires de l’Académie Royale des Sciences, in: Histoire de l’Académie Royale des Sciences (
1699
), pp.
112
126
.
9.
Which is not necessarily true even on the macroscopic level: see, for example,
David
Van Domelen
, “
Showing area matters: A work of friction
,”
Phys. Teach.
48
,
28
29
(
Jan.
2010
).
10.
E. P.
Bowden
and
D.
Tabor
,
The Friction and Lubrication of Solids
, 1st ed. (
Oxford University Press
,
Oxford
,
1954
), pp.
87
89
.
11.
Jonathan F.
Reichert
, “
How did friction get so ‘smart,’
Phys. Teach.
39
,
29
31
(
Jan.
2001
).
12.
T. P.
Doerr
, “
Note on ‘smart’ forces
,”
Phys. Teach
.
39
,
486
487
(
Nov.
2001
).
13.
Ronald
Newburgh
, “
Other ‘smart’ forces
,” letter to the editor,
Phys. Teach.
39
,
133
(
March
2001
).
14.
We initially used both thread and 28-gauge wire, but, as expected, there was no difference in tension so only string was used in later trials.
15.
See Ref. 7.
16.
Ibid.
AAPT members receive access to The Physics Teacher and the American Journal of Physics as a member benefit. To learn more about this member benefit and becoming an AAPT member, visit the Joining AAPT page.