For practical problem-solving, however, this is a case where we’re
better off forgetting the fundamental classification of interactions
at the atomic level and working with a more practical, everyday
classification of forces. In this practical scheme, we have three types
of forces that can occur between solid objects in contact:
A normal force,Fn, is perpendicular to the surface of con-
tact, and prevents objects from pass-
ing through each other by becoming as
strong as necessary (up to the point
where the objects break). “Normal”
means perpendicular.
Static friction,Fs, is parallel to the surface of contact, and
prevents the surfaces from starting to
slip by becoming as strong as neces-
sary, up to a maximum value ofFs,max.
“Static” means not moving, i.e., not
slipping.
Kinetic friction,Fk, is parallel to the surface of contact, and
tends to slow down any slippage once
it starts. “Kinetic” means moving, i.e.,
slipping.
self-check C
Can a frictionless surface exert a normal force? Can a frictional force
exist without a normal force? .Answer, p. 1055
If you put a coin on this page, which is horizontal, gravity pulls
down on the coin, but the atoms in the paper and the coin repel each
other electrically, and the atoms are compressed until the repulsion
becomes strong enough to stop the downward motion of the coin.
We describe this complicated and invisible atomic process by saying
that the paper makes an upward normal force on the coin, and the
coin makes a downward normal force on the paper. (The two normal
forces are related by Newton’s third law. In fact, Newton’s third law
only relates forces that are of the same type.)
If you now tilt the book a little, static friction keeps the coin
from slipping. The picture at the microscopic level is even more
complicated than the previous description of the normal force. One
model is to think of the tiny bumps and depressions in the coin
as settling into the similar irregularities in the paper. This model
predicts that rougher surfaces should have more friction, which is
sometimes true but not always. Two very smooth, clean glass sur-
faces or very well finished machined metal surfaces may actually
stickbetter than rougher surfaces would, the probable explanation
being that there is some kind of chemical bonding going on, and the
smoother surfaces allow more atoms to be in contact.
Finally, as you tilt the book more and more, there comes a point
where static friction reaches its maximum value. The surfaces be-156 Chapter 3 Conservation of Momentum