Friction
Reading: IS FRICTION A DRAG?
Friction is a common, everyday occurrence. We
deal with it in almost every aspect of our lives.
We may even take it for granted. Without
friction, erasure of our mistakes would be
difficult. Stopping our car or bicycle at a stop
sign would be almost impossible, as would
walking, keeping food on our fork, lighting a
match, or . . . well, let's not get carried away
with this. Friction is obviously important to us.
Friction also has some nasty qualities. It eats
our tires up and wears out our brakes. It is
responsible for those ear-piercing sounds
fingernails make when dragged across the
chalkboard. Friction makes doors squeak and
makes us work harder. Friction obviously has
its less desirable consequences.
For our purposes, friction is a force which
opposes the motion or attempted motion of two
solid surfaces across one another. Notice that
we are distinguishing it from drag, which is the
opposing force created as something travels
through a liquid or a gas.
What causes friction? Centuries of study have
yielded no completely reliable theory. The
common-sense notion that the surface roughness
or texture controls friction is countered by
experiments showing that extremely smooth
surfaces can yield large frictional forces. Until
recently scientists theorized that such strange
behavior came from weak chemical bonds
between surfaces, but that theory is not
supported by atomic-level friction experiments
made in the 1990's. For now, we must be
content with describing the factors that control
frictional force, without understanding the
underlying mechanisms.
Three primary factors affect how much
frictional force is available between two solid
surfaces: the force pressing the surfaces
together, the motion of the surfaces, and the
types of surfaces in contact. We will examine
each of these in turn.
When an object pushes against a surface, the
third law predicts the surface will apply an
opposing force. Since this reaction force is
always perpendicular or "normal" to the surface
itself, it is called the normal force.
Your lab work has shown that the normal force
is directly proportional to the frictional force.
Thus for the first time we see that a vertical
force could indirectly affect horizontal motion.
(Heavier objects press harder against floors,
causing increased horizontal friction.)
The amount of friction is also affected by
motion. In the lab you saw that the force
required to start a block sliding is greater than
that needed to maintain the slide. This is not an
inertial effect, but a real difference in the
frictional force. The amount of static friction
that is available is always greater than the
available kinetic friction. Static friction builds
up from zero to a maximum value until the
object begins to slide. During the slide,
however, kinetic friction remains constant even
if the speed varies.
Now suppose a ball of wood and a block of
wood of equal weight were briefly shoved so
they started moving across a table. Which
would you expect to stop first? If you thought
the block would stop first, you were right. The
ball would continue to roll after the block had
come to rest. This indicates that rolling friction
is smaller than either kinetic or static friction.
You might guess that this is because the ball has
less surface area contacting the table at any
point in time. However, scientists have found
that friction is not affected by the apparent area
of contact if variables such as weight and
material are controlled. The exact reason why
rolling friction is less than sliding friction is
unknown.
Finally, surface types affect friction. You know
that smoother surfaces often (but not always!)
yield less friction. You also know that
lubricants can greatly reduce friction. A layer
of oil between two surfaces reduces friction
because the oil forms a thin film and, even
though the layer is only a few molecules thick,
prevents the two surfaces from coming in
contact with one another. The friction between
sliding solid surfaces is thus replaced by the
smaller force of liquid drag.
Graphite, a carbon substance, is another
common lubricant. Graphite, however, is solid.
The molecular nature of graphite is such that
the graphite crystals move quite easily over each
other. When graphite is placed between two
surfaces, the adhesion between the graphite and
the surfaces is greater than that between the
graphite crystals themselves. When the surfaces
slide, it is the graphite crystals which are
actually skidding.
Another way to reduce friction is to use bearings. Ball bearings are present in the wheel
axles of many skateboards, bicycles, and automobiles. Since rolling friction is less than sliding friction, bearings alleviate much of the wear
that would occur in the wheel and the axle. Oil,
graphite, and bearings work well in reducing
friction, but friction is still present in systems
that employ them. An almost friction-free system is one that employs gases to separate two
surfaces. Examples of such systems include the
air tracks used in physics labs and air hockey
tables. Another way to reduce friction is to levitate surfaces magnetically.
We have seen that friction can be both helpful
and destructive. It is a complex phenomenon
which is still not fully explained. However, we
do know some fundamental rules about its
behavior which we can apply to a wide variety
of situations.
Questions
1. List the three primary factors that affect friction.
2. List two factors friction does NOT depend on.
3. Apply your knowledge of friction to list four ways to
increase the friction of a vehicle in icy weather.
4. Why do cars traveling on ice skid more easily on hills
than on level pavement?
8 Friction
Name
Worksheet
Inquiry Physics
answer all questions with several complete sentences
1.
When your textbook is at rest on the level desktop and no one is touching it, how much frictional force is
acting? Justify your answer.
2.
Now suppose the table is tilted but the book remains at rest. How does the friction change from what it was in
answer 1? Explain your answer.
3.
The rear of some sports cars feature curved surfaces called spoilers. Spoilers are designed so that air flowing
past them exerts a downward force. Use your understanding of friction to explain why this improves the
handling of the sports car.
4.
Concrete road dividers and barriers are replacing steel rails on our
nation's highways and bridges. The figure at right shows how a sideswiping car would interact with each type of barrier. Use your
knowledge of friction to speculate as to why concrete barriers are
preferred over steel barriers.
A Related Topic: Air Drag
We found out earlier in the course that all objects fall downward at the same acceleration (9.8 m/s2) in the absence
of air resistance. Drag, friction from the air, can have a significant effect on falling bodies. As a body falls with
increasing speed, drag builds up. This will reduce the net force on the falling body, making it accelerate at less than
9.8 m/s2.
It is possible for a body to fall fast enough that the upward push of drag balances the body's weight. If this happens,
the net force on the falling body will be zero, and it will no longer accelerate. The body has reached its terminal
velocity. Feathers are everyday objects that reach a terminal velocity very quickly when dropped. Coins, however,
are objects which do not show much effect from air resistance. A coin would have to drop for a few minutes before
its speed would be great enough for the air resistance to increase to its weight. Its terminal velocity might be as high
as 200 km/h downward.
The terminal velocity for a human skydiver varies from about 150 to 200 km/h downward (93 to 124 mph),
depending on weight and body position. A heavier person will have a higher terminal speed than a lighter one
because the larger weight is better at "plowing through" the air. A heavy and light skydiver can remain in close
proximity if the heavy person spreads out his or her limbs (like a flying squirrel) while the light person falls head or
feet first. A parachute's greater surface area greatly increases air resistance and lowers the terminal speed down to
15 to 25 km/h (9 to 16 mph). With a parachute, the "terminal" speed isn't deadly!
5.
Compare the size of weight and air drag before a falling body reaches terminal velocity.
6.
Compare the size of weight and air drag after a falling body reaches terminal velocity.
7.
After she jumps, a certain skydiver
reaches terminal speed after ten seconds.
The d vs. t graph at right shows her
displacement with and without air drag.
On the blank axes, sketch her velocity
vs. time with and without drag. Clearly
label your two lines.
8.
Does the skydiver gain more speed during the first second of fall or the ninth second of fall?
9.
Does the skydiver fall a greater distance in the first second of fall or the ninth second?
10. In the absence of air resistance, a ball that is thrown upward will return at the speed at which it was thrown. If
air resistance is a factor, will the speed with which it returns be greater, less than, or equal to the initial upward
speed? Explain your answer.