Regents Physics Information Sheet
Name
Friction
For an all encompassing definition of friction we can use: Friction is the force that results from two
surfaces move across each other opposing the motion of the two surfaces. Friction can appear in many forms,
but one thing is certain, Friction will always oppose the current motion of an object, or the potential motion an
object could have.
Friction is an interesting force. Without it we could not walk or write. And yet, we are always “neglecting”
it in certain types of problems. We want to eliminate it in automobile engines, but we need a lot of it when we
want to move the car along the road, but we then try to streamline the car so there is less of it once we are
moving at any substantial speed. Sometimes we even use something that has lower amounts of friction to
change from one form of friction to a different lesser one.
Regardless of what kind of friction you may encounter, one thing is true for all forms of friction. The
amount of the force of friction depends on how much force is holding surfaces together, or how much force is
involved in the interaction of the surfaces and the nature or “stickiness” of the surfaces.
We classify friction into three forms: Sliding, Rolling, and Fluid.
Sliding Friction is the type of friction that occurs when to solid surfaces move across each other. The amount
of friction depends on how much force is holding the surfaces together, and how “sticky” the surfaces are.
Sometimes really smooth surfaces can actually generate more friction than something that is only moderately
smooth. This type of friction comes in two forms: Static and Kinetic.
Static friction is the friction that exists when the two objects are resting on each other, or they are not
moving relative to each other. It is the kind of friction that holds a box in place on a tilted surface, or holds a
crate in place in the back of a pick-up when the driver gently accelerates the truck. It is also the friction that
keeps a refrigerator magnet on the side of a refrigerator! More on that later in the year.
Kinetic friction is the friction that exists when two surfaces are actually moving across each other. This
could simply be one object moving while the other remains stationary, or it could be both objects moving in
opposite directions.
Kinetic friction is always less than static friction. When something is at rest, the surfaces are more easily
able to make the very weak electromagnetic interactions that create the friction. Once the object is moving,
these physical interactions are harder to make so the friction once something is moving will be less.
The amount of sliding friction something will experience depends only on how much force is holding the
two surfaces together and the nature of the surfaces (how sticky, smooth, or rough) that are interacting. This
can be summed up in the equation:
, where Ff is the force of friction,  is the coefficient of friction,
and FN is the force that is pressing the two surfaces together. The coefficient of friction is the value that
represents the “stickiness” or nature of the surfaces.
Rolling Friction is the type of friction that results from a round surface rolling across another solid surface.
Interestingly, at the point of contact between the round object and the surface, the round object’s surface is at
rest relative to the other surface. Rolling friction is actually a form of “static” friction in that sense. Even
though the entire round object is moving, that particular point on the object is not moving relative to what the
round object is rolling over.
The amount of rolling friction depends on how much force is holding the round object against the other
surface, the nature of the two surfaces (another coefficient of friction), and the total area in contact between the
round object and the surface. That is why the rear tires on a dragster have a very large diameter and are very
wide, while the tires on a racing bicycle are very narrow.
Fluid Friction is the type of friction that results from a solid material moving through a fluid (either liquid or
gas). There are a lot of factors that affect how much fluid friction an object will experience. Fluid friction is
basically a form of “kinetic” friction because without the motion of either the fluid moving (i.e. a wind blowing
or water flowing) or the object moving through the fluid (a skydiver falling through the air, or a boat moving
through the water) there is no fluid friction.
The amount of fluid friction depends on the surface area exposed to the motion, the nature of the fluid the
object is moving through, and the speed of the motion. For gasses, the speed is squared to determine the
amount of fluid friction the object experiences. That’s why for very slow speed through the air, you do not feel
that much friction, but as your speed increases the amount of force you experience increases VERY
dramatically (at a squared rate, so doubling your speed quadruples the friction!!!)
The main affect that friction has on the motion of an object is always to oppose the motion of the object.
When an object is moving and friction is the only force acting, the object will always slow down since the
frictional force creates an acceleration that is opposite to the motion of the object. In order to keep an object
moving at a constant speed in the presence of friction, you need to apply a force in the direction of the motion
that is equal to the amount of friction the object experiences. If you want increase the speed of an object, you
must exert a force greater than the frictional force. Thus you are unbalancing the forces.
Since forces represent the interaction that is exchanging forms of energy, friction can always be thought of
as the force that that is taking energy away from the motion of the object. Friction is converting the mechanical
kinetic energy of the object (the energy of its motion through space) into a different, less useful form of kinetic
energy (Heat). That heat cannot be recovered to be made back into the mechanical kinetic energy of the motion
of the object. Thus we consider friction to cause a loss of energy whenever it is present. The energy really is
not “lost” it is simply converted to a form of energy no longer useful to maintaining the motion of the object.
Lubricants are materials that change sliding friction into fluid friction. They are materials that coat a
surface and act as a barrier that keeps the two surfaces from directly interacting. While there is still friction
present, the fluid friction of that the lubricant creates can be significantly less than the sliding friction that
would normally result. In some instances, when the fluid film is thin enough and under high enough pressure, it
loses all frictional forces within itself (becoming what is known as a superfluid) and creates conditions that are
very nearly frictionless. This is a very interesting area of materials science that a very large amount of research
has gone into recently.