Friction is a force that opposes motion between
two surfaces that touch each other.
You will find friction everywhere that objects
come into contact with each other.
The force acts in the opposite direction to the
way an object wants to move.
1. The type of surfaces in contact.
2. The amount of force pressing the surfaces
The amount of friction between two surfaces
What Creates More Friction?
• Rough Surfaces
– Rough surfaces have more microscopic hills & valleys.
– The rougher the surface, the greater the friction will be.
• Greater Force
– The amount of force exerted increases the amount of
– Changing the amounts of the surfaces that touch does
NOT change the amount of friction.
Friction is greater when surfaces are pushed
together with more force.
To the naked eye a material may seem smooth, such
as the polished mica pane shown below.
At the microscopic level, even the smoothest surfaces
are covered with dips and bumps.
Picture of extremely smooth mica with a scan probed
If you look at it at mica at the microscopic level, it will
The tips of the bumps are the only points that touch the
other material. Only a very small portion of the apparent
surface area is in contact with the other surface.
This causes extremely high pressure to form on the parts
that touch. The microscope ridges between two surfaces
can get stuck in the grooves of the other, effectively
creating a type of mechanical bond, or microweld, between
Friction is independent of surface area. Changing the
amounts of the surfaces that touch does NOT change
the amount of friction
kinetic (motion) energy
into thermal (heat)
There are two kinds of friction: static friction
and kinetic friction:
Static friction is the force between two stationary objects.
static friction is always greater than kinetic friction.
Static friction prevents an object from moving against a
surface. It is the force that keeps a book from sliding
off a desk, even when the desk is slightly tilted, and that
allows you to pick up an object without the object slipping
through your fingers.
In order to move something, you must first overcome the
force of static friction between the object and the
surface on which it is resting.
9, 10, 11
Once two objects are in motion it is easier to
keep them in motion.
When two surfaces are in motion with respect to
each other there is Kinetic friction.
There are several types
of kinetic friction,
including sliding friction
and rolling friction.
Sliding friction resists the motion of an object as it moves
along a surface.
Sliding friction, is created by flat objects moving
against each other.
Rolling friction hinders the motion of an object
rolling along a surface.
Another force must be present to keep an object
rolling. For example, a pedaling bicyclist provides
the force necessary to the keep a bike in motion.
When one surface is rolling over another surface
there is rolling friction.
When the Great Pyramids were being build in ancient
Egypt, they used logs as rollers under the giant blocks of
granite instead of trying to slide the rocks along the
Friction Can Be Harmful or Helpful
• Harmful friction
– Holes in socks or knees of jeans
– Wind & water cause erosion
• Helpful friction
– Friction between pencil & paper
– Without friction, you would just slip &
fall when you tried to walk
Rotors and brake shoes on a Porsche.
Brake pads in your car help produce friction to slow you down
We have contours in our skin's surface called friction ridges
that help the hand to grasp by increasing friction. Friction
ridges occur in patterns (fingerprint).
The skateboard has a layer of grip tape on its surface for
the sole purpose of creating friction between the
skateboarder's shoe and the skateboard.
This is what keeps the board under the skateboarder's feet
as he jumps into the air. This friction allows skateboarders to
flip the board, jump with the board, and stay on the board at
In much the same way, skateboarders turn in midair by
twisting their arms and legs in opposite directions. Upon
landing, a skater can use the friction between his or her
feet and the skateboard to twist the upper body back
A cracked and worn disc brake pad
In machines with moving parts, friction increases wear,
produces heat and reduces efficiency.
While friction allows us to
convert one form of motion to
another, it also converts some
energy into heat, noise, and
wear and tear on material.
Losing energy to these effects
often reduces the efficiency of
In automobiles and airplanes, friction converts some of the
energy in the fuel into heat, noise, and wear and tear on
the engine’s parts.
Excess frictional heat can damage an engine and braking
Averaged over urban and highway
conditions, the average mechanical
efficiency is around 52% for typical
Much of the energy is lost in overcoming
friction in operating the engine.
The types of friction include energy
necessary for pumping (moving the
intake and exhaust valves), energy to
overcome rubbing friction, and energy
required to operate accessories such as
the air conditioner.
More friction means more heat and less motion.
• Ways to reduce friction
– Lubricants- Substances applied to
surfaces to reduce the friction between
– Switching from sliding friction to
rolling friction by the use of ball
– Make surfaces that rub together
Friction is a force that opposes motion.
Standard friction equation
Friction = coefficient of friction x normal force.
Measures of friction are based on the type of materials that
are in contact.
Each material has what is known as a coefficient of
friction. The coefficient is a measure of how easily one
object moves in relationship to another.
When you have a high coefficient of friction, you have a lot
of friction between the materials.
Concrete on concrete has a very high coefficient of
A material such as Teflon (on most things) has a very low
coefficient. Teflon is used on surfaces where we don't want
things to stick; such as pots and pans.
How is friction measured?
= coefficient of kinetic friction
= coefficient of static friction
FN = the normal force
μ = Symbol for the Greek letter mu, used in
the formula to represent the coefficient.
The coefficient of kinetic friction between a block and
the level surface it slides on is 0.45.
If the mass of the block is 10.0 kg, what is the
minimum force needed to keep the block moving with
The applied force need only balance the kinetic
frictional force in order to maintain uniform motion.
Fk = µ kFN
Fk = µ kFN
Fk = (0.45)(10.0)(9.8
Fk = 44 N
A student pulls a 5.00 kg object and discovers that she
needs to exert 30.0 N of force before the object moves.
What is the coefficient of static friction between the
object and the surface on which it rests?
Since the applied force is
Equal to 30 N just before
Moving, the friction force
Must be equal in size (30 N).
A 7.30 kg box is at rest on a level table. The coefficient
Of static friction between the box and table is 1.03.
How big is the static frictional force?
There is no static frictional
force acting since there is no net
force attempting to move the
The static frictional force could,
at most be equal to μstaticFN