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• Frictionless situations rarely exist.
• We ignore friction to make calculations easier.
• So what is friction?
– Friction is a contact force that opposes the relative
motion of two surfaces in contact.
• Friction is ALWAYS opposite the direction of MOTION
• Examples:
– When you are standing still, what is the net force acting
on you?
• Zero (Newton's 1st law)
– How do you start moving when you begin walking?
(think forces)
• You push your foot backward with the muscles in your leg
– So how do you start moving forward?
• Your foot is in contact with the floor, so there are two surfaces
in contact. The force of friction opposes the motion of the two
surfaces relative to each other (friction doesn’t want them to
slide past each other). Since your foot is trying to go backward,
friction acts forward.
– So if friction is forward, and it opposes motion, why do I
go forward?
• Friction opposes the motion of your foot against the floor, not
the rest of your body…look at the example of me….
– How does friction help a car move?
• As the wheels turn, they push backward on the ground, and the
force of friction keeps the tire from sliding, so the car rolls
– So why is it harder to walk on ice than on grass?
• Ice has less friction than grass does
– What determines how much friction there is between two
• Something called the coefficient of friction ( μ  this is the
Greek letter mu)
• The coefficient of friction is different for every surface…
– Surfaces on a microscopic level are not actually smooth.
– When the rub against each other, they catch on all of
these uneven spots & resist the movement.
• Different surfaces have different degrees of roughness and
therefore different amounts of friction.
• 2 types of friction
– Static friction
• This friction exists when two surfaces are in contact, but not
sliding against each other
– Example: when I hold a box against the wall, it does not slide because
gravity is being canceled out by static friction
– Kinetic friction
• This friction exists when two surfaces ARE sliding against each
other (it is also called sliding friction)
– For example: When I pull this block of wood across a table top, the
force of kinetic friction acts opposite to the direction of motion.
– Static friction can vary…up to a maximum value
dependent upon the coefficient of static friction (μs).
• This means that it will always be equal in magnitude to the
force trying to move an object until that force reaches or
exceeds the maximum of static friction.
– Once an object starts sliding (or moving) kinetic friction
takes over & is constant in magnitude.
• Kinetic friction has only one value & is dependent upon the
coefficient of kinetic friction (μk).
– The coefficient of static friction is always larger than the
coefficient of kinetic friction between two surfaces.
Because of this, the maximum force of static friction will
ALWAYS be larger than the force of kinetic friction
• This is why when you pull on an object that is stationary, it
“jerks” to a start…
– Static friction must be overcome to get an object to start sliding, but
once it starts moving, it doesn’t need as much force to overcome kinetic
• So what factors affect the magnitude of the force of
– Type of surface (coefficient)
– How hard they are pressed together (FN)
• Equation:
Ff = μ∙FN
The force of friction is equal to the coefficient of friction
times the normal force.
Ff = μ∙FN
Rubber on concrete
Rubber on wet concrete
Wood on wood
Steel on steel (dry)
Steel on steel (with oil)
Teflon on steel
Ff = μ∙FN
1. A 25N wooden box is pushed across a wooden floor with a
constant velocity. Calculate the Friction Force.
2. A 25 kg Teflon object is sliding across a steel surface with a
constant velocity. Calculate the Ff.
3. A boy pulls a 52N sled across a cement sidewalk at a force
of 36N at a constant velocity. What is the coefficient of
kinetic friction?
4. Now the sled is being pulled over snow. The coefficient of
friction is 0.12. A person weighting 650N sits on the sled,
what force is needed to pull the sled across the snow at a
constant velocity?