# Friction and Newton Laws by nikeborome

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• pg 1
```									Friction and Newton’s
Laws
Review

• What is a force?
– Any push or pull
• What is the symbol for force?
–F
• What is the unit for force?
– Newton (N)
• What are the two types of forces?
– Long range and contact
Review

• What will an object do if the net force is zero?
– Move with constant motion
• What is required to make an object change its
motion?
– A force
Friction

• Now we can discuss the nature of friction and how it
effects the motion of objects.

• Sometimes, in physics, we discuss situations with
NO friction; but these situations rarely actually
exist.

• We ignore friction in many cases, simply to make
calculations easier.
Friction

• So what is friction?
– Friction is a contact force that opposes the relative
motion of two surfaces in contact.

• Basically, friction is the force caused by rubbing.

• Friction is ALWAYS opposite the direction of MOTION
Friction

• Let’s discuss some 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
Friction

– 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….
Friction

– Now, let’s think about a car. How does friction help a car
move?
• See the picture below

• As the wheels turn, they push backward on the ground, and the
force of friction keeps the tire from sliding, so the car rolls
forward.
Friction

– 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
surfaces?
• Something called the coefficient of friction ( μ  this is the
Greek letter mu)

• The coefficient of friction is different for every surface…
Friction

• Let’s see some more examples….
– If we look at two surfaces on a microscopic level, we see
they are NOT actually smooth….

– So when the rub against each other, they catch on all of
these uneven spots and resist the movement.
• Different surfaces have different degrees of roughness and
therefore different amounts of friction.
Friction

• There are actually two different kinds of friction
– Static friction
• This friction exists when two surfaces are in contact, but not
sliding against each other
– For 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 my table top, the
force of kinetic friction acts opposite to the direction of motion.
Friction

• Things you should know about these types of
friction…
– 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.
Friction

• Things you should know about these types of
friction…
– Once an object starts sliding (or moving) kinetic friction
takes over and is constant in magnitude.
• Kinetic friction has only one value and is dependent upon the
coefficient of kinetic friction (μk).
Friction

• Things you should know about these types of
friction…
– 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…
– you have to overcome static friction to get it to start sliding, but once it
starts moving, you don’t need as much force to overcome kinetic
friction.
Friction

• So what factors affect the magnitude of the force of
friction?
– Obviously, the coefficient of friction between the two
surfaces matters
• This means that the types of surfaces makes a difference.
– If you think about it; how hard the two surfaces are
pushed together makes a difference also.
• What is the name of the force that pushes two surfaces
together?
– The normal force
Friction

• So we get two factors that make a difference
– The type of surfaces, and how hard they are pushed
together

• This gives us an equation for the force of friction…
Ff = μ∙FN
The force of friction is equal to the coefficient of friction
times the normal force.
El fin de Friction

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