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Friction and Newton Laws

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					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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