Physics 001 Project 2 Friction

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Physics 001Project 2 Friction
Yi Zheng
Evan Gostowski
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Friction Definition

• Friction is the force that opposes the motion of
  an object.

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• Ff µFn
• This is the equation for the model of Coulomb
  Friction, which was named after Charles-Augustin
  de Coulomb.
• The equation relates the frictional force to the
  normal force and allows a numerical value to be
  assigned to the frictional force.

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• µ
• This is the coefficient of friction.
• It is a constant, but the number varies for
  different materials.
• For instance, the coefficient for rubber on
  concrete is different than the coefficient for
  paper on glass.
• Also, the coefficient of static friction is
  different than the coefficient of kinetic
  friction.

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• Static Friction
• Static friction is the frictional force acting on
  a stationary object.
• The coefficient of static friction is greater
  than the coefficient of kinetic friction with
  similar surfaces.
• When using the equation of form Ff µFn with
  static friction, the equals sign is misleading.
  The frictional force can actually be less than or
  equal to the frictional constant times the normal
  force. The number computed for Ff by the equation
  is the maximum possible frictional force. If a
  force of greater magnitude than the computed Ff
  acts on the object in the opposite direction,
  then static friction will cease because the
  object will begin to move.

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• Kinetic Friction
• Kinetic friction is the frictional force being
  exerted on a moving object.
• The coefficient of kinetic friction is less than
  the coefficient of static friction meaning that
  the frictional force on a moving object can not
  be as large as the frictional force on the same
  object while it is stationary and in otherwise
  similar conditions.
• Unlike the static frictional force, the kinetic
  frictional force is always the same when the
  touching surfaces are the same. The speed of the
  object is also irrelevant when looking at the
  kinetic frictional force. As the opposing force
  changes, the net force and velocity of the object
  also change. However, the kinetic frictional
  force remains unchanged.

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Example Static

• A car is driving on a flat road at 20 meters per
  second and the driver sees a deer and needs to
  come to a stop as soon as possible.
• If it does not skid, what is the shortest amount
  of time that the car could take to come to a
  stop?
• The car and passengers combined have a mass of
  2,000 kilograms, and therefore weigh about 20,000
  newtons if the gravitational constant is
  estimated as ten meters per seconds2.
• The coefficient of static friction between rubber
  and concrete is estimated at 1.
• If the brakes are applied in the most efficient
  manner using static friction the car will stop in
  about 2 seconds.

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Example Kinetic
Now we can see how long it would take the car to
come to a stop using kinetic friction. This time
all other aspects of the problem are the same
except the car will skid to a stop. The
coefficient of kinetic friction between rubber
and concrete is estimated at 0.8. If the
car skids to a stop, using kinetic friction, it
will stop in about 2.5 seconds.
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New Insight

• From the examples we have gained insight on how
  these frictional concepts apply to the world
  around us.
• We learned that static friction stops a car, or
  any other object, more quickly than kinetic
  friction does.
• This shows us that hitting the breaks harder will
  not always stop a car more quickly. When
  skidding, a car actually takes longer to stop.
• It also shows us that when pushing a couch, it is
  harder to get it started than it is to keep it
  moving.

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Conclusion

• This information helps us to understand friction
  on a deeper level.
• We now see the difference between the frictional
  force on a moving object and the frictional force
  on a stationary object.

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Citation Page

• "Applications of Newton's Laws Friction." 2 Dec.
  2008 lthttp//gardner.byu.edu/105w1/index_files/cla
  ss207.pdfgt.
• "Friction." Wikipedia. 2 Dec. 2008
  lthttp//en.wikipedia.org/wiki/frictiongt.
• 2 Dec. 2008 lthttp//images.google.com/imgres?imgur
  lhttp//www.mut.ac.th/physics/physicsmagic/pic2
  520magic/nfw1.gifimgrefurlgt.