Welcome to the Wonderful World of Friction

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Welcome to the Wonderful World of Friction
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What is friction?
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Friction is

• Friction A force that resists the relative
  motion or tendency to such motion of two bodies
  in contact.
• Source Dictionary.com

Or why this wont work very well
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Places where friction is good

• The soles of shoes
• Car tires
• The feet of table legs
• Rock climbing holds
• Brakes
• etc

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Places where friction is bad

• In engines
• In transmissions
• On the bottoms of skis and snowboards (To a
  point)
• When you are trying to move a box by pushing it
  along the floor
• etc

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What causes friction?
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Source of Friction
At the microscopic level even the smoothest of
surfaces is dotted with little mountain peaks.
The tips of the peaks are the only parts 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 pressures to form on the parts
that touch. This causes the two surfaces to
become welded almost at the points of contact

• Picture of extremely smooth mica with a scanned
  probe microscope

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Source of friction (continued)

• The true surface contact area is proportional to
  the normal force because the peaks will deform
  plastically when force is applied increasing the
  contact area
• Plastic deformation to change shape permanently
  without fracturing

• Other lesser reasons for friction
• Surface adhesion between pure metals
• Ploughing of one surface by the other harder one
• Elastic deformation

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Experiment Does the friction force depend on
surface area?

• We stick a 1kg mass on a piece of plexiglass on
  top of the clean surface and, using a Newton
  meter, pull the weight across the table with the
  string provided at constant speed.
• We put the same weight on a different piece of
  plexiglass with drastically different surface
  area and repeat
• What do you notice?

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Friction doesnt depend on apparent surface area
(much)

• You may have noticed a small dependence. This is
  partially due to the greater weight of the larger
  piece of plexiglass, and also partially due to
  the fact that there is a dependence of friction
  on surface area, just one that is small enough to
  be ignored under most circumstances.
• Why is the friction force approximately
  independent of surface area? Because as discussed
  previously the materials plastically deform at
  the peaks depending on the pressure increasing
  real surface area.
• RSA real surface area, F Normal Force, SA
  Surface area, F/SA P Pressure, C constant
  depending on the materials

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What is k and s?

• The coefficient of friction is different
  depending on if the objects are in motion
  relative to each other or not as in the graph.
  s is the coefficient of static friction and k is
  the coefficient of kinetic friction.

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Why?

• The slightly larger value for static friction
  results from irregularities and contaminants on
  the surfaces and is less accurate in general than
  the coefficient of kinetic friction
• Between (very) carefully cleaned surfaces the
  difference between the two coefficients
  disappears

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Finally something useful..

• The only formula you need for friction forces
• or
• Ff Fn

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What is the normal force?
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Newtons Second and Third Laws

• Newtons Second Law If there is an unbalanced
  force on an object it will accelerate according
  to FUnbalanced
• Newtons Third Law If you exert a force on an
  object it exerts an equal and opposite force on
  you

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Block at Rest on a Table
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Normal Force

• From Newtons third law we know that if gravity
  or some other force pushes an object (like a
  block) into a second object (like a table) that
  second object will be exerting an equal force
  back on the first.
• Normal force is the force the table exerts back
  on the block
• Normal force is always exerted perpendicular to
  the surface
• Friction Force is always parallel to the surface
• So if the table is horizontal and gravity is the
  only force on the block Fn -Fg

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Normal force on a hill

• Normal force is exerted perpendicular to the
  surface in accordance with Newtons Third law
• No unbalanced force so the block is stationary or
  at least not accelerating

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Typical Physics question

• Ancient Egyptians are pushing a stone block up
  the side of a pyramid of angle 20 degrees. What
  stone should they be using so they can take a
  break and not have it slide down, but have it
  still be not much over the coefficient of
  friction needed so they dont have to push so
  hard?
• Stone A .2
• Stone B .3
• Stone C .4
• Stone D .5

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Solution
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Experiment

• With the Newton meter find the coefficient of
  kinetic and static friction for the plexiglass
  and the plastic track.

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Wormbot?

• Wormbot is a crawling machine divided into three
  sections.
• Each section weights 3.67 kg
• Each section has a plexiglass bottom layer so
  they each have the same coefficient of friction
  that we found earlier for plexiglass on the
  surface.

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Can Wormbot gain a net movement in a direction?

• Why is this possible/impossible?

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If you have two weights with the coefficient of
friction connected by something that can expand
and contract can you gain a net movement?

• What if you have 4 weights with connections in
  between?

What if you have three weights but one is heavier
than the other two. How much heavier can it be?
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How much weight can it pull on flat ground with
the corner pulley? Does it matter where the
additional weight is attached?How does putting
the weight on plexiglass on the table and getting
it to tow it that way change things?
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If you were to put Wormbot on a table at an
angle what is the steepest angle he could climb?