Types of Friction by CS91lK


									Types of Friction

   Mr. V. Perez
  Grade 6 Science
•  The amount of friction between two surfaces depends on
   two factors:
1) The kinds of surfaces that the objects are made of
2) The force pressing the surfaces together
• If two surfaces are in contact, welding or sticking occurs
   where the bumps touch each other. Microwelds are
   where the bumps meet. They are the source of the friction.
                                        • To move one
                                          surface over the
                                          other, a force must
                                          be applied to
                                          overcome the
                                        • Once the object is
                                          moving, friction
                                          slows it or stops it
                    Static Friction
• If a box is too heavy for
  you to lift, push, or pull it,
  then it is not moving. We
  can also say that it has
  zero acceleration. Why?
                                                       The net
• Another force that cancels
  your lift, push, or pull                             force is 0
  (applied force) must be
  acting on the box
• The force is the friction
                                 • Static friction is the
  due to the microwelds
                                   frictional force that prevents
  between the bottom of the
                                   two surfaces from sliding
  box and the floor
                                   past each other
                   Static Friction
•   Arrows that show the direction of the frictional force are
    drawn in the opposite direction of the force (push or pull)
•   Why isn’t the potted tree able to move?
                          •   The net force is zero. This
                              means the push is equal to the
                              opposing force – which is the
                              static friction.
                          •   The force (or push) is not strong
                              enough to overcome the
                              microwelds between the bottom
                              of the pot and the floor
                          •   Baseball players use batting
                              gloves or pine tar to increase
                              static friction
• Lets say you ask a friend      Sliding Friction
  to help you move the box
• Pushing together, the box
• Enough applied force has
  been exerted to overcome
  the microwelds between
  the floor and the bottom
  of the box
• If you stop pushing, the
  box comes to a stop
• This is because the box        • Sliding friction is the
  slides across the floor, and     force that opposes the
  another force - sliding
  friction - opposes the           motion of two surfaces
  motion of the box                sliding past each other
                  Sliding Friction
•   In this example a force (in the form of a push) is put on
    the potted tree
•   The tree accelerates in the direction of the push (from
    right to left)
                              • What is the force that acts on
                                  the push?
                              • Sliding Friction
                              • Sliding Friction is the
                                  frictional force that prevents
                                  the potted tree from
                                  continuing to stay in motion
                              • Rubber pads are used under
                                  rugs to add sliding friction,
                                  which stops you from falling
• As a wheel rolls over a              Rolling Friction
  surface, the wheel digs
  into the surface, causing
  both the wheel and the
  surface to be deformed
• Static friction acts over
  the deformed area where
  the wheel and surface are
  in contact, producing a
  frictional force called           The wheels are deformed or “squared”
  rolling friction
• Much less force is            •   Rolling friction is the frictional
  needed to roll an object          force between a rolling object
  than to slide it (or drag it)     and the surface it rolls on
• This is why pens are          •   Tires on a car or bicycle will
  easier to use than pencils        get very hot due to rolling
                                                   Ball bearings
    Rolling Friction                               in these wheels
•    Rolling                                       greatly reduce
                                                   friction by
     Friction takes                                replacing
     place at the                                  sliding friction
     point of rolling                              with rolling
     contact due to                                friction
     the bent shape
     of the wheel and
     the ground
•    Skates, skateboards, bicycles, and automobiles use ball
     bearings to replace sliding friction with rolling friction
•    Grease or oil is often used on the ball bearings to limit the
     amount of friction (that is from unwanted heat energy)
•    Up to 20% of the energy output from an engine is lost to
     the heat from friction. Oil is used to limit the friction and
     prevents the parts of the engine from not working properly
      Fluid Friction
•   Fluid Friction opposes the
    motion of an object through
    liquids and gases.
•   You feel fluid friction when
    you stir a cake batter. The
    faster you stir, the greater
    the fluid friction.
•   Air resistance is the fluid
    friction on an object moving     A flying squirrel takes
    through air. Bicyclists,         advantage of air resistance to
    speed skaters, and skiers        slow its fall and increase the
    wear slick racing suits to       distance covered in the jump
    reduce air resistance.       •     Soap is a lubricant that
    Another example of air             is used on screws.
    resistance is a parachute.   •     It adds fluid friction to
                                       overcome static friction
                  Fluid Resistance
•    Fluid resistance is when a solid object is in contact with a
     fluid (such as a liquid or gas) and a force is applied to
     either the object or the fluid, there is a frictional force that
     resists the motion.
•    Examples of where fluid friction occurs include:
1)   water flowing through a hose
2)   an airplane flying through the atmosphere
3)   oil lubricating moving parts on an engine
4)   a parachute that is opening by a skydiver or a dragster
•    It is harder to move thick grease through a hose than it is
     to move water through a hose
•    It is harder to ride a bicycle on sand or gravel path than it
     is to ride it on concrete or black-top path because there is
     more resistance from sand and gravel
•    Fluid resistance has less resistance than sliding friction
    Air Resistance
• Air resistance is a friction-
  like force that opposes the
  motion of objects that move
  through the air
• Air resistance causes
  objects to fall with different
  accelerations and different
• The amount of air
  resistance on an object
  depends on the speed, size,
                                •   Air resistance, not the
  and shape of the object
                                    object’s mass, is why feathers,
• The larger the surface area       leaves, and pieces of paper
  exposed to the air – the          fall more slowly than pennies,
  greater the air resistance        acorns, and apples.
What will happen when the
hammer and feather are dropped
at the same time on the moon?
The hammer and the feather
will fall at the same rate
because there is no air
resistance, or friction, in a
What will happen when
the hammer and feather
are dropped at the same
time on Earth?
On Earth, the hammer
will land first because
there is more air
resistance on the feather
than the hammer.

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