Types of Friction by CS91lK

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```									Types of Friction

Mr. V. Perez
Friction
•  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
microwelds
• 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
• Pushing together, the box
moves.
• 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
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
friction
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
speeds
• 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
vacuum.
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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