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Friction
What is Friction?
• Friction - the force that opposes motion, or
makes it difficult for an object to move
across a surface.
• The amount of friction depends on the
surface type and the force pressing the two
surfaces together. Rougher surfaces will
usually result in greater friction and smooth
surfaces will result in less friction
Increasing or Decreasing
Friction
• People are always trying to increase of
decrease friction to perform everyday tasks.
• New tires on a car will help increase friction
between the car and the road.
• Wax on skies will allow them to slide over
the snow better.
Friction and Net Force
In the picture below, friction
between the box and the
surface is equal to 2
newtons. If you pull the box
to the right with 8 newtons
of force the net force will be
6 newtons to the left.
• Net Force - the
combination of all
forces acting on a
object.
Friction in a Fluid (Air)
• Air Resistance is also
called Drag and refers to
forces that oppose the
relative motion of an
object through a fluid (a
liquid or gas).
• Here the cyclists wear
tight fitting clothes to
reduce the amount of drag.
This allows them to move
through the air with less
resistance.
Friction in a Fluid (Liquid)
• When an object moves through a liquid it
encounters a force that opposes its motion. To
understand how much the opposing force will be
one needs to consider the viscosity of the liquid.
Viscosity is basically how much a liquid resists
flowing. To keep it simple, honey has a high
viscosity and water has a low viscosity. If you
dropped a marble in a cup of honey the marble
would encounter more friction (drag) than if you
dropped the same marble in water.
Viscosity
Honey has High Viscosity
Water has Low Viscosity
Static Friction
• Static friction is friction
between two solid objects
that are not moving
relative to each other. For
example, static friction
can prevent an object from
sliding down a sloped
surface or in the picture on
the left the box does not
move when the woman
pushes on it.
Sliding Friction
• Sliding friction is
when two objects slide
across from one
another. Examples
include the skis sliding
on the snow or sliding
a book across a desk.
• It is also known as
Kinetic friction
Kinetic Friction
• Kinetic friction occurs
when objects move.
In the pictures on the
left you see an
example of rolling
friction (car) and
sliding friction (the
tube).
Free Fall
• A free falling object is an object that is falling under the sole
influence of gravity. Any object that is being acted upon only
by the force of gravity is said to be in a state of free fall.
There are two important motion characteristics that are true
of free-falling objects:
* Free-falling objects do not encounter air resistance. (this
happens in a vacuum or in outer space)
* All free-falling objects (on Earth) accelerate downwards at a
rate of 9.8 m/s/s
Free Fall
• In the photo to the
right, gravity is the
force that is pulling on
the rock.
• t = time in seconds,
v = velocity
•
This photo shows that the rock will
speed up by 9.8 meters per second each
second it falls. The drag, or air
resistance (friction) on the rock will
eventually cause the rock to reach
terminal velocity (a point where the
rock will no longer gain speed).
http://www.physicsclassroom.com/class/1dkin/U1L5b3.gif
Free Fall
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Free-fall is a type of motion in which
the only force acting upon an object is
gravity. Objects which are said to be
undergoing free-fall, are not
encountering a significant force of air
resistance; they are falling under the
sole influence of gravity. Under such
conditions, all objects will fall with the
same rate of acceleration, regardless of
their mass.
On earth we have and atmosphere so all
objects will encounter air resistance
(friction) when the object falling
collides with air molecules.
If the objects that fall are of similar
shape they will fall at the same rate.
Newton’s 2nd law of motion helps us
understand this concept.
You can try this by dropping a golf ball
and a basket ball at the same time.
They should hit the ground at the same
time.
Free Fall
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Newton determined that F=ma (force =
mass times acceleration).
Using this formula it is also possible to
determine the acceleration of an object in free
fall and explains that acceleration due to
gravity (on earth) is 9.8 m/s/s. (in this
example we will round it to 10m/s/s).
Acceleration is determined by dividing force
by mass.
As you see the two objects if dropped at the
same time, from the same height will
accelerate to earth at the same speed and hit
the ground at the same time.
An objects that have large flat surface areas
will encounter more air resistance and will
fall at a different rate. A feather or a
parachute attached to an object will slow the
object down. This is where friction helps us.
What type of Friction?
Kinetic or static?
What type of Friction?
• What type of Friction?
Kinetic or Static?
• Kinetic, all objects are in
motion. The car is
experiencing rolling
kinetic friction, the tuber
and shark are experiencing
sliding kinetic friction.
How is Free Fall on Earth different
than Free Fall in outer space?
• ??????
How is Free Fall on Earth different
than Free Fall in outer space?
• On earth objects in free fall encounter the
force of gravity and some air resistance. In
outer space objects only encounter the force
of gravity and NO air resistance. This is
demonstrated in the hammer/feather
experiment that the Apollo 15 astronauts
conducted to prove Galileo’s theory.
(click or copy the link below to watch)
http://www.youtube.com/watch?v=5C5_dOEyAfk