What is a force? How does friction affect motion?
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- What is a force? How does friction affect motion?
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- Motion A change in position, also known as an objects
displacement
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- Speed (rate) Rate of change in position How fast or how slow
Measured in m/s (meters per second)
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- Velocity Speed with a direction Vector
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- Constant speed A speed that does not vary Graph a straight
line, no turns
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- Acceleration Rate of change in velocity Change in speed over
time Change in direction
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- Acceleration Units for acceleration m/sec 2 Acceleration refers
to increasing speed, decreasing speed (deceleration, negative
acceleration), or changing direction
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- Calculating Acceleration Acceleration = Final speed Initial
speed Time A plane travels from 0 m/s to 24 m/s in 3 sec. What is
its acceleration? Acceleration = 8 m/s 2
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- Calculating Acceleration, alt. Force = mass x acceleration
Acceleration = A force of 6 N is applied to an object that has a
mass of 3 kg. What is the acceleration? Acceleration = 2 m/s 2
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- What is a Force? A force is a push or a pull. All forces have
two properties: Direction and Size
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- A Newton (N) is the unit that describes the size (magnitude) of
a force.
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- The student is pushing down on the chair, but the chair does
not move. The floor is balancing the force by pushing on the chair,
which in turn pushes on the student.
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- A force between two surfaces that touch and are sliding, or
trying to slide across one another, for example when you try to
push a toy car along the floor. What is friction? Friction is a
force that opposes motion between two surfaces that touch each
other.
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- Friction always works in the direction opposite from the
direction the object is moving, or trying to move. It always slows
a moving object down. Moving Friction
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- The surface of any object is rough. Even an object that feels
smooth is covered with tiny hills and valleys. The contact between
the hills and valleys of two surfaces causes them to stick,
resulting in friction.
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- Friction converts kinetic (motion) energy into thermal (heat)
energy.
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- Principles of Friction Friction acts parallel to the surfaces
that are in contact. The direction that friction acts is OPPOSITE
the direction of the motion (or intended motion). F weight F normal
F applied F friction(k)
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- What Affects Friction? The amount of friction depends on:
Roughness of the surfaces Force pushing the surfaces together
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- What Creates More Friction? Rough Surfaces Rough surfaces have
more microscopic hills & valleys. The rougher the surface, the
greater the friction will be. Greater Force (caused by increased
mass) The amount of force exerted by the object increases the
amount of friction Increase the amount of force exerted by the
object by increasing the mass of the object
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- What Does Not Affect Friction? Surface Area Changing the
amounts of the surfaces that touch does NOT change the amount of
friction 4 N4 N4 N4 N 4 N4 N4 N4 N
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- What Does Not Affect Friction? 2 N Velocity Friction forces are
independent of speed or velocity The force of kinetic friction is
the same at 5 m/s as it is for 20 m/s. 5 m/s 20 m/s
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- Types of Friction There are two types of friction: Static
Friction the frictional force present just before motion begins.
Kinetic Friction the frictional force present with motion Note:
Static Friction is usually higher than Kinetic Friction
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- Static Friction Static friction occurs when force applied to an
object does not cause the object to move. In order to move
something, you must first overcome the force of static friction
between the object and the surface on which it is resting.
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- Kinetic friction occurs when force is applied to an object and
the object moves. Examples: Sliding Friction: pushing an object
across a surface Rolling Friction: between wheels and a surface
Fluid Friction: opposes the motion of objects traveling through a
fluid (air or water), also called drag force or air resistance
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- Reduce the contact area by using rollers/ball- bearings/wheels
Change the surfaces of the materials that are touching by using
lubrication like motor oil, wax, or grease Roll objects instead of
sliding them Create a cushion of air Eg. Like a hovercraft or air
hockey table Reducing Friction
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- Friction Can Be Harmful or Helpful Harmful friction Holes in
socks or knees of jeans Wind & water cause erosion Helpful
friction Friction between pencil & paper Without friction, you
would just slip & fall when you tried to walk
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- Friction can be a useful force because it prevents our shoes
slipping on the pavement when we walk and stops car tires skidding
on the road. Useful Friction
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- Ice causes very little friction, which is why it is easy to
slip over on an icy day. But this is a good thing for ice skating
and sledding.
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- Recall the Sliding Penny Why do things not continue to move at
constant velocity? Friction If the sliding penny slows down, whats
the force responsible? Friction
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- How could I keep it moving at a constant velocity? Reduce the
friction Do I need to apply a force to keep it moving? Why? No,
inertia will keep it moving once the initial force was
applied.
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- Measures of friction are based on the type of materials that
are in contact. Each material has what is known as a coefficient of
friction. The coefficient is a measure of how easily one object
moves in relationship to another. When you have a high coefficient
of friction, you have a lot of friction between the materials.
Concrete on concrete has a very high coefficient of friction. A
material such as Teflon (on most things) has a very low
coefficient. Teflon is used on surfaces where we don't want things
to stick; such as pots and pans. How is friction measured?
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- Standard Friction Equation Friction = coefficient of friction x
normal force. Normal force comes into play any time two objects are
in direct contact with one another, and always acts perpendicular
to the object that applies the force. The simplest example of the
Normal force can be seen in the situation of an object on a
horizontal surface. If the object is on a horizontal surface, then
Normal force = Weight
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- 2 N2 N2 N2 N Friction and the Normal Force 4 N The force
required to overcome or friction is proportional to the normal
force, n The force required to overcome static or kinetic friction
is proportional to the normal force, n. f k = k n f s = s n n 12 N
6 N n 8 N 4 N n
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- = coefficient of kinetic friction = coefficient of static
friction FNFN = the normal force kk ss = Symbol for the Greek
letter mu, used in the formula to represent the coefficient.
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- Sample Coefficients of Friction Material on Material s = static
friction k = kinetic friction steel / steel0.60.4 add grease to
steel0.10.05 metal / ice0.0220.02 brake lining / iron0.40.3 tire /
dry pavement0.90.8 tire / wet pavement0.80.7 Spring 200836
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- The coefficient of kinetic friction between a block and the
level surface it slides on is 0.45. If the mass of the block is
10.0 kg, what is the minimum force needed to keep the block moving
with uniform motion? The applied force need only balance the
kinetic frictional force in order to maintain uniform motion. F k =
k F N F k = (0.45)(10.0)(9.8 m/s 2 ) F k = 44 N
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- A student pulls a 5.00 kg object and discovers that she needs
to exert 30.0 N of force before the object moves. What is the
coefficient of static friction between the object and the surface
on which it rests? Since the applied force is equal to 30 N just
before moving, the friction force must be equal in size (30 N).
Therefore: = 30 N (5 kg)(9.8 m/s 2 ) = 0.61