Download coefficient of friction

Review
Force, Friction, Motion, Power and
Work
In science, a force is a push or a
pull.
 All forces have two properties:
 Direction and Size
 A newton (N) is the unit that
describes the size of a force.
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When the forces on an object produce a net
force of 0 N, the forces are balanced.
There is no change in the motion of the object.
When the net force on an object is not 0 N, the
forces on the object are unbalanced.
Unbalanced forces produce a change in motion
of an object.
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Every time you push on something, friction
pushes back. To get an object moving, you
have to overcome the friction force first. If
you’ve ever walked on pavement and
suddenly stepped on a patch of ice, you know
that different surfaces have different amounts
of friction.
What determines how much friction an object
experiences? Is it the type of material? The
object’s weight? The amount of surface area in
contact with the ground?
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Friction occurs because the surfaces
of any object is rough.
Even surfaces that appear smooth
cause friction.
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The rougher the surface, the greater the
friction.
Ex. Pavement is much rougher than grass,
more friction is produced when you slide on
the pavement than on grass.
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Objects with more mass need more
force to overcome friction, objects
with less mass need less force to
overcome friction.
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Sliding Friction- Ex. When you go sledding, or
apply the brakes on a bicycle or car.
Give an example of sliding friction
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Rolling Friction-Important when it
comes to means of transportation.
Anything with wheels using rolling
friction.
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Fluid Friction includes liquids and gases.
For example the friction between air and a fast
moving car is the largest force opposing the
motion of the car.
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Static Friction is when a force is applied to
the object but the object does not move.
Static friction disappears as soon as the
object moves.
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It is easier to drag an object over glass than
sandpaper. The reason for this is that the
sandpaper exerts more frictional resistance. In
many problems, it is assumed that a surface is
"smooth", which means that it does not exert
any frictional force. A "rough" surface is one
which will offer some frictional resistance. -
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The coefficient of friction (COF), often
symbolized by the Greek letter µ, which
describes the ratio of the force of friction
between two bodies and the force pressing
them together. The coefficient of friction
depends on the materials used; for example, ice
on steel has a low coefficient of friction, while
rubber on pavement has a high coefficient of
friction. Coefficients of friction range from near
zero to greater than one.
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Imagine that you are trying to push a book
along a table with your finger. If you apply a
very small force, the book will not move. This
must mean that the frictional force is equal to
the force with which you are pushing the book.
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The force of kinetic friction which is equal to
the coefficient of kinetic friction times the
normal force. Since the magnitude of the
applied force is greater than the magnitude of
the force of kinetic friction opposing it, the
block is accelerating to the left
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Use this equation:
Ff = µ * mg and solve for µ.
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µ = Ff / mg
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Find coefficient of friction, given force and mass
Q: An engine provides 5.0 kN of force to keep a 1600kg vehicle moving at a uniform speed. (Air resistance is
negligible.) What is the coefficient of rolling friction
between the tires and the road surface?
A: First convert 5.0 kN to Newtons: F = 5.0 kN = 5000
N. (Just like 5 kg = 5000 grams when measuring mass.)
You know it’s traveling at uniform speed, so this tells
us that the right force = left force. There is no net force,
because there is no acceleration!
F = 5000 N
m = 1600 kg
g = 9.8
=?
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A horizontal force of 400.0 N is needed to pull a
1760 N trunk across the floor at constant speed.
What is the coefficient of kinetic friction (µk)?
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What force must be exceeded to start a 1.35 kg
book moving across a desk if the coefficient of
static friction (µs) is 0.30?
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A 1500. N force is exerted on a 200.0 kg crate to
move it across the floor. If µk is 0.250, what is the
acceleration of the crate?
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Work is done when a force causes an object to
move in the direction that the force is applied.
The formula for work is: W = F X D
If there is no movement, there is no work
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Work is measured in Joules.
Rule for Work
Work (Joules) = Force (Newtons) X
Distance(meters)
W=FXd
Power is the rate at which work is done. Power is
measured in Watts.
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Rule for Power
Power (Watts) = Work (Joules) / Time (seconds)
P=W/T