Transcript Work, Energy & Power

Work, Energy & Power
There are many different TYPES of
Energy.
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Energy is expressed
in JOULES (J)
4.19 J = 1 calorie
Energy can be
expressed more
specifically by using
the term WORK(W)
Work = The Scalar Dot Product between Force and Displacement.
So that means if you apply a force on an object and it covers a
displacement you have supplied ENERGY or done WORK on that
object.
Scalar Dot Product?
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W  F  x  Fx cos
A product is obviously a result of
multiplying 2 numbers. A scalar
is a quantity with NO
Thie means that F and x MUST be parallel.
DIRECTION. So basically
To ensure that they are parallel we add the
Work is found by multiplying
cosine on the end where the cosine is the
the Force times the
angle between the direction of the force and
displacement and result is the direction of the displacement. If the force
ENERGY, which has no
and displacement are parallel, the angle is
direction associated with it. zero, and the cosine is one.
Work
The VERTICAL component of the force DOES NOT
cause the block to move the right. The energy imparted to
the box is evident by its motion to the right. Therefore
ONLY the HORIZONTAL COMPONENT of the force
actually creates energy or WORK.
When the FORCE and DISPLACEMENT are in the SAME
DIRECTION you get a POSITIVE WORK VALUE. The
ANGLE between the force and displacement is ZERO
degrees.
When the FORCE and DISPLACEMENT are in the
OPPOSITE direction, yet still on the same axis, you get a
NEGATIVE WORK VALUE. This negative doesn't mean
the direction!!!!
When the FORCE and DISPLACEMENT are
PERPENDICULAR, you get NO WORK!!! The
ANGLE between the force and displacement in this
case is 90 degrees.
The Work Energy Theorem
Up to this point we have learned Kinematics and
Newton's Laws. Let 's see what happens when we
apply BOTH to our new formula for WORK!
1. We will start by applying
Newton's second law!
2. Using the v2 equation !
3. An interesting term appears
called KINETIC ENERGY or
the ENERGY OF MOTION!
The Work Energy Theorem
And so what we really have is
called the WORK-ENERGY
THEOREM. It basically means
that if we impart work to an
object it will undergo a CHANGE
in speed and thus a change in
KINETIC ENERGY. Since both
WORK and KINETIC ENERGY
are expressed in JOULES, they
are EQUIVALENT TERMS!
" The net WORK done on an object is equal to the change in kinetic
energy of the object."
Example
W=Fxcos
A 70 kg base-runner begins to slide into second base when moving
at a speed of 4.0 m/s. The coefficient of kinetic friction between
his clothes and the earth is 0.70. He slides so that his speed is
zero just as he reaches the base (a) How much energy is lost
due to friction acting on the runner? (b) How far does he slide?
a) W f  K
Ff  Fn  m g
W f  0  1 m vo2   1 (70)(4) 2
2
2
W f  -560 J
 (0.70)(70)(9.8)
= 480.2 N
W f  Ff x cos
 560  (480.2) x(cos180)
x  1.17 m
Work Done By Friction
The frictional force of a surface always acts in
the opposite direction of an objects
displacement. This means that friction will
always do negative work on an object. An
object will always do positive work on the
surface
Work Energy
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A child pulls a 5kg sled with a rope angled 37 degrees
above the horizontal with a force of 10N. The coefficient
of friction is 0.1 How much work does he do? Friction?
How fast will the sled be moving after moving 4meters?
Work Energy
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Work Energy
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Work Energy
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How much work is done by gravity?
How much work is done by the normal force?
Work as the transfer of energy
Just as there is conservation of mass in the
universe. There is also a conservation of energy.
 Energy is neither created or destroyed
 Energy is transferred from one object to another
or changes from one form to another
 The change in an objects kinetic energy is the
result of the net work done on it.
 Energy Gained is positive work
 Energy lost is negative work
The Scalar Nature of Energy
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Energy is a scalar quantity. It does not have
direction. An object in motion only has
positive kinetic energy due to the equation
using v2
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For work-energy calculations, only the
speed of an object can be calculated, not
its velocity
Work Done by Gravity
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In this case,
because the
object is at rest
initially, the
change in
kinetic energy is
equal to the
final kinetic
energy
Solve for the final velocity of an object
after being dropped from height “h”
Work-Energy
Kinematics
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Potential Energy
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An object with kinetic energy can do work on
another object.
An object that can fall has the ability to do
work once it has fallen.
This energy by virtue of its position is its
potential energy.
Potential Energy Requires a
“Reference Level”
Potential Energy Due to Gravity
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