Physics 101 Chapter Energy Work
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Transcript of Physics 101 Chapter Energy Work
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Chapter 7 - Energy
Work, Power, Potential and Kinetic Energy and
Conservation of Energy
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Energy
What is energy? I dont know. We observe transfers of energy or
the effects of energy.
We cant see most forms, But we can calculate a
value for it in its many different forms. (Thermal,
electrical, chemical, mechanical, light, etc.)
Ability to do work? (This really tells us what it
does, not what it is.) It may go in and out of a system, but it is (as far as
we know) never lost from the universe.
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Related Concept: WORK
Work = net force x distance
FdW
Objects motion changes in the direction of the
applied net force (Fig. 7.1)
Amount of work done is proportionalto both the
applied net force and the distance the object moves
Units of Work:
Metric: Newton-meter (Nm)1 Nm = 1Joule (J)
British: Foot-pound (ftlb)
Fig 7.4
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Fig 7.1
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Fig 7.2
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Only the component of force
that is parallelto the directionof the motion does work
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Doing work
The object must move.
Only the part of the force that is in the direction of the motion
of the object does work.
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Two categories of Work
Work done against another force
Lifting against the force of Gravity
Pushing against the force of friction
Work done to change the speed of an object Speeding up or slowing down your car
Can energy be transferred from one object or
source to another?
Yes! When one object does work on another, this
istransfer of energy!
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Work is a Scalar Quantity
Work can have a sign, but the sign doesnt mean the direction
of the work as a vector
Work is positive if the net force would tend to cause the object to
speed up Net Force is in thesame direction as velocity .
Work is negativeif the net force would tend to cause the object to
slow downNet Force is in the oppositedirection as the velocity.
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What is the sign on the work done by gravity
on the roller coaster?
When determining the sign ignore the other forcesWork done by friction is negative both ways
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What is thesignof work done
byfriction?!!!
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Power
Power is howfasta person does work, or how
fast energy is transformed from one form into
another.
P = Work done/time interval
Units?
Joule/sec(J/s); 1 J/s =1 Watt (1 W).
1 kilowatt = 1 kW = 1000 W
1 hp is approximately 750 W or kW.
t
W
P
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More power (to ya)
One Watt of power is the work done in lifting 1N
(1/4 pounder) one meter in one second.
More power means the ability to do work faster,or doing the same amount of work in less time.
It also means we are using energy faster!
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When electricity flows through a light bulb, electricity
is converted to light and heat
The rate at which this conversion is made is called power
On the light bulb you will see the power rating, In this instance40 Watts
This means that 40 J of electrical energy is converted to light
and heat each second that the bulb is on.
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Mechanical Energy
Two most common forms:
Potential Energy. The ability to do workby virtue of
the positionor conditionof an object.
Kinetic Energy. The ability to do workbecause of the
motionof an object.
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Potential Energy
Stored energy: a stretched or compressed spring,a stretched bow, electrons in molecules (chemical
energy).
Gravitational potential energy: energy possessedby an elevatedobject.
To elevate the object one must do work against
the force of gravity (the weight of the object).
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Calculating Gravitational
Potential Energy
Lets lift a mass (m) straight up a distance h.
The upward force required to move m at constant velocity
is equal to its weight or mg.
So Work = F x d
Here: Force = mg
d = height (h)
therefore
PEg= (mg) x h > > > PEg= mgh
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Gravitational Potential Energy
Note: h is above some chosen reference level
See figure 7.4 and 7.6
The thing that is significant is that there is a
change in PE (a change in elevation).
PEcan be transformedinto kineticenergy, theenergy of motion.
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See figure 7.6. Increase or decrease i n
PEgis independent of the path taken to
get there. Path matters not!
Fig 7.6
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Fig. 7.6
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Fig 7.7
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Kinetic Energy
The energy of motion KE = mv2
Units?
(1 kg) (1 m/s)2= (1 kg) 1 m2/s2=
(1 kgm/s2) 1 m = 1 Nm = 1 J
It is the work requiredto bring an object from rest toa given speed(positive work), or the work required
by the object (or against it) the object can do whilebeing brought to rest(negative work).
Point: Work required to move or to stop motion!
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XXXKinetic Energy
2
21 mvKE
The KE of the object depends
On both the mass of the object
And its speed
Q. Object A and object B move with the same speed. If object B has
three times the mass of object A, how much more KE does it have?
A. KE is directly proportional to mass, so 3x the mass gives 3x the KE.
Q. Object A and object B have the same mass. If object B moves with three
times the speed of object A, how much more KE does it have?
A. KE is directly proportional to the speed squared, so 3x the speed gives
32x the KE or 9x the KE.
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Table, p.120
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Work and Kinetic Energy Work-Energy Theorem
Work doneby the net force equalsthe change in kinetic energyof
the object.
Worknet= KE
Work is not energy, but a way of transferring energy from one
place to another or one object to another.
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Work and Kinetic Energy
The final speed of the object depends on the
Net work done on the object
When net work is positive, the object
Will speed up and KE increases
When net work is negative,
The object will slow down and
KE decreases
When net work is zero, the object
Moves with constant velocity and
The KE doesnt change
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XXXWork and Kinetic Energy
Q. An 8-kg object is sliding across a floor at a speedof 10 m/s. What is the objects KE?
Q. Object B has twice the speed of object A and the same mass. Howmuch more work does it take to stop B than to stop A?
A. The work needed to do this is equal to the change in KE. When B has 2xthe speed it has 4x the KE. In stopping, it loses all the KE, so it loses 4xthe KE. Therefore it will require 4x the work to stop B.
JmvKE 400)10)(8(2
1
2
1 22
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Work and Kinetic Energy
A 50-N pail is being lifted 6-m by a rope with a tension of 75-N. Find
the change in the pails KE during this process. Is it an increase or decrease?
Find the work done by eachforce: the tensionand gravity.
WT
= T d = (75N)(6m) = +450J
Wg= Weight d = -(50N)(6m) = -300J (why is this negative?)
Wnet = WT+ Wg= 450J300J = +150J
Apply the work-energy formulaWnet= KE = +150J
This is an increasein KE. The rope puts in 450J of energy while
Gravity removes 300J of energy. This leaves a net gain of 150J of KE.
75 N
50 N
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Conservation of Energy
Energy cannot be created or destroyed; itmay be transformed from one form to
another, but the total amount never changes.
For a given system, it may be transferred in
or out.
Examples:
Figure 7.12 the circus diver
Pendulum
Vibrating Spring
Roller Coaster
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ME = PE + KE
Fig. 7.10
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ME = PE + KE
Fig. 7.14
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PE
PE + KE
PE + KEPE + KE
KE
ME = PE + KE
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First Class Lever
Fig. 7.15
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First Class Lever
Fig. 7.16
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Second Class Lever
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Third Class Lever
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Summary
It just keeps . . .
getting . . .
Worse!
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Law of Lever