Download Chapter 6 Work, Power and Energy

Chapter 6
Work, Power and Energy
What You Need to Know
Energy Facts
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There are different types of energy
Energy of all types is measured in
Joules
Law of Conservation of Energy – Energy
can be neither created nor destroyed,
merely changed from one form to
another
Types of Energy
(Unit Overview)
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Mechanical Potential Energy
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Energy of Position
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Kinetic Energy
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Energy of Motion
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If it moves it has kinetic energy
Heat Energy
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Gravitational
Elastic
Heat is a form of Energy Transfer
Other Forms of Stored Energy
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Chemical
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Fuels - usually release energy by combustion
Food – energy released by digestion
Electrical
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Generated from other forms of energy
Work
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The Physics definition of work requires
a displacement, i.e. an object must be
moved in order for work to be done!
The Applied force which causes the
displacement contributes to the work,
i.e. in order to contribute to the work,
the applied force must be parallel to the
displacement.
Work: A Mathematical Definition
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Work = (Force)(Displacement)
Units of Work = (Newton)(Meter)
1 Newton•Meter = 1 Joule
A Joule is a unit of Energy and it takes
energy to do work and work done on an
object either causes it to move (kinetic
energy) or is stored (potential energy)
Sample Problem
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What work is done sliding a 200
Newton box across the room if the
frictional force is 160 Newtons and the
room is 5 meters wide?
W = Ff • ΔX = (160 N)(5 m)
800 Joules
Elastic Potential Energy
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Bungee cords, rubber bands, springs
any object that has elasticity can store
potential energy.
Each of these objects has a rest or
“zero potential” position
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When work is done to stretch or compress
the object to a different position elastic
potential energy is stored
Elastic Potential Energy
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Top picture is “rest position”; x = 0
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This is a point where the elastic potential energy = 0
Bottom picture is “stretched position”
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Here elastic potential energy is stored in the spring
Us = ½ kx2 where k is the “spring constant” in N/m
Sample Problem
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What is the Elastic potential energy of a
car spring that has been stretched 0.5
meters? The spring constant for the car
spring is 90 N/m.
Ep = ½ kx2 = (½)(90 N/m)(0.5 m)2
=11.25 Joules
Where Does “K” Come From?
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K is measured in Newtons/meter. It is
defined as the force required to displace
a spring 1 meter. So:
K = F/x
Often K is determined by hanging a
known weight from the spring and
measuring how much it is stretched
from its rest postion.
Sample Problem
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A spring is hung from a hook and a 10
Newton weight is hung from the spring. The
spring stretches 0.25 meters.
What is the spring constant?
If this spring were compressed 0.5 meters,
how much energy would be stored?
If this spring were used to power a projectile
launcher, which fires a 0.2 kg projectile, with
what velocity would the projectile leave the
launcher? Assume 0.5 m compression.
Power
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Power = Work/time
= Joules/Second
Mathematically there
are two formulas for
Power:
Fd or since d  v
P
t
t
then
P  Fv
Problem Types
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Work
Work at an angle
Kinetic Energy
Gravitational Potential
Elastic Potential
Conservation
Power