Transcript Energy, Work, and Power

Energy
• According to Einstein, a counterpart to mass
• An enormously important but abstract
concept
• Energy can be stored (coal, oil, a watch
spring)
• Energy is something moving objects have
• How to deal with this idea???
Work
• Easiest to start with the notion of work
• Work = Force X Distance
• Lift a box from the floor, you apply a force
to overcome gravity
• Multiply that force by the distance through
which you apply the force and you calculate
the amount of work accomplished
Is this Work?
Work
• Unit is the JOULE
• A Joule is a newton-meter
Power
• The rate at which work is done
• Takes more power to run up the stairs than
to walk up the stairs, but the energy
consumed is the same in either case
work done
power 
time interval
Power
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Unit is the WATT
A Watt is a newton--meter per second
Think of 100-Watt light bulb
Bigger units are kilowatts and megawatts
Utility sells energy in kilowatt-hours
1 KWh = 1000 Joules/second times 3600
Seconds = 3.6 X 106 Joule
Potential Energy
• If we lift an object from the floor into the
air, it has the potential to do work for us
• This ability to do work is called
POTENTIAL ENERGY
• Other forms of potential energy include the
compression of a spring, the stored energy
in coal or oil, the stored energy in a uranium
nucleus
Potential Energy
• Gravitational potential energy is simple to
calculate
• Gravitational Potential Energy = weight X
height
PE  mgh
Gravitational Potential Energy
• Independent of Path to get there
Kinetic Energy
• The energy of moving objects
• Kinetic Energy = 1/2 Mass X Speed2
1
2
KE  mv
2
Energy Conversion
Energy Conversion
Work-Energy Theorem
• Work done on an object can give the object
either potential or kinetic energy or both
• If we do work on an object to lift it into the
air, we give it potential energy
• If we do work on an object and set it into
motion, we give it kinetic energy
• The work-energy theorem relates to the
second case
Work-Energy Theorem
• If we do work on an object and set it into
motion without changing the object’s
potential energy, the work done appears as
kinetic energy of the object
Work  KE
Conservation of Energy
• Perhaps the most important discovery of the
past two centuries
• In the absence of external work input or
output, the energy of a system remains
unchanged. Energy cannot be created or
destroyed.
• Remember from Einstein, that mass is a
2
form of energy
E  mc
Collisions
• Elastic Collisions conserve both momentum
and kinetic energy
• Inelastic Collisions conserve momentum by
energy is lost to heat
Machines
• A device that multiplies forces by taking
advantage of the definition or work and the
conservation of energy
• Work input = Work output
• Levers
Machines
Machines
Efficiency
work done
Efficiency 
energy used
In many machines, some energy is lost due
to friction. This may be metal-on-metal (oil
the parts to reduce friction) or air resistance
(energy loss moves molecules in the air
faster giving them kinetic energy).
Energy Sources
• For the earth, there are two energy sources,
the sun and radioactive decay in the earth’s
interior
• The earth receives about 1400 Joules/meter2
each second
• This is 1.4 kW per square meter
• Recover for use in plants (burn wood)
• Recover from wind
Man’s Need for Power
• Man can generate about 75 Watts to do
work
• Domesticated Animal about 750 Watts
• Machines limited by size
• Power plants generate electricity in the
hundreds of megawatt range
Universal Gravitation (Newton)
• Every mass attracts every other mass with a
force that is proportional to the product of
the two masses divided by the square of the
distance between the masses
• For distances, calculate from the CENTER
OF MASS
• For the earth, that is at the center of the
earth
Universal Gravitation
FG
m1 m2
d
2
Acceleration Due to Gravity
FG
m earthm object
2
Rearth
F  m object g
gG
mearth
2
Rearth
g  6.67  10
11
g  9.8 m/sec 2

6  10
24
6.4 10 
6 2
Inverse Square Law
Inverse Square Law
Weight and Weightlessness
Tides
Stretch is about one meter high.