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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
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
power work done
time interval
Power
• 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
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 form of energy E mc 2
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
Efficiency
Efficiency work done
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
Acceleration Due to GravityF G
mearthmobject
Rearth2
F mobject g
g Gmearth
Rearth2
g 6.67 10 11 6 1024
6.4 10 6 2
g 9.8 m/sec2