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Chapter 6Chapter 6
Work, Energy, PowerWork, Energy, Power
WorkWorkThe work done by force is defined
as the product of that force times the parallel distance over which it acts.
cosFssFW
The unit of work is the newton-meter, called a joule (J)
Work is a scalar
Work
F
s
)cos F(ssF
s cos
Work
F
s
)cos s(F sF
F cos
EnergyEnergyTypes of Energy
• Kinetic Energy = “Motion Energy”• Potential Energy = “Stored Energy”
Kinetic EnergyKinetic EnergyKinetic Energy is the energy
possessed by an object because it is in motion.
221 mvKE
Gravitational Gravitational Potential EnergyPotential Energy
Gravitational Potential Energy is the energy possessed by an object because of a gravitational interaction.
mghPEG
Properties of GravitationalProperties of GravitationalPotential EnergyPotential EnergyArbitrary Zero Point
You need to select a zero level
Independent of Path All that matters is the vertical height
change
Conservation of Conservation of Mechanical EnergyMechanical EnergyEnergy can neither be created
nor destroyed, but only transformed from one kind to another.
finalinital )PEKE()PEKE(
Works for systems with no losses (friction, air resistance, etc.)
Work Energy Theorem
The amount of kinetic energy transferred to the object is equal to the work done. KE = W Many of the problems can be worked
from here
Problem Solution Guidelines
Determine that energy can be conserved (no losses) Pick the zero level for potential energy
Pick two interesting places in the problem Write kinetic and potential energies at
these places Conserve energy
• (KE + PE)1 = (KE + PE)2
PowerPowerPower is the time rate of doing work.
Velocity* Force
t
W
Time
Done Work Power Average
Time
cetanDis*Force
PowerPower
The unit of power is a joule per second, called a Watt (W).
On to problems...
