Download Physics 108

Chapter 6
Work, Energy, Power
Work
The work done by force is defined
as the product of that force times
the parallel distance over which it
acts.
 
W  F  s  Fs cos 
The unit of work is the newtonmeter, called a joule (J)
Work is a scalar
Work

F


s
 
F  s  F(s cos )
Work

F

F cos 

s
 
F  s  s(F cos )
Energy
Types of Energy
• Kinetic Energy = “Motion Energy”
• Potential Energy = “Stored Energy”
Kinetic Energy
Kinetic Energy is the energy
possessed by an object
because it is in motion.
KE  mv
1
2
2
Gravitational
Potential Energy
Gravitational Potential Energy is
the energy possessed by an
object because of a gravitational
interaction.
PE G  mgh
Properties of Gravitational
Potential Energy
Arbitrary Zero Point
 You need to select a zero level
Independent of Path
 All that matters is the vertical height
change
Conservation of
Mechanical Energy
Energy can neither be created
nor destroyed, but only
transformed from one kind to
another.
(KE  PE)inital  (KE  PE)final
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. DKE = 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
Power
Power is the time rate of doing work.
Work Done
Average Power 
Time
W

t
Force * Dis tan ce

Time
 Force * Velocity
Power
The unit of power is a joule per
second, called a Watt (W).
On to problems...