Transcript Physics Chapter 5 Work & Energy

Physics Chapter 5
Work & Energy
Sections:1 5-1 Work
5-2 Energy
5-3 Conservation of Energy
5-4 Work, Energy & Power
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Work
A force that causes a displacement of an object does work on that
object
 W= F x d
 W is work, F is force, d is distance
 Unit is a Newton-meter or Joule, J
Work is done on an object only if the object moves due to the action of
an applied force
 Consider the situations of a mom & child
Work is done on an object only when components of the applied force
are parallel to the displacement of the object
If a force is applied at an angle , then only the vector component of
the force parallel to the direction of displacement is the force that does
the work on the object
 W = F x d (cos)
 If  = 0, then cos 0 = 1 & so W=Fd; if  = 90, then cos 90 = 0 &
W=0
If many constant forces act on an object, find the net work by finding
the net force
 Wnet = Fnet (d) (cos)
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Equations
 W= F x d
 W = F x d (cos)
 Wnet = Fnet (d) (cos)
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Work
 Work is a scalar
 Work is positive when direction of
displacement is same as direction of applied
force
 Ex: lift a box (applied force up, direction is up)
 Work is negative when direction of
displacement is opposite that of applied
force
 Ex: friction of box sliding on floor (applied force
moves box forward, but friction does work on box
in opposing direction)
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Kinetic Energy
 Kinetic energy – energy of an
object due to its motion
 Depends on speed & mass of object
 KE = ½ mv2
 KE is a scalar, unit is Joule, J
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Potential Energy
 Potential energy – energy associated with an
object’s position
 Gravitational potential energy – energy
associated with an object due to its position
relative to the Earth or some other gravity
source
 PEg = mgh
 M = mass, g = 9.81m/s2, h = height
 Unit is Joule, J
 FYI – electrical energy often measured in
kWh, 1 kWh = 3.6 x 106 J
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Elastic Potential Energy
 Elastic potential energy – energy in a stretched
or compressed elastic object
 Relaxed length – term for the length of a spring
when no external forces acting on it; when a
force compresses or lengthens a spring PE is
stored in spring (work is done to spring!)
 The amount of PE depends on distance spring is
compressed or stretched from relaxed length
 PEe = ½ kx2
 k = spring constant, x = distance compressed or
stretched
 Spring constant = a parameter that expresses
how resistant a spring is to being compressed
or stretched, unit is N/m
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Mechanical Energy
 Mechanical energy – the sum of KE & all
forms of PE
 ME = KE + PE
Energy
Non-mechanical
Mechanical
Kinetic
Potential
Gravitational
Elastic
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Conservation of Energy
 MEi = MEf
 Initial mechanical energy = final mechanical energy
 Friction is absent
 ½ mv2i + mghi = ½ mv2f + mghf
 Above equation rewritten using KE & PE
 In presence of friction, ME is not
conserved; b/c ME converted to
nonmechanical energy
 Non-mechanical energy includes:
electromagnetic energy, heat, light, nuclear
energy, chemical energy
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Work, Energy, & Power
 Net work = change in KE
 Wn = KE
 Work done by friction:
 Wf = ME
(Wf is work due to friction)
 Power = W/t
 If W = Fd, then P = W/t = Fd/t
 For work, remember there is a time interval
involved, so work done is done in a unit of
time.
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Power
 Since distance moved per unit time is speed
then an alternative equation for power is:
 P = Fv
 Watt is unit for power; 1 W = 1J/s
 1 horsepower = 746W
 Wattage of light bulbs
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Dim bulb ~40W
Bright bulb ~500W
Decorative light bulbs: indoor ~0.7W, outdoor ~7.0W
What about CFBs?
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