Energy
• conservation of energy
• work, energy, and power
• machines & efficiency
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Homework:
RQ: 3, 4, 5,10, 12, 13, 15, 18, 30.
Ex: 23, 26, 28, 37, 49, 62.
Problems: 1, 5, 6.
1
Energy & Work
• Energy is the capacity to do ___________
• Unit: joule = newton·meter (J = N·m)
• Work = force x distance (Fd) when force is
in direction of motion (or opposite to
motion)
• Ex. 50N pushes distance of 4 meters.
• W = (50N)(4m) = _______________
• /
2
Machines
• change an applied force by ___________
it, _________________________ it, or
______________________ its direction.
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Types:
inclined plane, screw, wedge
pulley, wheel
lever
3
levers
• Work input Fd = Work output Fd
F
__
F
=
d
__
d
• Ex. Your hand moves 100m, causes car to
rise 0.10m. The force amplification factor
is,
100m

 1000
0.1m
inclined plane
• Weight x height change
•
= Force x distance along plane
• Force along ramp _________ than Weight
• Ramp distance ____________ than height
change
• ADA Standards: Ramp must be at least
12x longer than vertical rise
• Ex. A 1ft vertical rise requires ______ of
ramp.
5
Machine Efficiency
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= (work output)/(energy input) x 100%
Ex: 10J are input and 9J are output.
Efficiency = (9)/(10) x 100% = _______
Ex: 5J are input and 4J are output.
Efficiency = (4)/(5) x 100% = ________
• /
6
Energy of Motion
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Called Kinetic Energy (KE)
KE = ½(mass)(velocity)2 = ½mv2.
Ex. 2000kg car moving at 2m/s.
KE = ½ (2000)(2)2 = ___________
7
Work & Energy
• Work transforms energy from one ______
to another
• Work = DKE
• Ex. Calculate distance 100N must act to
move a 2000kg car from rest to 2m/s:
• Work = Fd = (100N)(d) = 4000 J
• d = 4000J/100N = _____________
• //
8
Power
• Power is the ________ work is performed
• Power = work/time = _________________
• Unit: watt = joule/second = J/s
• Other Unit: horsepower
• 1 horsepower = 746 watts
• /
9
Energy & Power
• Energy = power x time
• Ex. A toy car has 1000 J of energy at full
charge.
• How long can it run at 100 watts? At 10
watts?
• Time = Energy/power
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= 1000J/100watts = 10 seconds
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= 1000J/10watts = 100 seconds/
10
Energy & Efficiency
• 1 gallon gasoline has ________________
• Engines only get a fraction of this:
• Ex. A 25% efficient car gets
(0.25)(138,000,000 J) = 34,500,000J out
of 1 gallon.
• A 20% efficient car gets 27,600,000J.
11
Mpg (20% Efficient Engine)
• Work = Force x distance
• Ex. 400N for 1600 meters (1 mile)
• Work = (400N)(1600m) = 640,000J for one
mile traveled (1mile/640,000J)
• Engine gets 27,600,000J per gallon
1mile 27,600,000 J
Mpg 
 43miles / gallon
640,000 J
gallon
• (at constant speed)
12
Stop & Go Mpg
• Energy is used to speed car, and all is
_________ to _________ when stopping
• Mpg much less in stop & go conditions
• /
13
Size, Shape & Mpg
• Block shape creates __________ air
friction than rounded shape car
• Larger vehicles experience ________ air
friction
• Air friction = Shape factor x Frontal Size
• Ex. At 60mph, an SUV can experience
about 4x more air drag than a small car.
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14
Speed & Mpg
• For a given vehicle, air friction increases
with the ____________
• Ex. If you double your speed, the air
friction will increase by a factor of 4.
• /
15
Potential Energy
• … is energy due to __________________
• Ex. Book standing on one end has more
potential energy than when lying flat
• Ex. A ball 1m above floor has more
potential energy than when on the floor.
16
Gravitational Potential Energy
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= mass x gravity x height
Ex. A 2kg ball is 1m above the floor
Grav. Pot. Energy = (2kg)(10N/kg)(1m)
= 20 joules
Ex. A 10kg sack of rice 0.5m above the
floor has Grav. Pot. Energy
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= (10kg)(10N/kg)(0.5m)
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= 50 joules
17
Conservation of Energy
• Energy cannot be created or destroyed;
but is __________________ from one
form into another – the total amount
staying the same.
• Ex. A falling object _____________
Gravitational Energy as it falls, but
___________ an equal amount of Kinetic
Energy.
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18
Mechanical Energy
• = sum of ___________________ Energy
• Ex. A glider slides down an inclined air
track. The Mech. Energy = KE + mgh =
constant as the glider moves to lower
heights h.
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19
Summary
• work = Fd (F along d)
• work = 0 (F perpendicular to d)
• Power = work/time = Fv
• KE = ½mv2. GPE = mgh
• work = change in KE
• total energy always conserved
• machines & efficiency
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