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Energy
• conservation of energy• work, energy, and power• machines & efficiency
• Homework:• RQ: 3, 4, 5,10, 12, 13, 15, 18, 30.• Ex: 23, 26, 28, 37, 49, 62.• Problems: 1, 5, 6.
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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) = _______________
• /
3
Machines
• change an applied force by ___________ it, _________________________ it, or ______________________ its direction.
• Types:
• inclined plane, screw, wedge
• pulley, wheel
• lever
levers• Work input Fd = Work output Fd
• Ex. Your hand moves 100m, causes car to rise 0.10m. The force amplification factor is,
FF
dd
=__ __
10001.0
100 mm
5
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.
6
Machine Efficiency
• = (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% = ________
• /
Energy of Motion
• Called Kinetic Energy (KE)
• KE = ½(mass)(velocity)2 = ½mv2.
• Ex. 2000kg car moving at 2m/s.
• KE = ½ (2000)(2)2 = ___________
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8
Work & Energy
• Work transforms energy from one ______ to another
• Work = KE
• 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 = _____________
• //
9
Power• Power is the ________ work is performed• Power = work/time = _________________• Unit: watt = joule/second = J/s
• Other Unit: horsepower• 1 horsepower = 746 watts• /
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
• = 1000J/100watts = 10 seconds
• = 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.
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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
• (at constant speed)12
gallonmilesgallon
J
J
mileMpg /43
000,600,27
000,640
1
Stop & Go Mpg
• Energy is used to speed car, and all is _________ to _________ when stopping
• Mpg much less in stop & go conditions
• /
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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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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.
• /
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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.
Gravitational Potential Energy
• = 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
• = (10kg)(10N/kg)(0.5m)
• = 50 joules17
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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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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.
• /
20
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