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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

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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.

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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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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 = _____________

• //

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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.

• /

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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