Energy conservation of energy work energy and power

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. 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. 50 N pushes distance of 4 meters. • W = (50 N)(4 m) = ________ • / 2

Machines • change an applied force by ______ it, _____________ it, or ___________ its direction. • • Types: inclined plane, screw, wedge pulley, wheel lever 3

levers • Work input Fd = Work output Fd F __ F = d __ d • Ex. Your hand moves 100 m, causes car to rise 0. 10 m. The force amplification factor is,

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 12 x longer than vertical rise • Ex. A 1 ft vertical rise requires ______ of ramp. 5

Machine Efficiency • • • = (work output)/(energy input) x 100% Ex: 10 J are input and 9 J are output. Efficiency = (9)/(10) x 100% = _______ Ex: 5 J are input and 4 J are output. Efficiency = (4)/(5) x 100% = ____ • / 6

Energy of Motion • • Called Kinetic Energy (KE) KE = ½(mass)(velocity)2 = ½mv 2. Ex. 2000 kg car moving at 2 m/s. KE = ½ (2000)(2)2 = ______ 7

Work & Energy • Work transforms energy from one ______ to another • Work = DKE • Ex. Calculate distance 100 N must act to move a 2000 kg car from rest to 2 m/s: • Work = Fd = (100 N)(d) = 4000 J • d = 4000 J/100 N = _______ • // 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 • = 1000 J/100 watts = 10 seconds • = 1000 J/10 watts = 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 J) = 34, 500, 000 J out of 1 gallon. • A 20% efficient car gets 27, 600, 000 J. 11

Mpg (20% Efficient Engine) • Work = Force x distance • Ex. 400 N for 1600 meters (1 mile) • Work = (400 N)(1600 m) = 640, 000 J for one mile traveled (1 mile/640, 000 J) • Engine gets 27, 600, 000 J per 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 60 mph, an SUV can experience about 4 x more air drag than a small car. • / 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 1 m above floor has more potential energy than when on the floor. 16

Gravitational Potential Energy • • • = mass x gravity x height Ex. A 2 kg ball is 1 m above the floor Grav. Pot. Energy = (2 kg)(10 N/kg)(1 m) = 20 joules Ex. A 10 kg sack of rice 0. 5 m above the floor has Grav. Pot. Energy • = (10 kg)(10 N/kg)(0. 5 m) • = 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. • / 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. • / 19

Summary • work = Fd (F along d) • work = 0 (F perpendicular to d) • Power = work/time = Fv • KE = ½mv 2. GPE = mgh • work = change in KE • total energy always conserved • machines & efficiency 20