1 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.

2 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 F d = Work output F d Ex. Your hand moves 100m, causes car to rise 0.10m. The force amplification factor is, F F d d = __

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 = ½mv 2. Ex. 2000kg car moving at 2m/s. KE = ½ (2000)(2) 2 = ___________ 7

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. 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 (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. / 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

16 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 joules 17

18 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. /

19 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 = ½mv 2. GPE = mgh work = change in KE total energy always conserved machines & efficiency