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 work. • 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) = 200 J • / 2

Machines • change an applied force by increasing it, decreasing it, or changing 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 less than Weight Ramp distance greater than height change ADA Standards: Ramp must be at least 12 x longer than vertical rise • Ex. A 1 ft vertical rise requires 12 ft of ramp. 5

ADA Ramp 6

Machine Efficiency • • • = (work output)/(energy input) x 100% Ex: 10 J are input and 9 J are output. Efficiency = (9)/(10) x 100% = 90% Ex: 5 J are input and 4 J are output. Efficiency = (4)/(5) x 100% = 80% • / 7

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 = 4000 J. 8

Work & Energy • Work transforms energy from one form 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 = 40 meters • // 9

Power • Power is the rate work is performed • Power = work/time = Force x velocity • Unit: watt = joule/second = J/s • Other Unit: horsepower • 1 horsepower = 746 watts • / 10

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/ 11

Energy & Efficiency • 1 gallon gasoline has 138, 000 J • 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. 12

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) 13

Stop & Go Mpg • Energy is used to speed car, and all is lost to heat when stopping • Mpg much less in stop & go conditions • / 14

Size, Shape & Mpg • Block shape creates more air friction than rounded shape car • Larger vehicles experience more 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. • / 15

Speed & Mpg • For a given vehicle, air friction increases with the speed 2. • Ex. If you double your speed, the air friction will increase by a factor of 4. • / 16

Potential Energy • … is energy due to position & orientation • 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. 17

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 18

Conservation of Energy • Energy cannot be created or destroyed; but is transformed from one form into another – the total amount staying the same. • Ex. A falling object loses Gravitational Energy as it falls, but gains an equal amount of Kinetic Energy. • / 19

Mechanical Energy • = sum of Kinetic and Potential 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 21

7 -Ex 43 • Force 10 N object hits ground with. • If object sits at rest on ground, Then the answer is 10 N (net force = 0, a = 0)

Explain energy conservation • Forms of energy: motion, potential, spring, thermal. • Slide a book across table, all motion energy disappears. ? ? • Motion energy Thermal energy • What about your Energy Lab? • Potential energy motion energy

Force & work? ? • Work is a product of force and distance • No distance, no work • no force, no work

frictionless slide • • total energy is 100 J, KE = 30 J, PE? KE + PE = 100 J 30 + PE = 100 PE = 70

calculate KE • • KE = ½ mv 2. Ex: m = 1000 kg, v = 10 m/s KE = ½ (1000)(10)2. = 500(100) = 50, 000 J

work-energy • as long as you really calculate the net work done on an object, that value will always be the change in KE of that object.

Net Work • net work = sum of all work done on an object • net work = (net force*)(d) • *parallel to motion • // 28

Work-Energy Theorem • net work on object = change in kinetic energy of object (KE, final – KE, initial) • Ex. 4 kg mass falls from rest from height of 1. 0 m. • Work done by gravity = Wh = (4 x 10)(1) = 40 J • net work, 40 J = ½mv 2 – 0 • 40 = ½(4)v 2. • 40 = 2 v 2. • v = square-root of 20 = 4. 5 m/s 29