Ch 8 Energy Notes

Name______________________________________Block______Date_______________Ch 8 Energy NotesMrs. PeckObjectives:Explain how sun is the ultimate source of energy for the planetDefine and describe work. 8.1Define and describe power. 8.2Define mechanical energy. 8.3Define kinetic energy and describe the work-energy theorem. 8.5State the law of conservation of energy. 8.6Describe simple machines and mechanical advantage. 8.7Explain why no machine can have an efficiency of 100%. 8.8Describe the role of energy in living organisms. 8.9Sun is the ultimate source of energy for the planet Earth8.1 Workwork- the product of the Force on and object and the distance thru wh/ the obj. is movedwork = force X distance SI unit: Joule (J) other unit: N . mW = F done joule of work done if force of 1 N exerted overdistance of 1 mkilojoules (kJ).......thousand joulesmegajoules (MJ) .....million joulesW = F dW ~ FW ~ dwork is done on an obj that moves: if no mvmt then no work donework is done on an obj:1. force exerted on obj (therefore changes velocity)2. object moves a distance2 categories of work: 1. work done against another force...moving it against an opposing force(eg. gravity)2. work done to change velocity of something(no work done if move object horizontally at constant velocity)? Mr. Manella has a mass of 100 kg, and stands on a pogo stick. How much work is done asMr. Manella compresses the spring of the pogo stick .05m?

8.2 Power 2power- rate at which work is done or Energy is transformed,equal to work done or Energy transformed per unit of timepower is how fast the work is donepower = work done P = W SI unit : Watt (W) other unit: J/stime intervalt1 watt is power expended when 1J of work isdone in 1 secondkilowatt (kW) 1000 wattsmegawatt (MW) 1,000,000 wattsP = WtP ~ WP ~ 1t( work = force X distance)P = W = Fd P ~ F W ~ F twice the force ....twice the powert t twice the time.......half the power*if have twice power then 1. doubled the force in same amount of time2. double the distance in same amount of time3. same work done in half the amount of timein US rate engine power in horsepower instead of watts1hp = .75 kW? Calculate the power expended when a 250 kg barbell is lifted 2.2m in 2 s?How much work is done to keep holding it up once it is already raised?

8.3 Mechanical Energy (ME) 3energy- the property of an object that enables it to do workenergy (E) is measured in Joules (J)many forms of energy: mechanical E thermal or heat E radiant Epotential E chemical Ekinetic Enuclear Emechanical energy- Energy due to position or movement of somethingmechanical E is potential E (PE) , kinetic E (KE), or combination of both8.4 Potential Energy (PE)potential energy- Energy of positionusually related to relative position of 2 thingseg. cement block raised 2 meters from floor..............block & floorcement block held outside of 3 story window.......block & groundcement block held outside of airplane.................block & ground*an object has potential energy because it is energy in a stored state...it has thepotential of doing workeg: bow string drawn back.............................bow has potential of doing work on the arrowrubber band stretched back.....................band has potential of doing work on a paper waspcement block raised above your foot ........block has potential of doing work on your foot!chemical energy- is potential energy located in the bonds of moleculessubstance with chemical energy has potential to do workeg: fossil fuels, electric batteries, food we eat (glucose and fat)*any substance capable of doing work through chemical action possesses PEFossil Fuels: 1. 2. 3.Fossil fuels: 1. plants transformed sun’s energy (radiant E) into chemical E stored in bondsof glucose during photosynthesis (chem E is PE)2. animals eat plants and store chemical energy.....fats and in liver3. plants & animals die....1000’s years heat & pressure convert organicmatter into fossil fuels4. burn fossil fuels to transform PE into KE...to do work...Electricity Production in USnuclear energynuclear fission.....heat water....produces steam....turn turbines.....turns generators....which generates electricitypower plant (coal)burn fossil fuel.....heat water....produces steam....turn turbines.....turns generators....which generates electricity

8.4 Potential Energy (PE) 4gravitational potential energy (PEg)-PE due to an elevated positionwork was required to elevate the object against gravity...transformed into PEamt of PEg is = work (force needed to lift it a given distance-height against gravity)PEg = W = Fd symbol: PEg SI unit: Joule (J)PEg = W = FdPEg ~ WPEg ~ FPEg ~ dupward force is equal to the objects weightw = mgPEg = mghh is the height in meters lifted above reference pt (ground, floor)PEg = mghPEg ~ mPEg ~ hPEg depends ONLY on weight of object and height of elevated positionPEg does NOT depend on PATH TAKEN to get object raised to height of elevatedpositionfig. 8.3 same PE in each boulder because same Force (w=mg= 100N) and same height (2m)the path getting up to 2 m does not influence the PE? If you lift a 2 kg box 3m, how much gravitational PE does the box have? What would be thePEg if you lift it twice as high?

8.4 Potential Energy (PE) 5chemical PEgravitational PEelastic PE- potential energy stored in elastic materials as the result of their stretching orcompressing.elastic PE can be stored in: rubber bandsbungee chordstrampolinesspringsarrow drawn into a bowamt. of elastic PE stored is related to the amt. of stretch of the devicethe more the stretch......the more the stored energy (elastic potential energy)springs are a special case: can have elastic PE due to compression or stretchingthe more the stretch or compression....the more the elastic PE8.5 Kinetic Energykinetic energy- energy of motion, equal to half the mass multiplied by the speed squaredan object moving is capable to do workenergy of motion: can be horizontal or vertical motionKE depends on object’s mass and on its speedKE= 1 mv 22SI unit: Joule (J)KE of a moving object = work needed to bring it to that speed from rest400 kg car moving at 10m/ s has what KE?KE= 1 mv 22a car has 20,000 J of KE....it took 20,000 J of work (Fd) to get it up to 10 m/sKE of a moving object = work object can do while being brought to resta car has 2000J of KE....it can do 20,000J of work on road while stopping (W=Fd)

8.5 Kinetic Energy 6KE is is directly proportional to square of velocity v 2 see fig. 8.5if velocity doubles........KE is quadrupled....takes 4 times the work to double the speedif velocity doubled................KE is increased by factor of 450 kg car is going 5 m/ s.....KE =50 kg car is going 10 m/ s....KE =car with v= 100 km/ h has twice velocity of same car with v = 50 km/ h.car going 100 km/ h has 4 times KE than it would going 50 km/ hcar going 100 km/ h would skid 4 times as far (when brakes lock) than going 50 km/ hif velocity triples....KE is 9 times as much.....takes 9 times the work to triple the speedif velocity triples...................KE is increased by factor of 9if velocity triples.......it will skid 9 times as farif velocity is quadrupled.......KE is 16 times as muchif velocity is quadrupled.................KE is increased by a factor of 16if velocity is quadrupled........it will skid 16 times as much? What is the kinetic energy of a 1000 kg car moving at 20 m/ s?? If the car above were moving with twice the speed, then what would be its newkinetic energy?? A diver had a kinetic energy of 15,000 J just prior to hitting the water. If the diver’smass was 50 kg, then what was her velocity?

work-energy theorem- theorem that states that whenever work is done, energy changes7work changes KE W = ∆E W = Fd ∆E = KE = 1 mv 22Fd = 1 mv 228.6 Conservation of Energywhen energy ∆’s from one form to another, it always transforms without a net loss or gainlaw of conservation of energy- energy cannot be created or destroyed, it can betransformed from one form into another, but the total amountof energy never changesPE in fossil fuels or in food (glucose or fat) can be transformed into KE & or thermal Ewhen gasoline combines with oxygen...chemical PE is ∆’d into molecular KE or thermal Erub hands together transform mechanical energy into heatthe total mechanical energy is equal to the sum of potential energy and kinetic energyfig. 8.7 sum of PE and KE remains constant at all positions of free fall. All PE is being transformedinto KE

8.7 Machines 8machine- a device for increasing a force or simply changing direction of a forcemachines multiply forces or ∆ direction of forceswork input = work outputFd (input) = Fd (output)actual mechanical advantage AMA- the ratio of output force to input force for a machineAMA = output F MA tells you how many times the machine multiplies force youinput F put into the machineideal mechanical advantage- IMA is the MA without friction, it is the theoretical MA based onlyon the geometry of the machine (distance)IMA= distance in (effort distance) IMA = ∆ dindistance out (resistance distance) ∆ doutsome E is always transformed into heat & no machine can ever be 100% efficient & cannever generate more E than is put into machineefficiency- the ratio of useful energy output to total energy input, or the % of the work inputthat is converted to work outputno machine is perfect, always get out less work than you put in. The more efficient themachine, greater work output you will get for your work input. Efficiency will always beless than 100%efficiency = work output = AMA X 100work input IMAhas no units, expressed as a percent

8.7 Machines 9machines multiply the forces put into them3 common simple machines: lever, pulley, inclinelever- simple machine, made of a bar that turns about a fixed pointfulcrum- the pivot point of a leverOutputInputdFFdd output is distance objgets liftedFoutput is Force of objwhich is force needed tolift its weight of objd input is distancethat i push down leverF input is force thati use to push down leverFd (input) = Fd (output)? How much force must you use to push down a lever 1m in order to lift an 8 kgrock up a distance of .8m?

8.7 Machines 10pulley- a type of lever that is a wheel with a groove in its rim, wh/ is used to ∆ thedirection of a force exerted by a rope or cable.MA is = to # of ropes that actually support objectincline plane- a simple machine that enables an object to be slid up at an anglewhich requires less force than lifting it verticallyd in is the dist that i actuallypush the obj. up the inclineFin is the force that i actuallyuse to push the obj up inclineFddF out is = to obj. weightit is the force needed tolift the object a given heightdout is goal heightit is the desiredheight to raise obj.? A 3m long ramp is resting on the back of a pickup truck that is 1m high. How muchforce is needed to push a 36kg chair to the top of the ramp.