Work in addition to Energy Physics 100 Chapt 5 Physicist’s definition of “work” dist Work =

Work in addition to Energy Physics 100 Chapt 5 Physicist’s definition of “work” dist Work = www.phwiki.com

Work in addition to Energy Physics 100 Chapt 5 Physicist’s definition of “work” dist Work =

Wolfson, Kate, Executive Producer has reference to this Academic Journal, PHwiki organized this Journal Work in addition to Energy Physics 100 Chapt 5 Physicist’s definition of “work” dist Work = F x dist A scalar (not a vector) dist Atlas holds up the Earth But he doesn’t move, dist = 0 Work= Fx dist = 0 He doesn’t do any work!

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Garcon does work when he picks up the tray but not while he carries it around the room dist is not zero, but dist is 0 Why this definition Newton’s 2nd law: F=m a Definition of work + a little calculus Work= change in ½mv2 A scalar equation A vector equation This scalar quantity is given a special name: kinetic energy Work = change in KE This is called: the Work-Energy Theorem

Units again Kinetic Energy = ½mv2 kg m2 s2 work = F x dist N m =kg m s2 m =1Joule same! Work done by gravity start end dist dist W=mg Work = F x dist = -mg x change in height = -change in mg h change in vertical height Gravitational Potential Energy Workgrav = -change in mgh This is called: “Gravitational Potential Energy” (or PEgrav) Workgrav = -change in PEgrav change in PEgrav = -Workgrav

If gravity is the only as long as ce doing work . -change in mgh = change in ½ mv2 0 = change in mgh + change in ½ mv2 change in (mgh + ½ mv2) = 0 mgh + ½ mv2 = constant Work-energy theorem: Conservation of energy mgh + ½ mv2 = constant Gravitational Potential energy Kinetic energy If gravity is the only as long as ce that does work: PE + KE = constant Energy is conserved Free fall (reminder) V0 = 0 t = 0s V1 = 10m/s t = 1s V2 = 20m/s t = 2s V3 = 30m/s t = 3s V4 = 40m/s t = 4s 75m 60m 35m 0m height 80m

m=1kg free falls from 80m V0 = 0 h0=80m t = 0s V1 = 10m/s; h1=75m t = 1s V2 = 20m/s; h2=60m 600J 200J 800J t = 2s V3 = 30m/s; h3=35m 350J 450J 800J t = 3s V4 = 40m/s; h4=0 0 800J 800J t = 4s mgh ½ mv2 sum 800J 0 800J 750J 50J 800J pendulum W=mg T Two as long as ces: T in addition to W T is always to the motion (& does no work) Pendulum conserves energy hmax E=mghmax E=mghmax E=1/2 m(vmax)2

Roller coaster Work done by a spring Relaxed Position F=0 F x I compress the spring (I do + work; spring does -work) Work done by spring = – change in ½ kx2 Spring Potential Energy Workspring = -change in ½ kx2 This is the: “Spring’s Potential Energy” (or PEspring) Workspring = -change in PEspring change in PEspring = -Workspring

If spring is the only as long as ce doing work . -change in ½ kx2 = change in ½ mv2 0 = change in ½ kx2 + change in ½ mv2 change in ( ½ kx2 + ½ mv2) = 0 ½ kx2 + ½ mv2 = constant Work-energy theorem: Conservation of energy springs & gravity mgh + ½ kx2 + ½ mv2 = constant Gravitational potential energy Kinetic energy If elastic as long as ce & gravity are the only as long as ce doing work: PEgrav + PEspring + KE = constant Energy is conserved spring potential energy example KineticE Spring PE grav PE

Two types of as long as ces: “Conservative” as long as ces as long as ces that do + & – work Gravity Elastic (springs, etc) Electrical as long as ces “Dissipative” as long as ces as long as ces that only do – work Friction Viscosity . -work change in PE -work heat (no potential energy.) (-)Work done by frictionheat Thermal atomic motion Heat energy= KE in addition to PE associated with the r in addition to om thermal motion of atoms Air solid

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Work-energy theorem (all as long as ces) Workfric = change in (PE+KE) Work done dissipative Forces (always -) Kinetic energy -Workfric = change in heat energy potential energy From all Conservative as long as ces -change in Heat Energy = change in (PE+KE) Workfric = -change in heat energy Work – Energy Theorem (all as long as ces) 0 = change in Heat Energy + change in (PE+KE) 0 = change in (Heat Energy+PE+KE) Heat Energy + PE + KE = constant Law of Conservation of Energy Energy conversion while skiing Friction: energy gets converted to heat Potential energy Potential energykinetic energy

Units again Heat units: 1 calorie = heat energy required to raise the temp of 1 gram of H2O by 1o C 1 calorie= 4.18 Joules Kg m2/s2 Food Calories 1 Calorie = 1000 calories = 1Kcalorie 1 Calorie= 4.18×103 Joules The Calories you read on food labels 8 x 105 J 7 x 106 J 2 x 106 J Power Rate of using energy: amout of energy elapsed time Units: Joule second 1 = 1 Watt Power = A 100 W light bulb consumes 100 J of electrical energy each second to produce light

Other units Over a full day, a work-horse can have an average work output of more than 750 Joules each second 1 Horsepower = 750 Watts Kilowatt hours energy time Power = energy = power x time power unit x time unit = energy unit Elec companies use: Kilowatts (103 W) hours (3600 s) 1 kilowatt-hour = 1kW-hr = 103 W x 3.6×103 s = 3.6×106 Ws J HECO charges us about 15 cents /kW-hr x

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