WORK and ENERGY Work Kinetic Energy Work Energy Theorem Potential Energy Conservation of Energy Power

WORK and ENERGYWORK and ENERGYWorkWorkKinetic EnergyKinetic EnergyWork Energy TheoremWork Energy TheoremPotential EnergyPotential EnergyConservation of EnergyConservation of EnergyPowerPower

WORKWORKWork is a transfer of energyWork is a transfer of energyForce causing an object to have a Force causing an object to have a displacement displacement Maximum work: F and d are parallelMaximum work: F and d are parallelMinimum work: F and d are perpendicularMinimum work: F and d are perpendicularW = FdW = FdUnits: NUnits: N∙∙m m SI Units: Joules (J) SI Units: Joules (J) ScalarScalarIndependent of pathwayIndependent of pathway

http://www.nu.ac.za/physics/1M2002/Energy%20work%20and%20power.htm

WORKWORK

Importance of signImportance of sign

+ W: F and d are in the same direction+ W: F and d are in the same direction

- W: F and d are in the opposite direction- W: F and d are in the opposite direction

http://www.physics.upenn.edu/courses/gladney/phys150

WORKWORK

Hewitt Physics

WORKWORK

Graphing WorkGraphing Work

Area under the curve of the F vs. d graph.Area under the curve of the F vs. d graph.

WORKWORK

Independent of pathwayIndependent of pathway

ENERGYENERGY

Ability to do workAbility to do work

Two typesTwo types Kinetic EnergyKinetic Energy Potential EnergyPotential Energy

Heat measures the transfer of energy.Heat measures the transfer of energy.

KINETIC ENERGYKINETIC ENERGYEnergy of motionEnergy of motionKE = KE = ½ m v½ m v22

Units: kg mUnits: kg m22 / s / s22 = Joule = JouleScalarScalarWork-Energy TheoremWork-Energy Theorem Net work is equal to the change in energyNet work is equal to the change in energy W = W = KE KE Fd = KEFd = KEff - KE - KE°°

Fd = Fd = ½ m v½ m vff2 2 - ½ m v- ½ m v°°

22 = ½ m (v = ½ m (vff2 2 - v- v°°

22))

Pg 184 #1-22Pg 184 #1-221)1) NoNo2)2) Yes +; no; Yes +; yes –Yes +; no; Yes +; yes –3)3) No; force decrease, No; force decrease,

distance increase; work distance increase; work constantconstant

4)4) Tension and Weight are Tension and Weight are perpendicular so no workperpendicular so no work

5)5) Longer skids are moving Longer skids are moving fasterfaster

6)6) Yes; no; yes; weight and air Yes; no; yes; weight and air resistanceresistance

7)7) 53 J; - 53 J53 J; - 53 J8)8) 2.4 x 102.4 x 1055 J J9)9) 47.5 J47.5 J

10) 6230 J; - 6230 J; 0.64010) 6230 J; - 6230 J; 0.64011) no, yes, yes11) no, yes, yes12) no; mass is + and v is 12) no; mass is + and v is

squaredsquared13) yes; depends on the distance13) yes; depends on the distance14) 1 to 2514) 1 to 2515) No work; motion and force 15) No work; motion and force

are perpendicularare perpendicular16) Speed double; work is 16) Speed double; work is

quadrupled…140 m.quadrupled…140 m.17) To climb work must be done 17) To climb work must be done

against gravity.against gravity.18) Work done by friction equals 18) Work done by friction equals

particle speeds decrease.particle speeds decrease.

POTENTIAL ENERGYPOTENTIAL ENERGY

Store energy Store energy Able to do work laterAble to do work laterUnits: kgmUnits: kgm22/s/s22 = Joules = JoulesScalarScalarTwo main typesTwo main types Gravitational potential energyGravitational potential energy Elastic potential energyElastic potential energy

GRAVITATION POTENTIAL ENERGYGRAVITATION POTENTIAL ENERGY

Energy possessed by object Energy possessed by object because of its position in a because of its position in a gravitational field.gravitational field.W = Fd W = Fd PE = mghPE = mghZero Gravitation Potential Energy Zero Gravitation Potential Energy is the point of reference is the point of reference

Pg 172 Sec Rev; pg 185 23-25Pg 172 Sec Rev; pg 185 23-25

1)1) 4.4 x 104.4 x 10-3-3 J J

2)2) 2.8 m/s2.8 m/s

3)3) 6.18 x 106.18 x 10-2-2 J J

4)4) Kinetic; Kinetic; nonmechanical; nonmechanical; kinetic, gravitational; kinetic, gravitational; elastic potentialelastic potential

5)5) Answers varyAnswers vary

23) a) 5400 J, 0 J, 5400 J23) a) 5400 J, 0 J, 5400 J

b) 0 J, -5400 J; 5400 Jb) 0 J, -5400 J; 5400 J

c) 2700 J, -2700 J, c) 2700 J, -2700 J, 5400 J5400 J

24) –19.6 J; 39.2 J; 0 J24) –19.6 J; 39.2 J; 0 J

25) 0.400 J; 0.225 J; 0J25) 0.400 J; 0.225 J; 0J

ELASTIC POTENTIAL ENERGYELASTIC POTENTIAL ENERGY

Potential energy stored in the Potential energy stored in the deformation (compression or stretched) deformation (compression or stretched) of an elastic object.of an elastic object.Hooke’s Law Hooke’s Law Restoring force Restoring force F = -kxF = -kx

W = Fd W = Fd

PE = PE = ½ k x½ k x22

Units: kg mUnits: kg m22 / s / s22 = Joule = Joule

CONSERVATION OF ENERGYCONSERVATION OF ENERGY

First Law of Thermodynamics First Law of Thermodynamics Energy is never created or loss; it is just Energy is never created or loss; it is just transfer from one form to another.transfer from one form to another.Energy before = energy after in an isolated Energy before = energy after in an isolated system.system.Second Law of ThermodynamicsSecond Law of ThermodynamicsTransfer of energyTransfer of energyMechanical Energy is the total energyMechanical Energy is the total energyTE = TE = KE + KE + PE (conserved)PE (conserved)

TE = TE = KE + KE + PE + WPE + Wf f (not conserved)(not conserved)


Energy is never created or loss; it is just Energy is never created or loss; it is just transfer from one form to another.transfer from one form to another.

Energy before = energy after in an isolated Energy before = energy after in an isolated system.system.

Mechanical Energy is the total energyMechanical Energy is the total energy

TE = TE = KE + KE + PE (conserved)PE (conserved)

TE = TE = KE + KE + PE + WPE + Wf f (not conserved)(not conserved)

Pg 186 #26-34, 37Pg 186 #26-34, 3726) nonmechanical; 26) nonmechanical;

mechanical; mechanical; mechanical; mechanical; mechanical; bothmechanical; both

27) KE Max at bottom27) KE Max at bottomPE Max at top; KE becomes PE Max at top; KE becomes

PE going up and PE PE going up and PE becomes KE going downbecomes KE going down

28) Conservation of energy 28) Conservation of energy and will not hit the and will not hit the instructor; give a push instructor; give a push then it will hitthen it will hit

29) Does work on it to move 29) Does work on it to move up; no work, d = 0; does up; no work, d = 0; does negative work against negative work against gravitygravity

30) KE max at the bottom; 30) KE max at the bottom; PE max at the topPE max at the top

31) No, energy is not 31) No, energy is not conservedconserved

32) Gravitational and 32) Gravitational and elastic; conservation of elastic; conservation of energy.energy.

33) 12.0 m/s33) 12.0 m/s

34) 10.9 m/s; 11.6 m/s34) 10.9 m/s; 11.6 m/s

37) 0.633 J; 0.633 J; 2.43 37) 0.633 J; 0.633 J; 2.43 m/s; 0.422 J, 0.211Jm/s; 0.422 J, 0.211J


Hewitt Physics


Hewitt Physics

NEXT-TIME QUESTIONNEXT-TIME QUESTION

Three baseballs are Three baseballs are thrown from the top of thrown from the top of the cliff along paths A, B the cliff along paths A, B and C. If their initial and C. If their initial speeds are the same and speeds are the same and there is no air resistance, there is no air resistance, the ball that strikes the the ball that strikes the ground below with the ground below with the greatest speed will follow greatest speed will follow path path

If two balls start simultaneously with the some initial speed, the ball to complete the journey first is along

If the initial speed equals 2 m/sec, and the speed of the ball at the bottom of the curve on Track B is 3 m/sec, then the speed of the ball at the top of the curve on Track A is

Two smooth tracks of equal length have, "bumps" - A up, and B down, both of the same curvature.

NEXT-TIME QUESTION

POWERPOWERRate work is done or which energy is Rate work is done or which energy is transferredtransferred

P = W / tP = W / t

= Fd / t= Fd / t

= F v= F v

Units: J/s = Watts (W)Units: J/s = Watts (W)

CONCEPTSCONCEPTSAs a consultant to the soft-drink industry, Dr. J is given the task of conducting the ultimate Pepsi As a consultant to the soft-drink industry, Dr. J is given the task of conducting the ultimate Pepsi taste test. This is Dr. J's tenth taste test, which puts him seven up on his nearest consultant, who taste test. This is Dr. J's tenth taste test, which puts him seven up on his nearest consultant, who had only done three. Of course Dr. J is very qualified, having been hooked on soft drinks had only done three. Of course Dr. J is very qualified, having been hooked on soft drinks (especially orange soda) since he was (especially orange soda) since he was NehiNehi to a pop bottle. Dr. J mounts a rather large container to a pop bottle. Dr. J mounts a rather large container of Pepsi on a ledge some 3 meters above the ground. A bullet of mass 5 grams is then fired into of Pepsi on a ledge some 3 meters above the ground. A bullet of mass 5 grams is then fired into the container, thus killing the taste. Not only that, but the Pepsi falls through the bullet hole onto the container, thus killing the taste. Not only that, but the Pepsi falls through the bullet hole onto the ground below (causing the taste to go flat). The wall of the container is 2 cm thick. The the ground below (causing the taste to go flat). The wall of the container is 2 cm thick. The velocity of the bullet changes from an initial value of 500 m/sec just before striking the container velocity of the bullet changes from an initial value of 500 m/sec just before striking the container wall to 5 m/sec upon leaving the container wall and entering the Pepsi. It finally fizzles out at a wall to 5 m/sec upon leaving the container wall and entering the Pepsi. It finally fizzles out at a point 25 cm from the container wall. point 25 cm from the container wall.

A. How much work does the container wall do on the bullet?A. How much work does the container wall do on the bullet?

How much work does the Pepsi do on the bullet? How much work does the Pepsi do on the bullet?

At what velocity does the Pepsi hit the floor? At what velocity does the Pepsi hit the floor?

Chapter 10: R pg 199Chapter 10: R pg 199

1) A force of 825 N is needed to push a 1) A force of 825 N is needed to push a car across a lot. Two students push the car across a lot. Two students push the car 35m.car 35m.a) How much work is done?a) How much work is done?b) After a rainstorm, the force needed to b) After a rainstorm, the force needed to push the car doubled because the ground push the car doubled because the ground became soft. By what amount does the became soft. By what amount does the work done by the students change?work done by the students change?29000J; work doubles29000J; work doubles


2) A delivery clerk carries a 34 N package 2) A delivery clerk carries a 34 N package from the ground to the fifth floor of an from the ground to the fifth floor of an office building, a total height of 15 m. How office building, a total height of 15 m. How much work is done by the clerk?much work is done by the clerk?

510 J510 J

3) What work is done by a forklift raising a 3) What work is done by a forklift raising a 583 kg box 1.2 m?583 kg box 1.2 m?

6900 J6900 J

Chapter 10: R pg 199-202Chapter 10: R pg 199-202

4) You and a friend each carry identical boxes to 4) You and a friend each carry identical boxes to a room one floor above you and down the hall. a room one floor above you and down the hall. You choose to carry it first up the stairs, then You choose to carry it first up the stairs, then down the hall. Your friend carries it down the down the hall. Your friend carries it down the hall, then up another stairwell. Who does more hall, then up another stairwell. Who does more work?work?Same amount of workSame amount of work5) How much work does the force of gravity do 5) How much work does the force of gravity do when a 25 N object falls a distance of 3.5 m? when a 25 N object falls a distance of 3.5 m? 88 J88 J

Chapter 10: R pg 202Chapter 10: R pg 2026) An airplane passenger carries a 215 N suitcase up 6) An airplane passenger carries a 215 N suitcase up stairs, a displacement of 4.20 m vertically and 4.60 m stairs, a displacement of 4.20 m vertically and 4.60 m horizontally.horizontally.a) How much work does the passenger do?a) How much work does the passenger do?b) The same passenger carries the same suitcase back b) The same passenger carries the same suitcase back down the same stairs. How much work does the down the same stairs. How much work does the passenger do now?passenger do now?903 J; -903 J903 J; -903 J7) A rope is used to pull a metal box 15.0 m across the 7) A rope is used to pull a metal box 15.0 m across the floor. The rope is held at an angle of 46.0 floor. The rope is held at an angle of 46.0 ° with the floor ° with the floor and a force of 628 N is used. How much work does the and a force of 628 N is used. How much work does the force on the rope do?force on the rope do? 6540 J6540 J


8) A worker pushes a crate weighing 93 N 8) A worker pushes a crate weighing 93 N up an inclined plane, pushing horizontally, up an inclined plane, pushing horizontally, parallel to the ground in the figure.parallel to the ground in the figure.a) The worker exerts a force of 85 N. How a) The worker exerts a force of 85 N. How much work does he do?much work does he do?b) How much work is done by gravity?b) How much work is done by gravity?c) The coefficient of friction is c) The coefficient of friction is = 0.20. = 0.20. How much work is done by friction?How much work is done by friction?340 J; -279 J; 130 J340 J; -279 J; 130 J

Answers: Chapter 10Answers: Chapter 101) 1) 800 J800 J

2) 12000 J2) 12000 J3) 59.9 kg3) 59.9 kg4) 1.86 x 105 J4) 1.86 x 105 J5) 0.80 J5) 0.80 J6) 25 N/m; 0.50 J6) 25 N/m; 0.50 J7) 600 J7) 600 J8) 826 J; 1.13 x 10 8) 826 J; 1.13 x 10 4 4 J; - 1.13 x 10 J; - 1.13 x 10 4 4 JJ9) 1.20 x 10 9) 1.20 x 10 4 4 JJ10) 58.7 degrees10) 58.7 degrees11) 1.8 x 10 11) 1.8 x 10 4 4 J J 12) no work12) no work13) 7.7 J13) 7.7 J14) 518 J14) 518 J


9) A box that weighs 575 N is lifted a 9) A box that weighs 575 N is lifted a distance of 20.0 m straight up by a rope. distance of 20.0 m straight up by a rope. The job is done in 10.0 s. What power is The job is done in 10.0 s. What power is developed in watts and kilowatts? developed in watts and kilowatts?

1150 W; 1.15 kW1150 W; 1.15 kW


10) A rock climber wears a 7.50 kg knapsack 10) A rock climber wears a 7.50 kg knapsack while scaling a cliff. After 30.0 min, the while scaling a cliff. After 30.0 min, the climber is 8.2 m above the starting point.climber is 8.2 m above the starting point.a) How much work does the climber do on the a) How much work does the climber do on the knapsack?knapsack?b) If the climber weighs 645 N, how much work b) If the climber weighs 645 N, how much work does she do lifting herself and the knapsack?does she do lifting herself and the knapsack?c) What is the average power developed by c) What is the average power developed by the climber?the climber?600 J; 5900 J; 3.3 W600 J; 5900 J; 3.3 W


11) An electric motor develops 65 kW of 11) An electric motor develops 65 kW of power as it lifts a loaded elevator 17.5 m in power as it lifts a loaded elevator 17.5 m in 35.0s. How much force does the motor 35.0s. How much force does the motor exert? exert?

1.3 x 101.3 x 1055 N N

Chapter 10: R pg 203Chapter 10: R pg 20312) Two cars travel the same speed, so that they move 105 km 12) Two cars travel the same speed, so that they move 105 km in 1 h. One car, a sleek sports car, has a motor that delivers in 1 h. One car, a sleek sports car, has a motor that delivers only 35kW of power at this speed. The other car needs its only 35kW of power at this speed. The other car needs its motor to produce 65 kW to move the car this fast. Forces motor to produce 65 kW to move the car this fast. Forces exerted by friction from the air resistance cause the difference.exerted by friction from the air resistance cause the difference.a) For each car, list the external horizontal forces exerted in it, a) For each car, list the external horizontal forces exerted in it, and give the cause of each force. Compare their magnitudes.and give the cause of each force. Compare their magnitudes.b) By Newton’s third law, the car exerts forces. What are their b) By Newton’s third law, the car exerts forces. What are their directions?directions?c) Calculate the magnitude of the forward frictional force exerted c) Calculate the magnitude of the forward frictional force exerted by each car?by each car?d) The car engines did work. Where did the energy they d) The car engines did work. Where did the energy they transferred come from?transferred come from?Road on car; air on car; 1200 N; 2200N; chemical energyRoad on car; air on car; 1200 N; 2200N; chemical energy

Answer Chapter 10: pg214Answer Chapter 10: pg214

15) 7400J15) 7400J16) 800 J; 600 J16) 800 J; 600 J17) -5.53 x 10 17) -5.53 x 10 3 3 J; no work; 5.53 x 10 J; no work; 5.53 x 10 3 3 J; no; 2.2 J; no; 2.2 kWkW18) 9000 J; 3.00 kW18) 9000 J; 3.00 kW19) 348 W; 696 W19) 348 W; 696 W20) 220 J; 110W20) 220 J; 110W21) 110 kJ; 3.14 kW21) 110 kJ; 3.14 kW22) 1.8 x 10 22) 1.8 x 10 4 4 J; 2.3 kWJ; 2.3 kW23) 160 W23) 160 W

Answer Chapter 10: pg214Answer Chapter 10: pg214

24) 54.7 m24) 54.7 m

25) 368 W25) 368 W

26) 90 kW26) 90 kW

27) 2890 N27) 2890 N

28) 2300 N28) 2300 N

Chapter 10: R pg 203Chapter 10: R pg 20312) Two cars travel the same speed, so that they move 105 km 12) Two cars travel the same speed, so that they move 105 km in 1 h. One car, a sleek sports car, has a motor that delivers in 1 h. One car, a sleek sports car, has a motor that delivers only 35kW of power at this speed. The other car needs its only 35kW of power at this speed. The other car needs its motor to produce 65 kW to move the car this fast. Forces motor to produce 65 kW to move the car this fast. Forces exerted by friction from the air resistance cause the difference.exerted by friction from the air resistance cause the difference.a) For each car, list the external horizontal forces exerted in it, a) For each car, list the external horizontal forces exerted in it, and give the cause of each force. Compare their magnitudes.and give the cause of each force. Compare their magnitudes.b) By Newton’s third law, the car exerts forces. What are their b) By Newton’s third law, the car exerts forces. What are their directions?directions?c) Calculate the magnitude of the forward frictional force exerted c) Calculate the magnitude of the forward frictional force exerted by each car?by each car?d) The car engines did work. Where did the energy they d) The car engines did work. Where did the energy they transferred come from?transferred come from?Road on car; air on car; 1200 N; 2200N; chemical energyRoad on car; air on car; 1200 N; 2200N; chemical energy

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WORK and ENERGY Work Kinetic Energy Work Energy Theorem Potential Energy Conservation of Energy Power