Unit 3 : Energy

1. Unit 3: Energy Energy can change from one form to another without a net loss or gain.

2. Work Work: • transfer of energy through motion • forceapplied overadistance (no move, no work) • unit of measure is joules(J) W: work (J) F: force (N) d: distance (m) W = Fd • Name 2 ways to increase work… • more force or more distance

3. golfer does work on club club does work on ball energy in golfer energy in club energy in ball Work is the transfer of energy by applying forceover a distance.

4. Work How much work is the lifter doing while holding it above her head? NONE!! because… The force applied is over zero distance (no motion) W = F x d

5. Work Brett’s backpack weighs 30 N. How much work is done on the backpack when he lifts it 1.5 m from the floor to his back? W = Fd F = 30 N d = 1.5 m W = (30 N)(1.5 m) W=45 J

6. Work • Sometimes work is done against another force. • An archer stretches her bowstring, doing work against the elastic forces of the bow. • When any object is lifted up, • work is done against the • force of gravity. • When you do push-ups, • you do work against your • own weight.

7. Power When carrying a load up some stairs, you do the same amount of work whether you walkorrun up the stairs. Power: • rateat which workisdone • unit of measure is watts (W) P: power (W) W: work (J) t: time (s) W t P = • Name 2 ways to increase power… • more work or less time(faster)

8. Power Rock climbers do a lot of work at a slowrate. Their power is small. A powerful lineman is STRONG (applies a big force) and FAST (moves objects in small times).

9. Power A figure skater lifts his 450 N partner 1.0 m in 3.0 s. How much power is required? W t F = 450 N d = 1.0 m t = 3.0 s P = W = Fd 450 J W = (450 N)(1.0 m) = P = 450 J 3.0 s P = 150 W

10. Power • A high-power engine does work rapidly. • An engine with twice the power of another engine does not necessarily produce twice as much work or go twice as fast. • Twice the power means the engine can do twice the work in the sametime, or the samework in half the time. • A powerful engine can get an automobile up to a given speed in less time than a less powerful engine can.

11. Quick Quiz! • Raising a car up in a service station requires work. Raising it twice as high requires… • half as much work. • the same work. • twice the work. • four times the work.

12. Quick Quiz. • Raising an auto in a service station requires work. Raising it in half the time requires • half the power. • the same power. • twice the power. • four times the power.

13. Forms of Energy When work is done by an archer in drawing back a bowstring... • something has been acquired that enables the object to do work. • It may be in different forms: • compression of atoms in an object • a separation of attracting bodies • rearrangement of electric charges in the • molecules of a substance.

14. Forms of Energy Energy: the ability todo work transferof energythrough motion unit of measure is joules(J) Mechanical Energy: energy due to the positionor the movement of something.

15. Forms of Energy • Potential Energy(PE) • stored energy • depends on position Elastic PE – stretched Chemical PE – in bonds Gravitational PE – height & weight Which boulder has greater gravitational PE? PE = mgh

16. Forms of Energy • The potential energy of the 100 N boulder with respect to the ground below is 200 J in each case. • lifted with 100 N of force

17. Forms of Energy • The potential energy of the 100 N boulder with respect to the ground below is 200 J in each case. • lifted with 100 N of force • pushed up a 4 m incline with 50 N of force

18. Forms of Energy • The potential energy of the 100 N boulder with respect to the ground below is 200 J in each case. • lifted with 100 N of force • pushed up a 4 m incline with 50 N of force • lifted with 100 N of force up 4 stairs, 0.5 m each same PE = mgh same W = Fd

19. Forms of Energy think! You lift a 100 N boulder 1 m. a. How much work is done on the boulder? W = Fd = 100 N·m = 100 J b. What power is expended if you lift the boulder in a time of 2 s? P = 100 J / 2 s = 50 W c. What is the gravitational PE of the boulder in the lifted position? PE = mgh PE = (100 N)(1 m) Relative to its starting position, the boulder’s PE is 100 J.

20. Forms of Energy • Kinetic Energy(KE) • energy of motion • depends on mass and velocity KE = ½m(2v)2 KE = ½mv2 KE = ½m(4)v2 • What happens to KE if you double the mass? • What happens to KE if you double the velocity? • It takes four times the work to double the speed. • An object moving twice as fast takes four times as much work to stop. 2x 4x

21. Forms of Energy • Energy: • the ability todo work • transferof energythrough motion • unit of measure is joules(J) PE KE WORK KE

22. Forms of Energy PE  KE

23. PE  KE Forms of Energy

24. PE  KE Forms of Energy Pendulum

25. Forms of Energy greatestgravitationalPE? greatestelasticPE? greatestKE?

26. Forms of Energy • Kinetic energy often appears hidden in different forms of energy, such as heat, sound, light, and electricity. • Random molecular motion is sensed as heat. • Sound consists of molecules vibrating in patterns. • Light energy originates from the motion of electrons within atoms. • Electrical energy is electrons in motion making electric currents.

27. Forms of Energy • Energy: • the ability todo work • transferof energythrough motion • unit of measure is joules(J) In what other forms can energy be stored or transferred? PE KE WORK KE

28. Forms of Energy The ability to do work. joules (J) THERMAL random motion of particles (heat) ENERGY MECHANICAL motion of objects (PE+KE) NUCLEAR stored in the nucleus ELECTROMAGNETIC ELECTRICAL radiated as waves (light) CHEMICAL motion of electric charges PE stored in bonds

29. Forms of Energy

30. Quick Quiz! • The energy due to the position of something OR the energy due to motion is called… • potential energy. • kinetic energy. • mechanical energy. • conservation of energy.

31. Quick Quiz. • After you place a book on a high shelf, we say the book has increased • elastic potential energy. • chemical energy. • kinetic energy. • gravitational potential energy.

32. Quick Quiz. • An empty truck traveling at 10 km/h has kinetic energy. How much kinetic energy does it have when it is loaded so its mass is twice, and its speed is increased to twice? • the same KE • twice the KE • four times the KE • more than four times the KE

33. Conservation of Energy To change the kinetic energy of an object, work must be done on the object. Work is required to start an object moving, or to bring it to rest. work-energy theorem: work equals the change in kineticenergy Work = ∆KE Fd = ∆½mv2

34. Conservation of Energy • If there is no change in an object’s kinetic energy, then no net work was done on it. • Push against a box on a floor. • If it doesn’t slide, then you are not doing work on the box. • On a frictionless floor, the work of your force times the distance appears as kinetic energy of the box. Fd = ∆½mv2 KE = 0 ½mv2 F d

35. Conservation of Energy (∑F)d = ∆½mv2 • With friction, the net force of your push minus the friction force is multiplied by distance to give the gain in kinetic energy. • If the box moves at a constant velocity, the net force and net work are zero, and, according to the work-energy theorem, ∆KE = 0. (no change in kinetic energy) ∑F = 0 ½mv2 ½mv2 F f f d

36. Conservation of Energy A car moving at twice the velocity of another has four times as much kinetic energy, and will require four times as much work to stop. Fd = ∆½mv2

37. Conservation of Energy A car moving at twice the velocity of another has four times as much kinetic energy, and will require four times as much work to stop. The friction force is nearly the same for both, so the faster car takes four times as much distance to stop. Fd = ∆½mv2 Fd = ∆½m(2v)2 F(4)d = ∆½m(4)v2 2 x v gives 4 x d

38. Conservation of Energy A car moving at twice the velocity of another has four times as much kinetic energy, and will require four times as much work to stop. The friction force is nearly the same for both, so the faster car takes four times as much distance to stop. Fd = ∆½mv2 Fd = ∆½m(4v)2 F(16)d = ∆½m(16)v2 4 x v gives 16 x d

39. Conservation of Energy think! A friend says that if you do 100 J of work on a moving cart, the cart will gain 100 J of KE. Another friend says this depends on whether or not there is friction. What is your opinion of these statements? Careful. Although you do 100 J of work on the cart, this may not mean the cart gains 100 J of KE. How much KE the cart gains depends on the net work done on it. (∑F)d = ∆½mv2

40. Conservation of Energy think! When the brakes of a car are locked, the car skids to a stop. How much farther will the car skid if it’s moving 3 times as fast? 9 times farther. The car has nine times as much kinetic energy when it travels three times as fast:

41. Conservation of Energy Open System: exchangematter and energy with surroundings (cup of hot coffee) Closed System: exchange only energy with surroundings (cup of hot coffee with a lid) Isolated System: NOexchange of matter or energy with surroundings (insulated cup of hot coffee)

42. Conservation of Energy Law of Conservation of Energy totalenergy in an isolatedsystem (universe) does notchange energy cannotbe createdor destroyed, but may change forms(conversion) open system (car) isolated system (universe)

43. Conservation of Energy PE = 10,000 J KE = 0 J When the woman leaps from the burning building, the sum of her PE and KE remains constant at each successive position all the way down to the ground. PE  KE PE = 7500 J KE = 2500 J PE = 5000 J KE = 5000 J PE + KE = 10,000 J PE = 2500 J KE = 7500 J PE = 0 J KE = 10,000 J

44. Conservation of Energy • energy may change forms(conversion)

45. Conservation of Energy • energy may change forms(conversion) List the energy converting steps in lighting a match: 1) muscles use chemical E to move the match, KE 2) friction converts KEof match to thermal E 3) thermal E triggers rxn releasing chemical E 4) chemical E converted to more thermal E and electromagnetic E in heat and light of the flame

46. Quick Quiz! • The work-energy theorem states that the work done on or by a system is equal to … • its potential energy. • its total mechanical energy. • its change in kinetic energy. • its change in an open system. Fd = ∆½mv2 Work = ∆KE

47. Quick Quiz. • When the brakes of a car are locked, the car skids to a stop. How much less will the car skid if it’s moving half as fast? • 1/2 as far • 1/4 as far • 1/8 as far • 1/16 as far Fd = ∆½mv2 Fd = ∆½m(1/2v)2 F(1/4)d = ∆½m(1/4)v2

48. Quick Quiz. • How is an open system similar to a closed system? They both… • exchange matter in and out. • exchange energy in and out. • exchange matter and energy in and out. • are identical to an isolated system.

49. Quick Quiz. • When your car engine burns gasoline to move the car, it is converting _____ energy into ______ energy. • mechanical, chemical • electrical, mechanical • potential, thermal • chemical, mechanical

50. Energy Resources