Energy

1. Energy • Something that enables an object to work is called energy. • What are some different forms of energy? • Potential • Electrical • Mechanical • Kinetic

2. Potential Energy • Potential Energy: energy that is stored and held in readiness to do work. • Any substance that can do work has potential energy. • Fossil Fuels • Electric Batteries • Food

3. Potential Energy • Work is required to elevate objects • This potential energy is called • Gravitational Potential Energy (GPE) • The amount of gravitational potential energy is equal to the work done against gravity by lifting it.

4. Energy • Energy is the physical agent that allows work to be done. • Energy has many forms: • In motion (KE) • In position (GPE) • In the physical/chemical properties of materials (EPE) • Energy, like work, is measured in Joules (J). A B

5. Which path will take the most work to get the ball to the top of the tower?

6. Gravitational Potential Energy • Work = Force x Distance • Upward Force = weight • Work = Weight x Distance • Work = mg x Distance • GPE = mg x Distance (height) • GPE = mgh

7. GPE Example The ball has a mass of 2kg, the tower is 3m tall. What is the GPE when the ball is on top of the tower? GPE = mgh

8. GPE Example The ball has a mass of 2kg, the tower is 3m tall. What is the GPE when the ball is on top of the tower? GPE = mgh

9. GPE Example The ball has a mass of 2kg, the tower is 3m tall. What is the GPE when the ball is on top of the tower? GPE = mgh

10. Which path will take the most work to get the ball to the top of the tower? They are all the same

11. GPE Example A cannon fires a 10kg cannon ball 150m into the air. What is the GPE at its highest point? h=150m

12. GPE Example A cannon fires a 10kg cannon ball 150m into the air. What is the GPE at when the cannonball lands back on the ground?

13. Kinetic Energy Kinetic Energy: is the energy of motion Note that KE quadruples when the velocity doubles

14. How far would a car skid? 30km/hr Skid 10m Skid 40m 60km/hr 120km/hr Skid 160m

15. Phy Gas Phy Gas Sample Problem #1 (KE) • A train (m = 340000kg) travels along a stretch of track with a velocity of 16m/s. Tunnel What is the KE of the train?

16. A train (m = 340000kg) travels along a stretch of track with a velocity of 16m/s. Phy Gas Phy Gas How much work is required to stop the train in 84.6s?

17. A train (m = 340000kg) travels along a stretch of track with a velocity of 16m/s. How much work is required to stop the train in 84.6s? Work-Energy Theorem Phy Gas Phy Gas

18. Elastic Potential Energy EPE: is the energy stored in a spring or flexible object • k is the spring constant (Units N/m) (material dependent) • Which spring would be harder to compress? • x is the distance compressed or stretched • (Units m) x

19. Compressing • When a spring is compressed from its neutral position, elastic potential energy is stored within the spring. • The energy required to compress a spring is given by Note: x is the displacement. x is the value of the number on the number line

20. Stretching • When a spring is stretched from its neutral position, elastic potential energy is stored within the spring. • The energy required to stretch a spring is given by

21. Dart-X Sample Problem 3 (EPE) Consider a small toy dart gun. The dart is pushed 0.05m into the spring (k = 330N/m) within the gun. • What is the EPE of the spring? Load

22. The Need For Algebra • Some energy problems will require algebraic manipulation in order to be solved. • Each energy equation can be broken down.

23. Conservation of Energy Example Recall the ball on top of the tower. What is the velocity of the ball if it falls from the tower?

24. Conservation of Energy Example What was the KE of the ball just before landing?

25. Energy was Conserved PE at the top of the tower KE at the bottom of the tower

26. The Law of Conservation of Energy • The Law of Conservation of Energy: in a closed and isolated system, the total energy remains constant. • Energy can not be destroyed. • Energy transforms, but the total amount never changes.

27. Conservation of Energy What energy transformation take place in this example?

28. Conservation of Energy Laser What energy transformation take place in this example?

29. Conservation of Energy Physics Tours, Inc. What energy transformation take place in this example? y

30. Conservation of Energy PE KE PE PE PE + KE PE + KE KE

31. GPE EPE KE Total Simple Harmonic Motion 12 1 11 2 10 3 9 4 8 X1 5 7 6 mgh1 V=0 X2 V=0 mgh0

32. GPE EPE KE Total Hooke’s Law 12 1 11 2 10 3 9 4 8 5 7 6

33. The Law of Conservation of Energy In terms of the energy: • In terms of the components:

34. Simple Harmonic Motion Simple Harmonic Motion: Motion caused by a linear restoring force that has a period independent of amplitude. Period: The time required to repeat one complete cycle Amplitude: Maximum displacement from equilibrium. 12 1 11 2 10 3 9 4 8 5 7 6

35. External Work (Wo) • You may have wondered what the Wo term was for. • Wo is any work done by the system or on the system during an energy transition. • In cases where energy is added to the system, Wo is positive(+). Examples include motors and muscles. • In cases where energy is lost by the system, Wo is negative(-). Examples include friction and air resistance. Now let's get crackin!

36. Kinetic Energy & Work-Energy Principle • Work-Energy Principle - the net work done on a body is equal to the change in its kinetic energy. Kinetic Energy Work Energy Theorem Units of W & KE Pronounced as a Joule

37. A B C Conservation of Energy What are the kinetic and potential energies at the following points? Explain why.

38. A C h B 3h/4 h/4 Conservation of Energy Example • A car’s engine (mcar = 1500 kg) puts 10,000 J of energy into getting the car to the top of a hill. • Calculate the GPE & KE of the car at the three points below.

39. KE GPE EPE RKE The MVE • A statement of the conservation of energy that includes most of the forms of mechanical energy. Energy Before Energy After

40. GPE EPE RKE KE W o Total Understanding the MVE - Launch • The projectile will be launched up from the ground. Key Factor What did the explosion do to the ball? Work-Kinetic Energy Principle? The net work done on a body is equal to the change in its kinetic energy.

41. 0.5 m 0.5 m 1.0 m Understanding the MVE – Free Fall Key Factor What is the ball’s final height? What is the ball’s GPE? The height is always equal to zero, and the GPE is always equal to zero at the Lowest Point. Total Energy GPE KE

42. MVE – Free Fall WS 11a #3 A beach ball is .82m above a picnic table which is .45m tall. The table is on a 2.9m platform. • Find GPE of the ball at the surface of the table • Find GPE of the ball at the top of the platform 0.82 m 0.45 m 2.9 m

43. GPE EPE KE W o Total Compressing

44. GPE EPE RKE KE W o Total Understanding the MVE – Hoops, Anyone?

45. Conservation of Energy Example • A ball, initially traveling at a velocity of 14m/s is rolled up a frictionless hill until it stops.. • How high up the hill did it go?

46. GPE EPE RKE KE W o 8-25 Total Understanding the MVE - Archery Key Factor What was the GPE of the arrow just as it struck the target? Why?

47. GPE EPE RKE KE W o Total Understanding the MVE – More Archery • Let’s get a bulls eye hit this time! What was the GPE of the arrow at the beginning and the end of the arrow’s flight?

48. GPE EPE RKE KE W o Total Understanding the MVE – In the Factory Key Factor What role did friction play in this problem? Friction resulted in the apparent loss of energy to the system. However, the energy is still accounted for as work other (WO).

49. MVE – Archery WS Intro #3 • An arrow (m=.15kg) is drawn back in a bow (k=1120N/m) a distance of .35m. • What is the EPE? • What is the GPE

50. Conservation of Energy Example • Roller Coaster • Down Hill Skier