Work and Energy Unit

1. Work and Energy Unit Chapter 9

2. Energy can change from one form to another without a net loss or gain. LAW OF CONSERVATION OF ENERGY!!! (You will learn to identify these transformations)

3. 9.1Work think! Suppose that you apply a 60-N horizontal force to a 32 kg package, which pushes it 4 meters across a mailroom floor. How much work do you do on the package?

4. 9.1Work think! Suppose that you apply a 60-N horizontal force to a 32-kg package, which pushes it 4 meters across a mailroom floor. How much work do you do on the package? Answer: W = Fd = 60 N × 4 m = 240 J

5. 9.7 Conservation of Energy Total Mechanical Energy Total Mechanical Energy Same energy transformation applies Non-mechanical Energy (dissipated) 10 J of PE does 8 J useful work on the arrow and 2 J of non-useful work on the molecules that compose the bow and string and arrow. The arrow has 8 J of KE as a result. The 2 J of heat can be called non-useful work (work that is not part of the object’s total mechanical energy). Dissipated energy (DE) is amount of energy transferred away from the total mechanical energy. More DE means less KE, which reduces TME, which means less speed!

6. 9.7 Conservation of Energy Total Mechanical Energy Total Mechanical Energy Non-mechanical Energy (dissipated) The 2 J of heat can be called non-useful work (work that is not part of the object’s total mechanical energy). Dissipated energy (DE) is amount of energy transferred away from the total mechanical energy. More DE means less KE, which reduces TME, which means less speed!

7. Energy • The ability to do work or cause change • Can be transferred into other forms • Is conserved (can neither be created nor destroyed) • SI Unit is Joules I can define energy

8. Work • Force times distance the force is applied (W = Fd) • When work is done, energy is transferred, stored or used • SI Unit is Joules  • Positive work is work done in the direction of motion. • Negative work does work against the object (in a direction opposite of motion) I can define work. Positive work? Negative work?

9. Watch the transfer of KE and PE. What happens to the PE when the skier moves down the hill? What happens to the KE and TME when the skier travels over the unpacked snow? What work is done?

10. 6.1Work = force × distance a) Did the weightlifter do work on the barbell and weights? b) Is the weightlifter currently doing work on the barbell and weights? c) Explain two ways that the work done by the weightlifter be increased. 1. 2.

11. 9.1Work = force × distance Did the weightlifter do work on the barbell and weights? • Yes, when he first lifted them above his head. Is the weightlifter currently doing work on the barbell and weights? No, the barbell and weights are not moving. • Explain two ways that the work done by the weightlifter be increased. • Increase the weight on the ends of the barbell • Increase the distance over which the weightlifter pushes the barbell and weights.

12. 9.1Work While the weight lifter is holding a barbell over his head, he may get really tired, but he does no work on the barbell. Work may be done on the muscles by stretching and squeezing them, but this work is not done on the barbell. When the weight lifter raises the barbell, he is doing work on it.

13. 9.1Work When the object moves. When is work done on an object? When is work not done on an object? When the object does not move.

14. Ramp Work: Force and Distance • To test the relationship between force and distance when using a ramp of different lengths • Use the equation W = Fd

15. 9.1Work Work has the same units as energy Joules Newton x meter J N x m • One joule (J) of work is done when a force of 1 N is exerted over a distance of 1 m (lifting an apple over your head).

16. Kinetic Energy • The energy of motion • KE = ½m x v2 • Different forms of KE (mechanical, electrical, thermal, electromagnetic or light) What is kinetic energy? What are the forms of KE?

17. Kinetic Energy KE increases with mass KE increases with speed

18. WIND ENERGY • Atmospheric pressure differences cause air particles to move.

19. SOUND ENERGY • Energy caused by compression of air particles.

20. ELECTRICAL ENERGY • Energy of moving charged particles.

21. THERMAL ENERGY • The energy of moving and vibrating molecules • Sometimes called heat.

22. LIGHT or RADIANT ENERGY • Energy that travels in waves as electromagnetic radiation and/or as photons.

23. 9.5Kinetic Energy When you throw a ball, you do work on it to give it speed as it leaves your hand. The moving ball can then hit something and push it, doing work on what it hits. WORK

24. 9.5Kinetic Energy If the speed of an object is doubled, its kinetic energy is quadrupled (22 = 4). • It takes four times the work to double the speed. • An object moving twice as fast takes four times as much work to stop and will take four times as much distance to stop.

25. Kinetic Energy • How does KE increase or decrease?   Increase or decrease the velocity or the mass!!!! Double the velocity, Quadruple the KE!!!!! Prove it: Calculate the KE of a 2500 kg car traveling at 20 m/s and at 40 m/s • KE at 20 m/s KE at 40 m/s • (500,000 J) (2,000,000 J)

26. Kinetic Energy More mass, same speed, more KE. Double the mass, double the KE Prove it: Calculate the KE of a 100 kg cart and a 200 kg cart, each traveling at 15 m/s • 100 kg cart at 15 m/s 200 kg cart at 15 m/s • (11,250 J) (22,500 J)

27. Potential Energy • Stored energy or the energy of position • Gravitational PE is based on height and mass • Gravitational PE is mass x gravity x height (GPE = mgh) • Increases in height cause increases in stored energy What is potential energy? How does GPE change?

28. 9.4Potential Energy Gravitational Potential Energy • Energy is stored in an object as the result of increasing its height. • Work is required to elevate objects against Earth’s gravity. • Example: Water in an elevated reservoir and the raised ram of a pile driver have gravitational potential energy.

29. 9.4Potential Energy The amount of gravitational potential energy possessed by an elevated object is equal to the work done against gravity to lift it. PE = mgh What is the gravitational PE of a 10.0 kg object at 4.00 m above the ground? mg is weight (in newtons) [mass (kg) x gravity (m/s2)] 10 kg x 9.8 m/s2 x 4 m = 392 J

30. 9.4Potential Energy The potential energy of the 100-N boulder with respect to the ground below is 200 J in each case. • The boulder islifted with 100 N of force.

31. 9.4Potential Energy The potential energy of the 100-N boulder with respect to the ground below is 200 J in each case. • The boulder islifted with 100 N of force. • The boulder ispushed up the 4-m incline with 50 N of force.

32. 9.4Potential Energy The potential energy of the 100-N boulder with respect to the ground below is 200 J in each case. • The boulder islifted with 100 N of force. • The boulder ispushed up the 4-m incline with 50 N of force. • The boulder islifted with 100 N of force up each 0.5-m stair.

33. 9.4Potential Energy think! You lift a 100-N boulder 1 m. a. How much work is done on the boulder? b. What power is expended if you lift the boulder in a time of 2 s? c. What is the gravitational potential energy of the boulder in the lifted position?

34. Other forms of PE • Other forms of PE (Chemical PE, Elastic PE, Electric PE, Magnetic PE, Nuclear PE) • Changes in position in a force field changes the PE (gravitational fields, magnetic fields and electric fields) What are the forms of potential energy?

35. 9.4Potential Energy Elastic Potential Energy—potential to do work • Energy stored in a stretched or compressed spring or material. • When a bow is drawn back, energy is stored and the bow can do work on the arrow. • These types of potential energy are elastic potential energy.

36. CHEMICAL POTENTIAL ENERGY • Energy due to the bond position between molecules (stored during bonding). • Potential chemical energy is released from chemical reactions (burning, for example). • Fuels, Food, Batteries, for example.

37. 9.4Potential Energy Name three examples of potential energy.

38. Difference between kinetic energy and potential energy Kinetic energy   The energy of motion Potential energy The energy of position or stored energy

39. Mechanical Energy • The sum of the KE and PE in a system: (total ME = KE + PE) • Describes energy associated with the motion of objects • The KE and GPE are conserved for moving objects (neglecting friction, drag, vibrations and sound) What is mechanical energy?

40. Mechanical Energy = PE + KE • The total mechanical energy = 100 J 100 J = 100 J PE + 0 J KE 100 J = 50 J PE + 50 J KE 100 J = 0 J PE + 100 J KE

41. Non-MechanicalEnergy • Energy not associated with the motion of objects • Typical examples are vibrations, sound and heat • Referred to as dissipated energy or waste energy • Can be “observed” at the molecular level • Path of energy transfer that reduces the KE of the object What is non-mechanical energy?

42. Starter 1-10 • Energy transfers • Energy transfers into different forms • PE transfers to KE • When you fall, your PE decreases and your KE increases • Half way down PE = KE • As the person falls, the PE and KE flip • Energy is not destroyed (start and end with 10,000 J)

43. Starter 1-10 • Gravity is involved (GPE-- gravitational PE) • Max KE can never exceed the Max PE • As PE decreases, KE increases • PE stored is used when object is in motion • No energy is “lost” from PE to KE • The farther the diver falls, the greater the KE • Energy transforms from PE to KE from point A to point B

44. Starter 1-10 • As the KE increases, the PE decreases • As the diver hits the bucket, all PE has been transferred to KE • Work produces energy • Energy changes throughout the dive • At the top of the platform, all GPE and no KE • The diver always possesses 10,000 J of energy (energy is conserved) • Inverse relationship between PE and KE

45. Starter 1-10 • Energy is constantly changing as the diver falls • No energy is lost during the dive; it transfers from PE to KE • All PE has been transferred to KE • The PE decreases • 2500 J change in PE or KE • As the PE decreases, the KE increase (gravitational PE) • PE is inversely related to KE

46. Total Mechanical Energy • Total mechanical energy = kinetic energy + potential energy • TME = KE + PE • 100 J = 0 J + 100 J • At rest, no KE, no motion • 100 J = 50 J + 50 J • In motion, 50 J of PE transferred, object now has 50 J of KE. • 100 J = 100 J + 0 J • In motion, no potential energy (100 J transferred to KE) • In each case, the total mechanical energy is the same. As the PE decreases, the KE increases.

47. Indicate where: • KE is at a minimum and maximum • GPE is at a minimum and maximum • The speed is greatest • The speed is least • Energy is being stored and released Positions 1 and 5 are at the same height 1. Explain how energy transforms and is conserved as the pendulum swings back and forth 2. What happens as the KE increases? 3. What happens as the GPE increases?

48. KE min KE min PE max PE max V = 0 m/s V = 0 m/s transformation of PE to KE (release) transformation of KE to PE (storage) KE max PE min V = maximum

49. Analyzing KE and PE farthest Distance (from motion detector) closest time

50. Horsepower