1. 1 Work, Energy & Power CHAPTER 17

2. 2 Specific Instructional Objectives At the end of the lesson, students should be able to: –Show understanding of the Physics concept of Work –Correctly identify Work from given situations –Recall and show understanding of the formula to calculate work done –Solve related problems involving work

3. 3 Work What does WORK mean to you? Are you doing WORK when… –Lifting weights? –Walking with a big bag of grocery in your hand? –Completing your homework assignment? –Writing an essay?

4. 4 Physics concept of WORK WORK is done only when a constant force applied on an object, causes the object to move in the same direction as the force applied.

5. 5 Physics concept of WORK What IS considered as work done in Physics: –You push a heavy shopping trolley for 10 m –You lift your school bags upwards by 1 m

6. 6 Physics concept of WORK What is NOT considered as work done: –You push against a wall –Jumping continuously on the same spot –Holding a chair and walking around the classroom

7. 7 Physics concept of WORK WORK can be calculated by: Work done = Constant x Distance moved force (N) in the direction of force (m) W = F x s Units: [J] [N] [m] SI Unit for Work is JOULE (J)

8. Calculating work done 2 factors are involved S = distance moved External force, F must be applied Object must move in the direction of the force, s

9. S = distance moved F = 300 N s = 5.0 m work done W = F x s = 300 x 5.0 = 1500 J = 1.5 kJ

10. 10 More Examples of WORK You are helping to push your mother’s heavy shopping cart with a force of 50 N for 200 m. What is amount of work done? Work done, W = F x s = 50 x 200 = 10,000 J or 10 kJ (kilo-Joules)

11. 11 More Examples of WORK: Jack put on his bag-pack of weight 120 N. He then starts running on level ground for 100 m before he started to climb up a ladder up a height of 10 m. How much work was done? From Physics point of view, no work is done on pack at level ground. Reason: Lift is perpendicular to movement. Work is done on pack only when Jack climbs up the ladder. Work done,W = F x s = 120 x 10 = 1200 J or 1.2 kJ

12. Doing work is a way of transferring energy. 1 Joule = 1 newton metre 1 J = 1 N m 1 Joule = 1 newton metre 1 J = 1 N m work done = energy transferred The Joule is defined as the amount of work done when a force of 1 Newton moves an object a distance of 1 metre in the direction of the force

13. Test yourself 1. In each of the following examples, explain whether or not any work is done by the force mentioned. a)You pull a heavy sack along the ground. b) The force of gravity pulls you downwards when you fall. c) The tension in a string pulls on a stone when you whirl it around at a steady speed. d) The contact force of the bedroom floor stops you from falling into the room below. 2. A man of mass 70 kg climbs stairs of vertical height 2.5 m. Calculate the work done against the force of gravity. More Examples of WORK:

14. Test yourself 3. A stone of weight 10 N falls from the top of a 250 m high cliff. a) Calculate how much work is done by the force of gravity in pulling the stone to the foot of the hill. b) How much energy is transferred to the stone? More Examples of WORK:

15. Force, distance and direction For force to do work, there must be movement in the direction of the force More Examples of WORK:

16. horizontal component (F x ) of 50 N F x = F cos 30 o = 50 cos 30 o = 43. 301270189 Work done = (F x ) x s = 43. 301270189 x 10 = 433.01270189 = 4.3 x 10 2 J More Examples of WORK:

17. h = 4.0 m Doing work against gravity A 80.0 kg man is climbing a stair as shown in the diagram on the right. Given the dimensions calculate for the work done by the man upon reaching the last step above. Force More Examples of WORK:

19. F d Quick review: Doing/Not Doing work????

20. 20 Specific Instructional Objectives At the end of the lesson, students should be able to: –Show understanding of the Physics concept of Kinetic Energy (KE) –Recall and show understanding of the formula –Distinguish situations involving KE –Solve related problems involving KE

21. 21 Energy – Quick Re-cap Energy is the capacity to do work SI Unit: Joule (J) Many forms Common ones: –Kinetic –Potential –Electric –Chemical –Solar –Nuclear

22. 22 Kinetic Energy (KE) A form of energy that a body in motion possess. A body a rest, will it possess any KE? Examples: –Bullet shot out from pistol –Helicopter flying at 120km/h

23. 23 Kinetic Energy (KE) The amount of KE of a moving body depends on: –Mass of body (kg) –Velocity (ms -1 ) When either mass or velocity of moving body is increased, KE will also increase.

24. 24 Kinetic Energy (KE) Formula: SI Unit: Joule [ J ] … same as Work Done Kinetic Energy = x Mass x (Velocity) 2 KE = x m x v 2 Units: [ J ] [kg] [ms -1 ] 2

25. 25 Kinetic Energy (KE) KE = ½  m  v 2 Mass = m kg Velocity, V

26. 26 Examples of KE Find the KE of an empty van of mass 1000kg moving at 2m/s. Find the KE of van when it is loaded with goods to give a total mass of 2000kg, and moving at 2m/s. Find KE of unloaded van when it speeds up to 4m/s. KE of van at 2m/s = ½ x 1000 x (2) 2 = 2000 J = 2 kJ KE of van at 2m/s = ½ x 2000 x (2) 2 = 4000 J = 4 kJ KE of van at 2m/s = ½ x 1000 x (4) 2 = 8000 J = 8 kJ

27. 27 Kinetic Energy (KE) Formula: KE = ½ mv 2 From the formula, what can you infer about the change in KE when… –Mass doubles –Velocity doubles KE doubles KE increases by FOUR times

28. 28 Examples of KE A motorcycle accelerates at 2m/s 2 from rest for 5s. Find the KE of motorcycle after 5s. Mass of motorcycle is 200 kg. Velocity of motorcycle after 5s,a = (v-u) t v = 2(5) + 0 = 10m/s KE of motorcycle at 10m/s= ½ x 200 x (10) 2 = 10,000 J = 10 kJ

29. 29 Specific Instructional Objectives At the end of the lesson, students should be able to: –Show understanding of the Physics concept of Gravitational Potential Energy –Recall and understand the formula –Distinguish situations involving GPE –Solve related problems involving GPE

30. 30 Potential Energy Potential energy is the energy possessed by an object as a result of its POSITION or CONDITION. Two common kinds: –Gravitational PE –Elastic PE (not in syllabus)

31. 31 Elastic PE Energy that can be possessed by an object due to its CONDITION. Examples: “Slinky” … when stretched or compressed Spring … when stretched or compressed Rubber band … when stretched Balloon with air … when compressed

32. 32 Gravitational PE Energy that can be possessed by an object due to its POSITION. In Physics, ground level is normally assumed to be at ZERO GPE. Any object that is at ground level has ZERO GPE. If object is lifted a certain height above ground, its GPE has increased.

33. 33 Gravitational PE Examples: –When a chair lifted from ground a distance of 1m –You sitting on the 3 rd storey of this building

34. 34 Gravitational PE Can be calculated with: GPE = mass  gravitational  height above acceleration ground level = m  g  h Units: [J] [kg] [m/s 2 ] [m] SI Units of GPE : Joule [J] Ground, 0 GPE Distance from ground, h Object on top of building, of mass, m g earth

35. 35 Example of GPE You lifted your bags to the top of your table. What can you say about the GPE of your bag? –Zero, increase, decrease Lift the same bag on the Moon. What happens to GPE? –Zero, increase, decrease Will the GPE be the same on Earth and Moon? –Same, less on Moon, more on Moon?

36. 36 Examples of GPE You lifted a set of books of mass 3kg, for 2m. What is the GPE gained by the books? Take g=10m/s 2. Find the work done by you to lift the books. GPE = mgh = 3  10  2 = 60 J Work done, W = F  d (F = weight of books) = (m  g)  d = 3 x 10 x 2 = 60 J (Note: same as GPE)

37. 37 Conservation & Conversion of Energy

38. 38 Specific Instructional Objectives At the end of the lesson, students should be able to: –Show understanding of conservation & conversion of energy –Correctly distinguish situation involving energy conservation & conversion –Solve related problems

39. 39 At the end of the lesson, students should be able to: –Show understanding of conservation & conversion of energy –Correctly distinguish situation involving energy conservation & conversion –Solve related problems Objectives

40. 40 Energy of an object can be thought of as the sands in an hourglass! Energy always remain same or fixed in quantity! But this sand can change position, from the top to bottom and bottom to top! Likewise energy can change in form eg. From KE  PE Conservation of Energy

41. 41 Conservation of Energy CANNOTNote that energy CANNOT be created nor destroyed! So what does this mean when viewed in context of the Earth?

42. 42 Conservation of Energy Conversion of energy is the term used to denote change in energy from one form to another. Eg. –Burning candle: Chemical  Heat, Light –Fuel: Chemical  Heat  KE  Electricity –Nuclear explosion: Nuclear  Heat, light –Spring: Elastic PE  KE

43. 43 Conversion of Energy For O-Levels, we are only concerned with: KE  GPE And such situations are only found when a moving object is at the same time undergoing changes in height

44. 44 Conversion of Energy Eg. of KE  PE Roller-coaster Falling object

45. 45 Free Falling object model An object in free fall means the object is falling freely, under the influence of gravity When the object is at the highest position, the GPE is at maximum and KE is zero. When the object is falling, the GPE decreases as it loses height, and the KE increases At the lowest position, the KE is at maximum and GPE is zero.

46. 46 Eg. of Conversion of Energy A car of 800 kg is moving at an average speed of 5 m/s. The traffic light changed to red and so the driver stepped on the brakes to bring the car to a quick, sudden and screeching halt. Find energy of moving car and what form of energy is this? –KE. KE = ½ mv 2 = ½ x 800 x 5 2 = 10,000 J. What energy does the car possesses when it stops? –None. What happened to the original energy of the moving car? –KE has changed to Sound and Heat Energy.

47. A stunt person slides down a cable that is attached between a tall building and the ground. The stunt person has a mass of 85 kg. The speed of the person when reaching the ground is 20 m s −1. Calculate: a)the change in gravitational potential energy of the person b) the final kinetic energy of the person c) the work done against friction d) the average friction acting on the person. Try this..

48. 1.Calculate how much gravitational potential energy (GPE) is gained if you climb a flight of stairs. Assume that you have a mass of 52 kg and that the height you lift yourself is 2.5 m. 2. A climber of mass 100 kg (including all the equipment he is carrying) ascends from sea level to the top of a mountain 5500 m high. Calculate the change in her gravitational potential energy (GPE) Try this..

49. 3. Calculate the increase in kinetic energy of a mass 800 kg when it accelerates from 20 m/s to 30 m/s. 4. Calculate the change in KE of a ball of mass 200 g when it bouncs. Assume that it hits the ground with a speed of 15.8 m/s and leaves it at 12.2 m/s. Try this..

50. The diagram shows a child on a swing. The mass of the child is 35 kg. The child is raised to point A and then released. She swings downwards through point B. a) Calculate the change in gravitational potential energy of the child between A and B. b) Assuming that air resistance is negligible, calculate the speed of the child as she passes through the equilibrium position B. c) The rope stays taut throughout. Explain why the work done by the tension in the rope is zero. Try this..

51. A bullet of mass 30 g and travelling at a speed of 200 m s −1 embeds itself in a wooden block. The bullet penetrates a distance of 12 cm into the wood. Using the concepts of work done by a force and kinetic energy, determine the average resistive force acting on the bullet. Work done by resistive force  initial kinetic energy of bullet Try this..

52. h v Y X An object of mass m passes a point X with a velocity v and slides up a frictionless incline to stop at point Y which is at a height h above X A second ball of mass 0.5m passes X with a velocity of 0.5v. To what height will it rise? Try this..

53. 10.0 m 1.0 m A body of mass 1.0 kg initially at rest slides down an incline plane that is 1.0 m high and 10.0 m long. If the body experiences a constant resistive force of 0.5 N over the slope, what is the KE of the body at the base of the plane? Gain in KE = loss in PE – Work against resistive force Try this..

54. Power = work done time taken J s Watts (W) - is the rate of working 1 joule per second ( scalar just like energy ) Power

55. It’s common to say that a strong person is “powerful” In Physics, strength, or force, and power are NOT the same. Large forces may be exerted w/o any movement and thus NO WORK is done and the power is zero. Large rock resting on the ground is not moving and yet exerts a large amount of force. Power

56. Consider a force F which moves a distance x at constant speed v in the direction of the force, in time t W = F s Dividing both sides by t W = F s tt but W/t = power and s/t = speed P = F v Power = force x speed Power

57. 57 END