Work, Energy,Power (1).pptx presentation

work power

1. Work , Energy and Power

2. WORK

3. What is Work? The term work was introduce in 1826 by the French mathematician Gaspard Gustave Coriolis as " weight lifted through a height" which is based on the used of early steam engines to lift buckets of water out of flooded ore mines.

4. Work is defined as the force (F) exerted in an object times the distance (x) moved in the direction of the force. The work done by the force is equal to the product of the force and the displacement. W = Fx

5. For work to be done, three conditions must be satisfied : 1 . There must be a force (F) exerted on an object and made it move. 2. There must be a displacement (x ) either in the same or opposite direction as the applied force. 3. There must be a component of force along the direction of the displacement.

6. The SI unit of Work is newton-meter (N-m) or also known as joule (J) which is defined as the work expended by a force of 1N through a distance of 1m. 1 joule = 1 newton- meter The SI unit of work is named after James Prescott Joule ( 1818-1889), an English physicist who showed the quantitative relationship of heat and work. He is one of the outstanding physicist of the 19th Century.

7. The non-SI unit of Work includes dyne-cm or erg and foot-poundal (ft-lb)

8. Sample Problems

9. Example: 1. If a 50 pound force is exerted on a crate at it moves 10 feet across the floor in the direction of the force then the work done is 50 pounds times 10 feet . Given : F = 50 lbs x = 10 ft W = ? Formula: W = Fx = (50lbs) (10ft) W = 500 ft-lbs

10. 2. How much work is done when a 50 kg block is pushed by a horizontal force of 60 N through a distance of 25 m? Given: F = 60 N d = 25 m W =? Formula: W = Fd = (60 N) (25 m) W = 1,500 J

11. On the other hand, one of the most common ways of doing work is raising a body. The work done in raising a body against the force of gravity is equal to the weight of the body multiplied by the height through which the body has been raised . A body of mass m kg through a height h expressed in meters. If the body is raised with no acceleration, then the upward force in raising the body is equal to the weight in newton, where W = mg h

12. Therefore, the work done is equal to weight multiplied by height (vertical distance). where: Weight = mass x acceleration due to gravity W = mg W = mgh where : m= mass of the object h = the distance where the body was raised g = acceleration due to gravity which is 9.8 m/  

13. Examples: 1. Suppose a person who weighs 600 newtons goes up the stairway from the ground floor to the third floor of a building. If the distance between the floors is 4 meters, the person raises himself through a height of 8 meters. Given: m = 6 00 N h= 8 m Formula: W= mgh = ( 6 00 N ) (9.8 m / ) ( 8 m) W= 4 7,040 J  

14. 2. How much work is required to raise a load of 50 kg from the ground to the tenth floor of a building 30 meters above the ground? Given: m = 50 kg h = 30 m g = 9.8 m/ Formula: W = mgh = ( 50 kg) (9.8 m/ )(30 m) = ( 490 N) (30 m) = 14,700 J  

15. 3. Find the work done when a person weighing 60 N clubs a tower 50 m high. Given: m = 600 N h= 50 m g = 9.8 m/s Formula: W= mgh = (600 N ) (9.8m/s) (50 m) W = 294 000 J

16. 4. A laborer who weighs 600 newtons is to carry 5 sacks of rice from the ground to the second floor of a storehouse. If the distance between floors is 3.5 meters and the weigh of each sack is 50 kg of force, find the total work done against gravity if the laborer carries only one sack at a time. Solution: The laborer will have to make five trips to the second floor. The force he exerts is equal to the sum of his weight and the weight of one sack.

17. Since the weight of each sack= 50 kg x 9.8 m/ = 490 N Work for each trip = ( 600 N + 490 N) x 3.5 m = 1,090 N x 3.5 m = 3,815 J Hence , the total work done = 5x 3,815 J = 19,075 J  

18. Work can either positive or negative: if the force has a component in the same direction as the displacement of the object, the force is doing positive work. If the force has a component in the direction opposite to the displacement, the force does negative work.

19. Example: If you pick a book off the floor and put it on a table, for example you're doing positive work on the book because you supplied an upward force and the book went up. If you pick the book up and place it gently back on the floor, you're doing negative work because the book is going down but you're exerting an upward force acting against gravity.

20. ENERGY

21. What is energy? Energy is defined as the property of matter that is manifest as a capacity to perform work such as causing motion or the interaction of molecules.

22. Forms of Energy 1. Chemical energy - the energy of mixture of chemicals that can do work that happens during chemical reaction Ex . Food energy 2. Electrical energy - most important energy in modern technological world. It is linked with the basic structure of an atom. 3. Radiant energy caused by the accelerated electric charges or magnetic fields. 4. Nuclear energy - energy release during the splitting or fusing of atomic nuclei. 5. Solar energy - is the radiation produced by nuclear fusion reactions deep in the sun's core.

23. 6 . Light energy - an energy visible to human eye that is radiated in moving particles. 7. Thermal energy - energy in transit that flow flows from a substance at a higher temperature to the substance at a low er temperature. 8. Wind energy - energy contained in the force of the winds blowing across the surface of the earth. 9. Mechanical energy - is due to the position of something or the movement of something.

24. Mechanical Energy Mechanical energy is the energy that is possessed by an object due to its motion or due to its position. It is the energy acquired by objects upon which work is done. Mechanical Energy is divided into two classes: 1. Potential Energy 2. Kinetic Energy

25. Potential Energy Potential energy ( stored energy of position) is an energy possessed by a body due to its position, shape or configuration. 2 Principal Types 1. Gravitational potential energy 2. Elastic potential energy

26. Potential Energy Gravitational potential energy is possessed by a body by virtue of its position, usually relative to the ground. The gravitational potential energy of a body is taken usually as a zero when it is at ground level. Example: 1. A physics book at rest on the top shelf of a locker possesses mechanical energy due to its vertical position above the ground.

27. 2. A barbell lifted high above a weightlifter's head . 3. A heavy block is raised by an engine to a height of 10 meters from the ground. The potential energy of a body of mass at a height from the ground is equal to the work done in rising the body from the ground to that height. Hence , P.E grav = mgh w here : m = mass in kg h = height in m g = acceleration due to gravity which is 9.8 m/  

28. Energy and work are expressed in the same unit since work is also the amount of energy transferred.

29. Example 1 . Find the increase in potential energy of a body when it is raised through a height of 12 m if the mass of the body is 40 kg. Given: h = 12 m m = 40 kg g = 9.8 m/ P.E grav = ?   Formula & solution: P.E grav = mgh = (40 kg) (9.8 m/ ) (12 m) = (392 N) (12m) = 4,704 J  

30. Elastic Potential Energy Elastic potential energy is the energy stored in elastic materials as the result of stretching or compressing. It can be stored in rubber bands, bungee cords, trampolines, springs, an arrow drawn into a bow, air compressor, air brakes of trains etc. The amount of elastic potential energy is such a device is related to the amount of stretch of the device- more stretch, the more stored energy.

31. Kinetic Energy Kinetic energy ( energy in motion) is the energy associated with motion. An object may have kinetic energy because it is moving as a whole or because it is rotating or both. Kinetic energy (KE) is the energy possessed by bodies in motion. It is equal to one-half the product of object's mass (m) and the square of its velocity ( ) . KE = m Kinetic energy depends upon the mass of the moving object.  

32. Sample Problems

33. Problem:  1. A 55 kg man runs at a speed of 4 m/s. Find his kinetic energy . Given: m = 55 kg v = 4 m/s KE = ? Formula: KE = m = ( 55 kg) (4 m/s = ( 55 kg) (16 ) = ( 880 J) = 440 J  

34. Law of Conservation of Energy The law of conservation of energy states energy can neither be created nor destroyed, it can only be transformed from one form to another. The law of conservation of energy is one of the basic laws of physics and therefore governs the microscopic motion of individual atoms in a chemical reaction.

35. Examples: Water can produce electricity. Water falls from the sky, converting potential energy to kinetic energy. This energy is then used to rotate the turbine of a generator to produce electricity. In this process the potential energy of a water in a dam can be turned into kinetic energy which can then become electric energy. When you push a book across the table, the energy from your moving arm is transferred from your body to the book, causing the book to move. A cat sitting on the highest branch of a tree has what is known as potential energy. If he falls off the branch and falls to the ground, his potential energy is now being converted into kinetic energy.

36. 4. Fingers hitting the piano keys transfer energy from the player’s hand to the keys. 5. When a moving car hits a parked car and causes the parked car to move, energy is transferred from the moving car to the parked car.

37. POWER

38. Power Power is defined as the rate of doing work. It is thus distinct from work and energy, with which the term is often confused. Power bears the same relation to work as velocity near the distance. The average power is defined by the relationship: average power = Power (P) is work (W) done divided by the time (t) it takes to do the work. Formula: P = W = Pt t =  

39. Power is measured in watts (W), named in honor of the Scottish mathematician and engineer James Watt, who greatly inspired the steam engine. 1 watt = 1 J/s

40. James Watt used a horse to estimate the value of power. He compared the power to his steam engine with the power of a horse. He found that the horse can lift or pull 550-lbs weight to a distance of one foot in one second. One horsepower ( hp ) equals 746 watts. 1 hp =746 watts= 550 ft-lbs

41. The units of power are the units of work divided by time, which are joules/second in SI unit. Consistent with the practice of using a new name when a unit is added, this unit is called a watt (W). The kilowatt, 1000 watts is also commonly used. The basic power unit in the British system is the ft-lb/sec. the horsepower is derived from this units: 1 horsepower = 550ft-lb/sec = 33, 000 ft-lb/h kW = 1000 W 1MW= 1, 000, 000

42. When a constant a force (F) performs work (W) on an object and moves it at constant rate, the power developed is equal to the product of the force (F) and velocity (v). In equation: P = = = F ( ) = Fv or P = Fv The derived formula for will be P = and velocity will be v = This equation reveals that a powerful machine is both strong (big force) and fast ( big velocity). A powerful person or machine must have a great force, to make an object move a great distance, in a short period of time.  

43. Example: An electric motor lifts an elevator that weighs 2.40 x N a distance of 18.0 m in 30.0 s. What is the power of the motor in watts? In kilowatts? Given: F = 2.40 x N x = 18.0 m t = 30.0 s P = ? Solution: P = = = = = W = kW  

44. Machines

45. The Principles of Machines

46. Machines in general may be looked upon as devices for transforming energy. The applications of simple machines such as hand tools are often for the purpose of multiplying force, that is for obtaining a larger output force than one exerts on the tool. However, a machine cannot supply more output energy than it is given as input energy. i.e. the output energy this follows the conservation of energy principle: energy cannot be created by the machine.

47. Machine - G reek word “machos”- expedient or something that makes work easy. -Roman word “ machina ” which means “trick” or “device”.

48. A machine is device that transfers energy from one place to another or transforms it from one form to another form. It is used to increase the force, or to increase the speed, or simply to change the direction of forces. But it does not change the amount of work done.

49. Machine is describe as the complex mechanical device powered by an engine or electric motor and designed to perform useful labor saving tasks.

50. A common misconception is that machines are used to do a task with less work than would be needed to do the task without the machine. In fact, due to friction, you do more work with a machine than without it for the same task. No machine can do the work on its own. Man must do an input work on the machine so that the machine can do the output work.

51. Input work (Wi) or the work done by man on the machine, is usually greater than the output work ( Wo ) or the work done on the machine. only ideal machine can transfer all the energy so the output work equals the input work. Real machines have friction between moving parts, so some work is wasted due to heat, and the output work becomes lesser than the input work.

52. Since we know that work equals : W = Fx then = and = Where: = input force = the force you exerted or applied on a machine = output force = the force exerted by the machine = input distance = displacement caused by man = output distance = displacement caused by the machine.  

53. The major benefit of a machine is that the input work can be done lesser input force, but a greater input distance. Since, work equals force multiplied by distance, then, large force times small distance equals small force times large distance. It means that no machine can increase the force and the speed at the same time. A machine which has the advantage of increasing force has the disadvantage of decreasing the distance (and the speed), and vice versa. If the machine is a force multiplier, it cannot be a speed multiplier and vice versa.

54. Example: We need 1000 J of work to lift a 1000 N load to a height of 1 m. if this load is pushed up on an inclined plane (with negligible friction) that is 4 m long, it will require only 250 N to do it. ( In reality, friction between the load and the inclined plane will make the applied force greater than 250 N.) Proof : = = (1000 N) (1m) = 1000 J = = ( 250 N) (4m) = 1000 J  

55. Moving heavy objects, such as a pile of rocks, can be made much easier with machines. Machines can be simple, like a shovel, or more complicated, like a bulldozer. Simple machines are usually made up of only a few parts. They can be combined to make a complex machine. This chapter will look at types of simple machines and how they can be combined to make a more complex machine.

56. Simple machines Simple machines are able to increase a force when energy is added to them. This means that they make work easier. Simple machines can also change the direction of the force or make things go faster. There are different types of simple machines, including levers, inclined planes, wedges, pulleys, wheels and axles.

57. Complex machines A complex machine is a machine that is made up of more than one simple machine. The simple machines work together to make work easier. Cars and bicycles are examples of complex machines. You can see some of the different simple machines that you have read about by looking at a bicycle.

58. A bicycle has wheels and axles that move the bike along. The handlebars, handbrakes and brake pedals are all types of levers. Many bikes also have gears that allow the rider to change the turning force. You can change the gears to make it easier to ride a bike up an inclined plane. The pedals need to be turned more but it is easier to travel uphill.

59. Levers A lever is a bar or rod that sits and turns on a fixed support called a  fulcrum . Levers are good for lifting. Force is applied to one end of the lever to move a load at the other end. Levers can be used to increase force or to change a small movement into a bigger one. Examples of levers include crowbars, see-saws, fishing rods, wheelbarrows and a pair of scissors. With these levers, heavy objects can be moved much more easily.

60. There are three orders of levers, defined by whether the fulcrum, load or effort is in the middle. A first order lever, with the fulcrum located in the middle, is what you have when you use a pair of scissors or a pair of pliers to work with something. A second order lever, with the load located in the middle, is what you see when you use a pair of nutcrackers or a wheelbarrow. A third order lever, with the effort located in the middle, is what you see when you use a fishing rod or a pair of tweezers or tongs with a spring at one end.

61. Inclined planes and wedges An inclined plane is the name for a slope or ramp which allows large weights to be raised to a higher level with a smaller effort. A road winding around a mountain is an inclined plane and so is a ramp. A ramp is longer than the height of a step but it is much easier to push a wheelbarrow along the ramp than to pull it up a step. Another example of an inclined plane is a wedge.

62. A wedge is an inclined plane that pushes objects apart. An axe, knife and a chisel are wedges because their blades are sloped like an inclined plane. These simple machines multiply the effort that you apply and make it easier to cut things. A saw is another type of wedge. Each of the teeth along the blade acts like a separate wedge so much less effort or force is needed to cut the wood.

63. Wheels, axles and gears Wheels are found in all sorts of machines, particularly in most forms of transport. A wheel is actually considered a type of lever that moves in a complete circle. The fulcrum is at the centre of the wheel. A wheel needs an axle to hold it so that it can turn. An axle is a small wheel at the centre of larger wheel. The axle turns with the larger wheel and can move a load a great distance. You can see an axle when you look at a bicycle. The axle is the smaller wheel at the centre of the larger wheels.

64. Gears are also wheels. Gear wheels have teeth around the edge that fit into teeth on other gears or wheels. These toothed wheels turn other toothed wheels. Gears are used to make things turn faster or slower and to increase the turning force. You can find gears in cars, bikes, windmills and various toys with motors.

65. Pulleys Pulleys also use wheels to work. A pulley is a simple machine which is made up of a grooved wheel, an axle and a rope that can be moved freely over the wheel. Pulleys are used to make lifting loads easier. A pulley works by pulling downwards on a rope that is stretched over a wheel. The load is then lifted upwards. Two or more pulleys can be used together to make it easier to lift even heavier loads. Examples of machines with pulleys include washing machines, clothes dryers and cranes.