1. CHAPTER 5 WORK

2. What is work? Have you done any work today? To many people the word work means something they do to earn money or for students studying for a test or doing homework.

3. The word work in Physics is used in a different way. Press you hand against the surface of you desk as hard as you can. Have you done any work?

4. The answer is NO! It doesn’t matter how hard you pushed or how tired you got you still didn’t do any work Remember a force is a push or pull.

5. A force is a push or pull. In order for work to be done, a force must make something MOVE!! WORK – Is the transfer of energy that occurs when a force makes an object move.

6. There are 2 conditions that have to be satisfied for work to be done on an object.  The applied force MUST make the object move  The movement much be in the same direction as the applied force.

7. If you pick up a suit case or a pile of books from the floor. You do work on the suitcases or books. The books move upward in the direction of the force you are applying.

8. If you hold the books in your arms without moving the books, you are NOT doing any work on the books. You are still applying an upward force to keep the books from falling, but no movement is taking place. Upward force hold the books, but no movement upward.

9. When you are carrying books while walking you may think that your arms are doing work. After all you are exerting a force on your arms and your books are moving.

10. BUT the force exerted by your arms on the book is upward, but the books are moving horizontally. The force you exert is at right angles to the direction the books are moving. You are NOT DOING WORK

11. How are work and energy related? When work is done a transfer of energy always occurs. Think about how you feel after carrying a heavy box up a flight of stairs. Remember as the height of an object above Earth’s surface increases, the Potential Energy of the object increases

12. Remember Energy is the ability to cause change. Another way to look at it is that Energy is the ability to do work. If something has energy, it can transfer energy to another object by doing work on that object. When you do work on an object you increase its energy.

13. When you carry something you transfer the chemical energy in your muscles to the object Energy is always transferred from the object that is doing the work to the object on which the work is done.

14. Answer : B If you push on a wall are you doing work on the wall? A.Yes B.No

15. Is the weight lifter doing work on the bar bells. A.Yes B. No Answer: A

16. Calculating Work The amount of work done depends on the amount of force exerted and the distance over which the force is applied. When a force is exerted and an object moves in the direction of the force the amount of work can be calculated. Work (joules) = applied force (N) x distance (m) W = F x d

17. You push a refrigerator with a force of 100 N. If you move the refrigerator a distance of 5 m, how much work do you do? A.20 J B.50 J C.500 J Answer : C

18. A force of 75 N is exerted on a 45 kg couch and the couch is moved 5 m. How much work is done in moving the couch? A.375 J B.225 J C.3375 J Answer : A

19. A lawn mower is pushed with a force of 80 N. If 12,000 J of work are done in mowing a lawn, what is the total distance the lawn mower was pushed? A.960,000 m B.150 m C..0067 m Answer : B

20. The brakes on a car do 240,000 J of work in stopping the car. If the car travels a distance of 50 m while the brakes are being applied, what is the total force the brakes exert of the car?  4800 N  5000 N  3000 N Answer : A

21. You and a friend want to see who can push the heavy box the fastest.

22. The box weighs the same, you both push it the same distance. What is the difference? Time

23. Your friend has more POWER than you do. POWER = is the amount of work done in one second. It is a RATE – the rate at which work is done. Power (watts) = Work (J)/ time (s) P = W/t

24. You do 900 J of work in pushing a sofa. If it took 5 s to move the sofa, what was your power?  4500 W  180 W  300 W Answer : B

25. To lift a baby from a crib 50 J of work are done. How much power is needed if the baby is lifted in 0.5 s?  25 W  250 W  100 W Answer : C

26. If Pegasus’s power is 130 W, how much work is done by the horse in 10 minutes.  78,000 J  1300 J  13 J Answer : A

27. Power and Energy Doing work is a way of transferring energy from object o another. Just as power is the rate at which work is done, power is also the rate at which energy is transferred.

28. When energy is transferred, the power involved can be calculated by dividing the energy transferred by the time needed for the transfer to occur.. Power (watts) = energy transferred (joules)/ time (sec) P = E/t

29. For Example: When the lightbulb is connected to an electric circuit, energy is transferred from the circuit to the lightbulb filament. The filament converts the electrical energy supplied to the lightbulb into heat and light. The power used by the lightbulb is the amount of electrical energy transferred to the lightbulb each second.

30. Find the force the reindeer exert in pulling Santa’s sleigh 20 m if 1,500 J of work are done.  30,000 N  75 N  100 N Answer : B

31. A helicopter’s engine produces 100kW of power. How much work does the engine do in 5 s?  500 J  20 J  500,000 J Answer: C

32. __________ is the rate at which work is done. A) Force B) Effort C) Power D) Efficiency

33. Answer : C Power is measured in __________. A) joules B) newtons C) watts D) m/s

34. Answer : C If a sheep applies a force of 10 N to a scooter and pushes it 10 m in 10 s, how much power did the sheep deliver? A) 30 W B) 1 000 W C) 10 W D) 100 W

35. Machines Machine – is a device that makes doing work easier. A machine doesn’t have to have an engine, or many moving parts. Scissors, knives, doorknobs are all simple machines and make our lives easier.

36. Machines can make work easier by increasing the Force that can be applied to an object. A screwdriver increases the force you apply to turn a screw.

37. A second way that machines can make work easier is by increasing the distance over which a force can be applied. A leaf rake is an example of this type of machine.

38. Machines can also make work easier by changing the direction of an applied force. A simple pulley changes a downward force to an upward force.

39. A car jack is an example of a machine that increase an applied force. The upward force exerted by the jack is greater than the downward force you exert on the handle. However the distance you push downward is greater than the distance the car is pushed upward

40. Why does a mover push the heavy furniture up the ramp instead of lifting it directly into the truck? Because less force is needed to move the furniture

41. The amount of work done to raise the cart from the floor to the seat top is dependent upon the force applied to the cart and the displacement caused by this force.

42. Because work is a product of force and distance (W=fd), The work done by the jack is not greater than the work you do on the jack. The jack increases the applied force, but it doesn’t increase the work done.

43. The work done in lifting an object depends on the change in the height of the object. The SAME AMOUNT OF WORK IS DONE whether the mover pushes the furniture up the long ramp or lifts it straight up. If she uses a ramp to lift the furniture, she moves the furniture a longer distance with less force.

44. Changing Directions Some machines change the direction of the force you apply. When you use the car jack, you are exerting a force downward on the jack handle. The force exerted by the jack on the car is upward. The direction of the force is changed.

45. Some machines change the direction of the force that is applied to them in another way. The wedge shaped blade of an ax is one example. When you use an ax to split wood, you exert a downward force as you swing the ax toward the wood. The blade changes the downward force to a horizontal force that splits the wood apart.

46. When you use a crowbar to pry a lid off of something you are using a machine to try to move something that resists being moved.

47. Two forces are involved when a machine is used to do work. You exert a force on the machine and the machine then exerts a force on the object you are trying to move. The force that is applied to the machine is called the INPUT FORCE ( F in ) The force applied by the machine is called the OUTPUT FORCE (F out )

48. When you try to pull a nail out with a hammer, you apply the input force on the handle. The output force is the force the claw applies to the nail.

49. Two kinds of work need to be considered when you use a machine- -The work done on the machine -The work done by the machine The work done by you on the machine is called the INPUT WORK (W in) The work done by the machine is called the OUTPUT WORK (W out)

50. Remember that energy is always conserved. When you do work on the machine you transfer energy to the machine. The machine transfers energy to the object. Because energy cannot be created or destroyed, the amount of energy the machine transfers to the object cannot be greater than the amount of energy you transfer to the machine. A machine cannot create energy, so W out is NEVER greater than W in.

51. The machine DOES NOT transfer all of the energy it receives to the object. Some energy changes to heat due to friction. The energy that changes to heat CANNOT be used to do work, so W out is always less than W in

52. Remember work is calculated by multiplying force by distance. W = Fd The input work is the product of the input force and the distance over which the input force is exerted. The output work is the product of the output force and the distance that force moves the object.

53. Suppose a perfect machine could be built where there is no friction. None of the input work would be converted to heat. For this IDEAL machine the input work would equal the output work. So…. W in = W out. Fd in = Fd out

54. A hammer claw moves a distance of 1 cm to remove a nail. If the output force of 1,500 N is exerted by the claw of the hammer, and you move the handle of the hammer 5 cm you can find the input force by: W in = W out F in d in = F out d out F in (0.05m) = (1,500 N)(0.01m) F in (0.05m) = 15 Nm F in = 300 N

55. Mechanical Advantage Machines like the car jack, the ramp and the claw hammer make work easer by making the output force greater than the input force. The ratio of the output force to the input force is the MECHANICAL ADVANTAGE of the machine. This can be calculated by the following equation: Mechanical Advantage = output force(newtons) input force (newtons) MA = F out F in

56. The Mechanical Advantage of a machine without friction is called the IDEAL MECHANICAL ADVANTAGE

57. EFFICIENCY For real machines, some of the energy put into a machine is always converted into heat by frictional forces. For that reason, the output work of a machine is always less than the work put into the machine.

58. EFFICIENCY = is a measure of how much of the work put into a machine is changed into useful output work by the machine. A machine with HIGH efficiency produces less heat from friction so more of the input work is changed to useful output work

59. Calculating Efficiency To calculate the efficiency of a machine the output work is divided by the input work. Efficiency is expressed as a percentage by this equation: Efficiency (%) = output work (joules) input work (joules) Efficiency = W out / W in

60. In an ideal machine there is no friction and the output work equals the input work. So the efficiency of an ideal machine is 100% In a real machine there is always friction, so the efficiency of a real machine is always going to be LESS than 100%

61. Machines can be made more efficient by reducing friction. You can add a lubricant, such as oil or grease. The lubricant fills in the gaps between the surfaces and allows the surfaces to slide past each other more easily.

62. Answer : B Calculate the mechanical advantage of a hammer if the input force is 125 N and the output force is 2,000 N.  12  16  30

63. Answer : A How does a ramp make it easier to move a heavy object a certain distance? A) by decreasing the amount of force required to move the object B) by decreasing the amount of work required to move the object C) by decreasing the distance the object moves D) by changing the direction in which the object moves

64. Answer : C A lever is an example of a machine that __________. A) is complex and has many moving parts B) increases the distance an object moves C) increases the force applied to an object D) decreases the work required to move an object

65. Answer : C Calculate the mechanical advantage of a hammer if the input force is 125 N and the output force is 2000 N. A..0624 B. 250,000 C. 16

66. Answer : A Find the efficiency of a machine that does 800 J of work if the input work is 2400 J. A. 33 % B. 30 % C. 25 %

67. Answer : C The amount by which a machine multiplies an effort force is called a ____ A. Efficiency factor B. Fulcrum C. Mechanical advantage D. Resistance force

68. Simple Machine SIMPLE MACHINE = a machine that does work with only one movement of the machine. There are 6 types of simple machines.  Lever  Pulley  Wheel and axel  Inclined plane  Screw  Wedge.

69. LEVER- is a bar that is free to pivot or turn around a fixed point ( called the FULCRUM) INPUT ARM – is the distance from the fulcrum to the point where the input force is applied. OUTPUT ARM – is the distance from the fulcrum to the point where output force is exerted by the lever. Output arm Input arm

70. Teeter-totter A teeter-totter is a lever that children use as a play thing. Since the children are usually approximately the same weight, the fulcrum is placed in the center of the board. A child sits on each end of the board, and they take turns lifting each other off the ground.

71. 3 CLASSES OF LEVERS The differences among the three classes of levers depend on the locations of the fulcrum, input force and the output force

72. FIRST CLASS LEVER In the first class lever the fulcrum is located between the input and the output forces. The output force is always in the OPPOSITE DIRECTION to the input force in a first class lever. The screwdriver used to open a paint can is an example of a first class lever

73. 2 nd CLASS LEVER For a 2 nd class lever the output force is located between the input force and the fulcrum. Wheelbarrow is an example of a 2 nd class lever. You apply an upward input force on the handles and the wheel is the fulcrum. For a 2 nd class lever the output force is always greater than the input force.

74. 3 rd CLASS LEVER For a 3 rd class lever the input force is applied between the output force and the fulcrum. A baseball bat is an example of a 3 rd class lever. The batter applies the input force with the right hand and the left hand is the fulcrum. The output force is ALWAYS LESS than the input force in a 3 rd class lever. INSTEAD – the distance over which the output force is applied is increased.

76. The IDEAL MECHANICAL ADVANTAGE or IMA, can be calculated for any machine by dividing the input distance by the output distance. For a lever the input distance is the length of the input arm and the output distance is the length of the output arm. The IMA of a lever can be calculated using this formula: IMA = L in L out

77. A PULLEY is a grooved wheel with a rope, chain or cable running along the groove. A pulley can change the direction of the input force or increase the input force, depending on whether the pulley is fixed or movable. A system of pulleys can change the direction or the input force and make it larger.

78. FIXED PULLEY = is attached to something that doesn’t move, like a ceiling or wall. Because the fixed pulley only changes the direction of the force the IMA = 1

79. MOVEABLE PULLEYS = A pulley in which one end of the rope is fixed and the wheel is free to move. Unlike the fixed pulley, a movable pulley DOES MULTIPLY FORCE.

80. Suppose a 4 N weight is hung from the movable pulley. The ceiling acts like someone helping you to lift the weight. The rope attached to the ceiling will support half of the weight – 2 N. You need to exert only the other half of the weight - 2N in order to lift the weight. The output force exerted on the weight is 4 N and the applied input force is 2 N. Therefore the IMA of the movable pulley is 2

81. THE BLOCK AND TACKLE – is a system of pulleys consisting of fixed and movable pulleys. The IMA of a pulley system is equal to the number of rope segments that support the weight. You can increase the IMA by increasing the number of pulleys in the system.

82. WHEEL AND AXEL – is a simple machine consisting of a shaft or axle attached to the center of a larger wheel, so that the wheel and axel rotate together. Door knob, screwdrivers and faucet handles are examples.

83. WHEEL AND AXLE – is a simple machine consisting of a shaft or axle attached to the center of a larger wheel, so that the wheel and axel rotate together.. WHEEL AND AXLE – is a simple machine consisting of a shaft or axle attached to the center of a larger wheel, so that the wheel and axel rotate together. Door knob, screwdrivers and faucet handles are examples. Axle Wheel

84. The length of the input arm is the radius of the wheel. The length of the output arm is the radius of the axle. The IMA of a wheel and axle is given by this equation: IMA = r w / r a

85. INCLINED PLANES – a sloping surface such as a ramp that reduces the amount of force required to do work.

86. You do the same work by lifting a box straight up or pushing it up an inclined plane. By pushing the box up an inclined plane, the input force is exerted over a longer distance compared to lifting the box straight up. As a result the input force is less than the force needed to lift the box straight upward. The IMA of an inclined plane can be calculated from this equation. IMA = length of slope height of slope IMA = l/ h

87. A SCREW – is an inclined plane wrapped in a spiral around a cylindrical post. If you look closely at the screw you will see that the threads form a tiny ramp that runs upward form its tip. You apple an input force by turning the screw. The output force is exerted along the threads of the screw. The IMA is larger if the threads are closer together. But more turns are needed to drive or screw it into some material.

88. A screw is an inclined plane wrapped around a post. Click on the light bulb to learn more.

89. WEDGE - is an inclined plane with one or tow sloping sides. It changes the direction of the input force.

90. COMPOUND MACHINE Some machines you use every day are made up of several simple machines. Tow or more simple machines that operate together form a COMPOUND MACHINE.

91. What is the IMA of a car’s steering wheel if the wheel has a diameter of 40 cm and the shaft its attached to has a diameter of 4 cm? IMA = r w /r a IMA = 10

92. A lever has an IMA of 4. If the length of the input arm is 1.0 m, what is the length of the output arm? IMA = l in /l out IMA =.25 M

93. A 6 M ramp runs from a sidewalk to a porch that is 2 m above the sidewalk. What is the ideal mechanical advantage of this ramp? IMA =l/h IMA = 3

94. Answer : D In which of the following situations do your arms do work on books? A) holding a heavy stack of books while standing B) carrying a heavy stack of books C) dropping a stack of books onto a table D) picking up a pile of books from the floor

95. Answer : C When work is done, there is always a(n) __________. A) constant rate B) single movement involved C) transfer of energy D) increase in the mechanical advantage

96. Answer : 38 An average golfer hits a golf ball with a force of 3800 N. If the club contacts the ball for a distance of 0.01 m, how much work is done? Use remotes to answer

97. Answer : 80 W A streetlight does 4800 J of work in 60 s. How much power does it use? Use remotes to answer.

98. Answer : 54 J You apply a force of 18 N to a dresser and move it 3 m. How much work did you do on the dresser? Use remotes to answer

99. Answer: C Power is measured in __________. A) joules B) newtons C) watts D) m/s

100. Answer : 10 w If you apply a force of 10 N to a box and push it 10 m in 10 s, how much power did you deliver? Use remotes to answer

101. Answer : 20 W A human brain does 700 J of work in 35 s. How much power do you use? Use remotes to answer

102. Answer : D Which is true of an axe blade? A) It changes the direction of the force from horizontal to vertical. B) The force exerted is an upward force. C) The force exerted is a horizontal force. D) It changes the direction of the force from vertical to horizontal.

103. Answer :C Which of the following describes a first-class lever? A) resistance force located between the effort force and fulcrum B) effort force located between the resistance force and fulcrum C) fulcrum located between the effort and resistance forces D) effort force and resistance force in same location