CHAPTER 12 NOTES - Work, Power, and Simple Machines 1. An example of work being done is a person pushing a lawn mower and cutting the grass. 2. Work divided by time equals power. 3. The work input of a crowbar is equal to the force applied to the handle times the distance the handle moves. 4. Machines multiply force or distance. 5. Without friction there would be equal work input and output. 6. A ramp is a simple machine called the inclined plane. 7. Sharpening a wedge increases the mechanical advantage and decreases the effort force. 8. For work to be done, the direction of the applied force must be the same as the direction of motion. 9. Work is force times distance. 10. The amount of work done when a force of 300 N moves an object a distance of 2.0 m is 600 N-m. 11. A unit of power is a watt. 12. Work output equals the resistance force times the distance through which the output force is applied. 13. The number of times a machine multiplies effort force is mechanical advantage. 14. An inclined plane that moves is called a wedge. 15. The work done by a person lifting a 100-N suitcase a distance of 2.0 m is 200 N-m. 16. The efficiency of a machine can be increased by reducing friction. 17. When an inclined plane is used, a smaller force is exerted over a longer distance. 18. A unit used to measure work is a newton-meter. 19. If a machine has a mechanical advantage of one, it may change the direction of the force. 20. One joule of work is the same as one newton-meter. 21. A screw multiplies effort by acting through a long distance. 22. The place at which a lever is supported is called a fulcrum. 23. In a second-class lever, the resistance force is between the effort force and fulcrum. 24. A wedge is a moving inclined plane. 25. A chain, belt, or rope wrapped around a wheel is called a pulley. 26. A simple machine made up of two circular objects of different sizes is called a wheel and axle. 27. A wheelbarrow is a second-class lever. 28. In a third-class lever, the effort force is between the resistance force and the fulcrum. 29. No machine has 100 percent efficiency due to friction. 30. The comparison of work input to work output is called efficiency. 31. You do not do work on an object if you hold it for a period of time. 32. Work is the product of force times distance. 33. It is not necesarily true that all machines make work easier by changing the direction of the applied force. 34. The efficiency of a machine is never 100 percent. 35. The length of an inclined plane is never shorter than its height. 36. The fixed point about which a lever is free to pivot is called the fulcrum. 37. More work output than work input is not the result of using a compound machine. 38. When a lever is used to pry up a rock, the rock's weight does not represent the effort force. 39. A machine is any device which makes work easier. 40. A wheel and axle is not an example of a compound machine. 41. Work is defined as a force acting through a distance. 42. One joule is equal to one newton-meter. 43. The rate at which work is done is called power. 44. Power is measured in watts. 45. The minimum number of different forces involved in the use of machines is two. 46. The force applied by the machine is called the output force. 47. The force applied to the machine is called the effort force. 48. The ratio of the work output of a machine to the work input is called the efficiency of the machine. 49. A jar lid is an example of a simple machine called a screw. 50. The six simple machines are the screw, the pulley, the lever, the wedge, the wheel and axle, and the inclined plane. 51. A movable pulley multiplies effort force. 52. A bicycle is an example of a combination of simple machines called a compound machine. 53. A see-saw is an example of a first-class lever. 54. The efficiency of a machine is reduced by the force called friction. 55. The fulcrum is between the effort force and the resistance force on a first-class lever. 56. A wheelbarrow is an example of a second-class lever. 57. A pulley that is attached to a structure is called a fixed pulley. 58. A moving inclined plane is called a wedge. 59. A doorknob is an example of a simple machine called a wheel and axle. 60. A fixed pulley only changes the direction of the effort force. 61. Explain who is doing more work and why: a bricklayer carrying bricks and placing them on the wall of a building being constructed, or a project supervisor observing and recording the progress of the workers from an observation booth. Work is defined as a force applied to an object, moving that object a distance in the direction of the applied force. The bricklayer is doing more work. 62. How much work is done in pushing an object 7.0 m across a floor with a force of 50 N and then pushing it back to its original position? How much power is used if this work is done in 20 sec? Work = force x distance Work = 50 N X 7 m = 350 N-m or J each way; therefore 700 N-m or J total. Power = work / time Power = 700 N-m / 20 sec = 35 W 63. Using a single fixed pulley, how heavy a load could you lift? Since a fixed pulley has a mechanical advantage of one, it will only change the direction of the force applied to it. You would be able to lift a load equal to your own weight, minus the negative effects of friction. 64. Give an example of a machine in which friction is both an advantage and a disadvantage. One answer might be the use of a car jack. Advantage of friction: It allows a car to be raised to a desired height without slipping. Disadvantage of friction: It reduces efficiency. Another is the braking system of a car: The advantage is that the friction generated by pressing a brake show against a brake drum or the brake pad against the brake rotor to stop a vehicle. The disadvantage would be the heat generated as a by-product of the friction. 65. Why is it not possible to have a machine with 100% efficiency? Friction lowers the efficiency of a machine. Work output is always less than work input, so an actual machine cannot be 100% efficient. 66. What is effort force? What is work input? Explain the relationship between effort force, effort distance, and work input. The effort force is the force applied to a machine. Work input is the work done on a machine. The work input of a machine is equal to the effort force times the distance over which the effort force is exerted.