Work and Machines


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Work and Machines

  1. 1. Which activity require the least amount of work?  Carrying heavy books home  Reading a 300-page novel  Skiing for 1 hour  Lifting a 45 kg mass  Holding a steel beam in place for 3 hours  Jacking up a car
  2. 2.   Book M: Chapter 4
  3. 3. By the end of this section, you should be able to:  Determine when work is being done on an object.  Calculate the amount of work done on an object.  Explain the difference between work and power.
  4. 4.  Work is the transfer of energy to an object by using a force that causes the object to move in the direction of the force.  One way you can tell that work is being done is that energy is transferred.
  5. 5. a force doesn’t always result in work being done.  Applying  For work to be done on an object, the object must move in the same direction as the force is applied.
  6. 6.  Work depends on distance as well as force.
  7. 7. Work = Force · distance  The unit used to express work is the newtonmeter (N · m), which is more simply called the joule.
  8. 8. Read the procedure on p.97
  9. 9.
  10. 10.  It may take you longer to sand a wooden shelf by hand than by using an electric sander, but the amount of energy needed is the same either way. Only the power output is lower when you sand the shelf by hand.
  11. 11. p.99 #3-9
  12. 12. Read through p.114-115 On a new sheet of paper, follow Step 2. Copy the Data Collection Table into your notes.
  13. 13. Why do we use machines?
  14. 14. By the end of this section, you should be able to:  Explain how a machine makes work easier.  Describe and give examples of the forcedistance trade-off that occurs when a machine is used.  Calculate mechanical advantage.  Explain why machines are not 100% efficient.
  15. 15. A machine is a device that makes work easier by changing the size or direction of a force.
  16. 16.  
  17. 17.  The work that you do on a machine is called work input.  The work done by the machine on an object is called work output.  Machines allow force to be applied over a greater distance, which means that less force will be needed for the same amount of work.
  18. 18.  Machines make work easier by changing the size or direction of the input force.  When a machine changes the size of the force, the distance through which the force is exerted must also change.
  19. 19.  
  20. 20. Math Practice p.103
  21. 21.  The less work a machine has to do to overcome friction, the more efficient the machine is.  Mechanical efficiency is a comparison of a machine’s work output with the work input.
  22. 22.
  23. 23.  
  24. 24. p.105 #3-10
  25. 25. What type of machine can be found on at least half of the people in this room right now?
  26. 26. By the end of this section, you should be able to:  Identify and give examples of the six types of simple machines.  Analyze the mechanical advantage provided by each simple machine.  Identify the simple machines that make up a compound machine.
  27. 27.  A lever is a simple machine that has a bar that pivots at a fixed point, called a fulcrum.  With a first-class lever, the fulcrum is between the input force and the load.
  28. 28.  The load of a second-class lever is between the fulcrum and the input force.
  29. 29.  The input force in a third-class lever is between the fulcrum and the load.
  30. 30.  
  31. 31. A pulley is a simple machine that consists of a wheel over which a rope, chain, or wire passes.  A fixed pulley is attached to something that does not move.
  32. 32.  Unlike fixed pulleys, movable pulleys are attached to the object being moved.  When a fixed pulley and a movable pulley are used together, the pulley system is called a block and tackle.
  33. 33.  A wheel and axle is a simple machine consisting of two circular objects of different sizes.
  34. 34.  The mechanical advantage of a wheel and axle can be found by dividing the radius (the distance from the center to the edge) of the wheel by the radius of the axle.
  35. 35.  An inclined plane is a simple machine that is a straight, slanted surface.  The mechanical advantage (MA) of an inclined plane can be calculated by dividing the length of the inclined plane by the height to which the load is lifted.
  36. 36.  A wedge is a pair of inclined planes that move.  Mechanical advantage of wedges can be found by dividing the length of the wedge by its greatest thickness.
  37. 37.  A screw is an inclined plane that is wrapped in a spiral around a cylinder.  The longer the spiral on a screw is and the closer together the threads are, the greater the screw’s mechanical advantage is.
  38. 38.  Compound machines are machines that are made of two or more simple machines.  The mechanical efficiency of most compound machines is low, because compound machines have more moving parts than simple machines do. Thus, there is more friction to overcome.
  39. 39.  
  40. 40. p.113 #3-9