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# Unit 25 Work And Power

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Work and Power

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### Unit 25 Work And Power

1. 1. Unit 25<br />Work, Power, and Machines<br />
2. 2. Work, Power & Simple Machines<br />
3. 3. Work<br />Weight<br />Lifting Force<br />Work- A force acting through a distance.<br />The distance that the object moves must be in the same direction as the force applied to the object.<br />What force is having to be overcome by the lifting force?<br />Weight!<br />Movement Distance<br />
4. 4. Work<br />Pushing Force<br />Distance Moved<br />Work- A force acting through a distance.<br />The distance that the object moves must be in the same direction as the force applied to the object.<br />What force is having to be overcome by the pushing force?<br />Friction!<br />
5. 5. Lifting Force<br />Movement Distance<br />Work is not done every time a force is applied. <br />Work is done only when a force moves an object in the same direction as the applied force.<br />Is the man doing work when he lifts the box?<br />Is the man doing work when he holds the box?<br />Is the man doing work if he carries the box?<br />
6. 6. Lifting Force<br />Movement Distance<br />Work is not done every time a force is applied. <br />Work is done only when a force moves an object in the same direction as the applied force.<br />Is the man doing work when he lifts the box?<br />Yes.<br />Is the man doing work when he holds the box?<br />Is the man doing work if he carries the box?<br />
7. 7. Lifting Force<br />Movement Distance<br />Work is not done every time a force is applied. <br />Work is done only when a force moves an object in the same direction as the applied force.<br />Is the man doing work when he lifts the box?<br />Yes.<br />Is the man doing work when he holds the box?<br />Is the man doing work if he carries the box?<br />No!<br />No!<br />(But he IS applying a force) <br />(But he IS applying a force) <br />
8. 8. Work<br />No!<br />But I did expend energy applying a force!<br />Think about pushing on wall that does not move. A force is applied but the wall has not moved a distance. <br />Is work done?<br />
9. 9. Calculating Work<br />Work = force (N) x distance (m)<br />W = F x D<br />The unit for work is a Joule (J). <br />If you lifted an object weighing 1N through a distance of 1m, you did 1 Joule of work:<br /> W = FD = 1N x 1m = 1 J<br />A Newton is about ¼ of a pound. <br />Is a Joule of work…very much work?<br />No!<br />
10. 10. Calculating Work<br />If you lifted an object weighing 200 N through a distance of 0.5m, how much work would you do?<br />Work = force x distance = 200 N x 0.5m = 100 J.<br />
11. 11. Power<br />Power is the rate at which work is done, or the amount of work per unit of time.<br />Two men can move a lot of sand using shovels…<br />
12. 12. Power<br />Power is the rate at which work is done, or the amount of work per unit of time.<br />Two men can move a lot of sand using shovels…<br />…but a front-end loader can do it in less time…<br />…because the front-end loader has more POWER.<br />
13. 13. Power<br />Power is the rate at which work is done , or the amount of work per unit of time.<br />Power = work / time or <br />Power = force x distance / time<br />Because work = force x distance<br />
14. 14. Power<br />The unit for power is watt (W). One watt is equal to 1 joule per second (1 J/sec).<br />Large quantities of power are measured in kilowatts (kW). <br />One kilowatt equals _____ watts.<br />One million watts is a megawatt<br />1000<br />
15. 15. Machines<br />A machine is a device that makes work easier.<br />A machine is ANY device that helps you to do something.<br />They can be “simple” or “compound”<br />
16. 16. Machines : How do machines affect work?<br />
17. 17. Machines : How do machines affect work?<br />What are other examples of machines?<br />Catapult<br />Wrench<br />Saw<br />Sythe<br />Plough<br />Pulley<br />Combine<br />
18. 18. Machines : How do machines affect work?<br />Hand Sewing and a Sewing Machine take the same amount of work to make a dress. But, the sewing machine is faster<br />
19. 19. Automatic Factory Looms<br />
20. 20. Machine Efficiency:Input and Output<br />There are always two types of work involved in using a machine. <br />Input work is the work that goes into the machine.<br />Output work is the work that comes out of the machine.<br />
21. 21. Machine Efficiency: Input and Output<br />The efficiency of a machine can be calculated:<br /> Efficiency = (work output / work input) x 100<br />This is easy to remember…think about it… <br />If you put 100 Joules of work into a pencil sharpener, but only got 80 Joules of work out, the pencil sharpener is 80% efficient:<br />(80 Joules / 100 Joules) x 100 = 80% efficiency<br />
22. 22. Machine efficiency can never be greater than or equal to 100% because some energy is always lost.<br />Friction makes every machine less efficient.<br />
23. 23. Machine Efficiency<br />The friction in a machine “wastes” energy in the form of heat.<br />Machines with the smallest amount of friction are the most efficient.<br />
24. 24. Machines<br />Machines make work easier because they change the size or the direction of the force put into the machine.<br />
25. 25. Bolts and Work<br />It is a lot easier to remove a bolt with a wrench than your fingers.<br />The longer the wrench, the easier it is to exert the force to remove the bolt.<br />
26. 26. Determining How Helpful a Machine Is<br />Besides the efficiency of a machine we also can determine how helpful a machine is.<br />
27. 27. Determining How Helpful a Machine Is<br />Effort Force<br />Resistance Force<br />What we mean by how helpful is how many times the machine multiples the effort force to overcome the resistance force<br />
28. 28. Determining How Helpful a Machine Is<br />The number of times a machine multiplies the effort force is called the mechanical advantage.<br />This tells you how much force is gained by using the machine. <br />The more times the machine multiples the effort force, the easier it is to do the job.<br />
29. 29. Six Simple Machines<br />WHEEL & AXLE<br />
30. 30. Lever<br />Lever : A lever is a rigid bar that is free to pivot , or move about a fixed point. The fixed point is called the fulcrum. <br />See Saw<br />Hand Truck<br />Wrench<br />Crow Bar<br />
31. 31. Inclined Plane<br />An inclined plane decreases the size of the effort force needed to move an object.<br />A ramp that reduces force necessary to climb<br />
32. 32. Wedge<br />Wedge: An inclined plane that moves.<br />In a wedge, instead of an object moving along the inclined plane, the inclined plane itself moves to raise the object. <br />Ice Wedging<br />
33. 33. Wheel and Axle<br />A wheel and axle is a simple machine made up of two circular objects of different sizes. The wheel is the larger object. It turns around a smaller object called the axle.<br />
34. 34. Wheel and Axle<br />The mechanical advantage depends on the radius of the wheel and of the axle.<br />
35. 35. Screw<br />Screw : A screw is inclined plane wrapped around a central bar or cylinder to form a spiral.<br />
36. 36. Pulley<br />A pulley is a rope, belt, or chain wrapped around a grooved wheel. <br />A pulley can function in two ways. It can change the direction of a force or the amount of force.<br />
37. 37. Compound Machines<br />Two or more moving parts<br />They can increase efficiency, but still can never get to 100%.<br />