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Work and Power

Work and Power. Section 14.1. The everyday use of the word work and the physics use of the word work are two very different things. What is work?. What is work?. In science, work is the product of force and distance Work requires motion Work depends on the direction.

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Work and Power

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  1. Work and Power Section 14.1

  2. The everyday use of the word work and the physics use of the word work are two very different things. What is work?

  3. What is work? • In science, work is the product of force and distance • Work requires motion • Work depends on the direction

  4. If force and motion are in the same direction… The work done is maximized Work and direction

  5. If force and motion are NOT in the same direction… Only the force acting in the direction of motion is doing work Work and direction

  6. If there is no force acting in the direction of motion… NO WORK IS BEING DONE Any part of a force that does not act in the direction of motion does no work on an object. Work and direction

  7. Calculating work • Units called a Joule (J) • 1 J = 1 N•m

  8. Calculate • It is easy to calculate the work done by a weight lifter who lifts a 1600-newton barbell over his head. Assume that the barbell is lifted to a height of 2.0 meters. = 3200 J

  9. What is power? • Power is the rate of doing work • To increase power… • Either increase the amount of work done • Or do the same amount of work in less time

  10. I HAVE THE POWER!!!

  11. Calculating power • Units are Watts (w) • 1 w= 1 J/s

  12. Calculate • You exert a vertical force of 72 newtons to lift a box to a height of 1.0 meter in a time of 2.0 seconds. How much power is used to lift the box? 36 W • Your family is moving to a new apartment. While lifting a box 1.5 m straight up to put it on a truck, you exert an upward force of 200 N for 1.0 s. How much power is required to do this? 300 W

  13. Calculate • You lift a book from the floor to a bookshelf 1.0 m above the ground. How much power is used if the upward force is 15.0 N and you do the work in 2.0 s? 7.5 W • You apply a horizontal force of 10.0 N to pull a wheeled suitcase at a constant speed of 0.5 m/s across flat ground. How much power is used? (Hint: The suitcase moves 0.5 m/s. Consider how much work the force does each second and how work is related to power.) 5 W

  14. James Watts Horsepower is the amount of power that one horse exerts Horsepower

  15. 14.1 Key Concepts • Describe the conditions that must exist for a force to do work on an object • Describe how to calculate the work done on an object • Describe power, and explain how to calculate power • Compare the units of Joules, Watts, and Horsepower as they relate to work and power

  16. Work and Machines Section 14.2

  17. Machines

  18. Machines • A machine is a device that changes a force • Machines make work easier to do by… • Changing the size of a force • Changing the direction of a force • Changing the distance over which a force acts

  19. You have to turn the jack handle with greater force… But have to turn it less of a distance to raise the car Increasing force

  20. Boat oar travels a greater distance than the boat moves… But it takes less force than to move the whole boat Increasing distance

  21. The direction of the oars is not the same direction that the boat moves Changing direction

  22. Work Input and Output • The work done by a machine is always less than the work done on a machine • Why? Friction!

  23. Work Input Vocab • Force exerted on a machine is called the input force • Input distance-distance the input force acts through • Work Input- input force multiplied by input distance

  24. Put it all together

  25. Work Output Vocab • The force exerted by the machine is called the output force • Output distance- distance the output force is driven through • Work output- Output force multiplied by output distance

  26. Increase Work Output? Increase the Input

  27. 14.2 Key Concepts • Describe what a machine is and how it makes work easier to do • Relate the work input to a machine to the work output of a machine.

  28. Mechanical Advantage and Efficiency Section 14.3

  29. How does this work?

  30. What is a mechanical advantage? • Mechanical advantage of a machine is the number of times that a machine increases an input force • Not the same as an actual mechanical advantage • Ratio of input force to output force

  31. Actual Mechanical Advantage

  32. And an ideal? • Ideal mechanical advantage (IMA) is the mechanical advantage that would exist if friction did not • AMA < IMA

  33. Putting it all together Types Is less than Is the mechanical advantage that avoids Has the formula

  34. Calculating mechanical advantage • It’s much easier to calculate ideal mechanical advantage • Can ignore friction

  35. Calculate • A woman drives her car up onto wheel ramps to perform some repairs. If she drives a distance of 1.8 meters along the ramp to raise the car 0.3 meter, what is the ideal mechanical advantage (IMA) of the wheel ramps? 6

  36. Calculate • A student working in a grocery store after school pushes several grocery carts together along a ramp. The ramp is 3 meters long and rises 0.5 meter. What is the ideal mechanical advantage of the ramp? 6

  37. Calculate • A construction worker moves a crowbar through a distance of 0.50 m to lift a load 0.05 m off of the ground. What is the IMA of the crowbar? 10

  38. Calculate • The IMA of a simple machine is 2.5. If the output distance of the machine is 1.0 m, what is the input distance? 2.5 m

  39. Efficiency • Efficiency is the percentage of work input that becomes work output • Will efficiency ever be 100%? Why or why not?

  40. Calculate • You have just designed a machine that uses 1000 J of work from a motor for every 800 J of useful work the machine supplies. What is the efficiency of your machine? 80% • If a machine has an efficiency of 40%, and you do 1000 J of work on the machine, what will be the work output of the machine? 400 J

  41. How to increase efficiency? • LOWER FRICTION!

  42. 14.3 Key Concepts • Compare a machine’s AMA to its IMA • Explain how to calculate the IMA and AMA of various machines • Explain why the efficiency of a machine is always less than 100% • Describe how to calculate a machine’s efficiency • Create a sample problem for calculating a machine’s efficiency

  43. Simple Machines Section 12.4

  44. Simple?

  45. Lever Wheel and Axle Inclined Plane Wedge Screw Pulley Simple machines

  46. The Lever • A lever is a rigid bar that is free to move around a fixed point

  47. Output arm- distance between the output force and the fulcrum Lever Vocab Input arm- distance between the input force and the fulcrum OUTPUT ARM INPUT ARM Fulcrum- a fixed point that the bar rotates around FULCRUM

  48. First Class Lever • Fulcrum is always between input force and output force

  49. First Class Lever • Input distance does not have to equal output distance • Mechanical advantage can be: • 1, <1, or >1

  50. First class lever examples

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