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WORK, POWER, AND MACHINES

WORK, POWER, AND MACHINES. W. d. F. WHAT IS WORK?. transfer of energy to a body by application of a force that causes body to move in direction of force. W = F  d SI units: joules (J). 1 J = 1 N•m = 1 kg•m 2 /s 2. W. F. d. WORK.

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WORK, POWER, AND MACHINES

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  1. WORK, POWER, AND MACHINES

  2. W d F WHAT IS WORK? • transfer of energy to a body by application of a force that causes body to move in direction of force. • W = Fd SI units: • joules (J). • 1 J = 1 N•m = 1 kg•m2/s2

  3. W F d WORK Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N? GIVEN: W = ? F = 190 N d = 2.0 m WORK: W = Fd W = (190 N) (2.0 m) W = 380 J

  4. W F d WORK A crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do on the girder? GIVEN: W = ? F = 5200 N d = 25 m WORK: W = Fd W = (5200 N) (25 m) W = 130,000 J or 1. 3 x 105 J

  5. W F d WORK An apple weighing 1 N falls through a distance of 1 m. How much work is done on the apple by the force of gravity? GIVEN: W = ? F = 1 N d = 1m WORK: W = Fd W = (1 N) (1 m) W = 1 J

  6. W p t POWER • rate at which work is done or energy is transformed. • power = work/time SI Unit: • watts (W) • watt = 1 J/s

  7. W p t POWER It takes 100 kJ of work to lift an elevator 18 m. If this is done in 20 s, what is the average power of the elevator during the process? GIVEN: p = ? W = 1 x 105 J t = 20 s WORK: p = W/t p = 1 x 105 J / 20 s p = 5 x 103 W or 5 kW

  8. W p t POWER While rowing across the lake during a race, John does 3960 J of work on the oars in 60.0 s. What is his power output in watts? GIVEN: p = ? W = 3960 J t = 60 s WORK: p = W/t p = 3960 J / 60 s p = 66.0 W

  9. W p t POWER Using a jack, a mechanic does 5350 J of work to lift a car 0.500 m in 50.0 s. What is the mechanic’s power output? GIVEN: p = ? W = 5350 J t = 50 s WORK: p = W/t p = 5350 J / 50 s p = 107 W

  10. MACHINES Machines • multiply and redirect forces. • help people by redistributing work put into them. • change either size or direction of input force. ALLOWS SAME AMOUNT OF WORK TO BE DONE BY • either decreasing distance while increasing force or • by decreasing force while increasing distance.

  11. FORCE AND WORK

  12. id of od if ma ma MECHANICAL ADVANTAGE • tells how much a machine multiplies force or increases distance. • mechanical advantage = output force = input distance input force output distance

  13. id od ma MECHANICAL ADVANTAGE Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high. GIVEN: ma = ? id = 5.0 m od = 1.5 m WORK: ma = id/od ma = 5.0 m / 1.5 m ma = 3.3

  14. id od ma MECHANICAL ADVANTAGE Calculate the mechanical advantage of a ramp that is 6.0 m long and 1.5 m high. GIVEN: ma = ? id = 6.0 m od = 1.5 m WORK: ma = id/od ma = 6.0 m / 1.5 m ma = 4

  15. of if ma MECHANICAL ADVANTAGE Determine the mechanical advantage of an automobile jack that lifts a 9900 N car with an input force of 150 N. GIVEN: ma = ? of = 9900 N if = 150 N WORK: ma = of/if ma = 9900 N / 150 N ma = 66

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