Power, by definition, is the rate of doing work; or the rate at which energy is transferred. P = W / tP = W / tP = W / tP = W / t Power is a scalar.

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Presentation transcript:

Power, by definition, is the rate of doing work; or the rate at which energy is transferred. P = W / tP = W / tP = W / tP = W / t Power is a scalar quantity. “How FAST you do work!”

P = W / tP = W / tP = W / tP = W / t VARIATIONS: Power= energy/time Power= Fd/t Since W=Fd………. Power= Fd/t Since P=F(d/t) Power=Fv avg and d/t=v avg, then …. Power=Fv avg

The SI unit of power is the Watt, named in honor of James Watt James Watt James Watt James Watt. One Watt, W, of power is the power achieved when 1.0 J of work is done or 1.0 J of energy is transferred in a time of 1.0 s. *A kilowatt(kW) is commonly used for electrical power *Developed 1 st practical steam engine!

P = W / tP = W / tP = W / tP = W / t *In Watt’s time, steam engines were replacing horses for work (mines/mills) and the HORSEPOWER was developed to showcase that the performance of the steam engine, compared to that of horses, was much more efficient. Horsepower British Eng..... Horsepower 1 HP = 746 Watts British Engineering Units?

Horsepower 1 HP = 746 Watts 100 Watt light bulb: 0.13 hp 100 W(1hp/746 W)= 0.13 hp Horsepower Horsepower typically used for large “machines” Watts Watts are used in small “machines” 100 Horsepower car engine: 100 hp(746 W/1 hp)= 74,600 W 580 hp !

An industrial crane is capable of lifting 1 metric ton vertically 25 m in 9 sec (const. vel.). How much useful work is done each second? Metric ton=1,000 kg D=25 m, t=9 sec Useful work/sec=power P=W/t P=W/t or Fd/t or mgd/t (1,000 kg)(9.8)(25 m) (9 sec) (9 sec)=P 27,222 W or 36.5 hp=P

Power output 27,222 W or 36.5 hp=P If the crane has 70 hp motor, how efficient is the crane? Efficiency is a percentage used to describe the amount of work, energy, or power put out by a machine compared to the amount that it consumes. ε =(W output /W input )x100% ε =(E output /E input )x100% ε =(P output /P input )x100% =(36.5 hp/70 hp)x100% =(36.5 hp/70 hp)x100% = 52% efficient = 52% efficient HW #5-6 (mini-lab) *Ave car only about 30% *Today’s bicycles about 95% *Human body about 25%

P=W/t….W=Fd…..P=Fd/t…P=mgd/t 1. Weigh yourself (convert to Newtons) *This is “mg” 2. Stairwell height is 4.15 m. *This is “d” 3. “t” is the time to run up stairs (stopwatch) 4. Answer in Watts…convert to horsepower (put on board)