IN TEXT CHAPTER 10-12

Power and work and efficiency

Additional Note

In case you did read in your text, which I would prefer if you did you may have missed the following definition…

Mechanical energy: Is the sum of potential and kinetic energy in a system.

Same rules apply to any type of energy or system, that because the law of ______________ tells us that __________ before has to be the same as ___________ after

Pa-Pa-Pa Power Wheels!!!

We all realize I think that it takes power to get stuff done, stuff like work, but what is power?

Great question: Power is how much work we do, over how long we take to do it!

P= W/t where w= work and t=timeThe unit of measure for power is the watt

(named after James Watt) and given as a w.“What’s a watt Mr. Caddy?” Who said that?

They get a cookie.A watt is 1J of energy transferred over 1

second. OR a watt is a J/s.

Example EZ

How much power is needed to lift a cup 2m that has a mass of 3 kg in 2 seconds?

P=w/tWork is F x dF is mgSo P= fd/t or 3 x 9.8 x 2/2P= 29.4 W or J/sAs a side note, not that you don’t know this

but a kilowatt is 1000w

Example harder

An electric motor lifts an elevator 9m in 15 seconds but exerting an upward force of 12000N. What is the power required in kW?

Known: 9m, t=15s, f=12000NUnknown: Power, workP= w/t , w=fdSo again 12000x 9 / 15 = 7200W7200W but I want kW so divide by 1000 and

7.2 kW

Efficiency

Efficiency: The rate or ratio or percent of how effectively something or someone turns input energy into useful energy.

Eff %= (useful energy/ input energy) x 100This applies to anything…like work etc,

output/input…..kinetic/potential…..

Example Ez

Calculate the efficiency of a light bulb in converting energy if its given 1000W of energy and only 60W of that is used to illuminate the room?

Eff= 60/1000 = 0.06x 100 = 6%

Example Harder

Suppose when you were at the top of the hill, your height was 25 m, and when you coasted down the hill, your 68kg body had a final speed of 13m/s. How efficient was I at converting types of energy?

Write out what you knowGiven

m = 68kg ∆h = 25 v = 13m/s g = 9.80m/s2

Need: Efficiency = output x 100% input

Efficiency = Ek x 100%

Ep

Gravitational Potential EnergyEp = mg∆h

= (68kg)(9.80m/s2)(25m)= 16677J

Kinetic Energy Ek = ½ mv2

= 0.5(68kg)(13m/s)2

= 5746J Efficiency = output x 100%

input

Efficiency = 5746J x 100% 16677J

Efficiency = 34.4564% Efficiency = 34%

Problems in Text

You can now do Page 262 4-8 work type problemsPage 264 9-14 power problemsPage 278-281 review questions on work,

power and efficiency Page 291 4-8 potential energy questionsPage 297 15 -15 kinetic and potential energy

questionsPage 300, 19-21 putting together mo, kinetic

and conservation