1. IN TEXT CHAPTER 10-12 Power and work and efficiency

2. 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

3. 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=time The 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.

4. Example EZ How much power is needed to lift a cup 2m that has a mass of 3 kg in 2 seconds? P=w/t Work is F x d F is mg So P= fd/t or 3 x 9.8 x 2/2 P= 29.4 W or J/s As a side note, not that you don’t know this but a kilowatt is 1000w

5. 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=12000N Unknown: Power, work P= w/t, w=fd So again 12000x 9 / 15 = 7200W 7200W but I want kW so divide by 1000 and 7.2 kW

6. 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 100 This applies to anything…like work etc, output/input…..kinetic/potential…..

7. 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%

8. 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 know Given  m = 68kg  ∆h = 25  v = 13m/s  g = 9.80m/s 2 Need: Efficiency = output x 100% input Efficiency = E k x 100% E p

9. Gravitational Potential Energy E p = mg∆h = (68kg)(9.80m/s 2 )(25m) = 16677J Kinetic Energy E k = ½ mv 2 = 0.5(68kg)(13m/s) 2 = 5746J Efficiency = output x 100% input Efficiency = 5746J x 100% 16677J Efficiency = 34.4564% Efficiency = 34%

10. Problems in Text You can now do Page 262 4-8 work type problems Page 264 9-14 power problems Page 278-281 review questions on work, power and efficiency Page 291 4-8 potential energy questions Page 297 15 -15 kinetic and potential energy questions Page 300, 19-21 putting together mo, kinetic and conservation