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Today

Today. Quick review of power and efficiency Then practice problems. Power & Efficiency. How fast can you work? How well do you work?. Units. Work Joules (J) Also Newton-meter (N ·m) Energy Joules (J) Power Watt (W) Also Joules/Second (J/S). POWER is ….

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Today

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  1. Today • Quick review of power and efficiency • Then practice problems

  2. Power & Efficiency How fast can you work? How well do you work?

  3. Units • Work • Joules (J) • Also Newton-meter (N·m) • Energy • Joules (J) • Power • Watt (W) • Also Joules/Second (J/S)

  4. POWER is … …the rate at which work is done.

  5. Calculating Power Power (W) = Work(J) / Time(s) SI unit = Watts (W), or J/s

  6. Efficiency • Efficiency describes how well a machine works • How much usesul work we get out compared to how much total work we put in • Law of Conservation of Energy – some turns to heat, or light (think FRICTION…) • We use lubricants (engine oil) to minimize friction and increase efficiency

  7. Efficiency • Efficiency = (work out / work in) * 100 • expressed as a percent (no units) • Actual efficiency will ALWAYS be less than 100%

  8. Practice #1: Charlie pulls a sled across level snow with a force of 300 N. If the sled moved 36 m in 90 s, how much power did Charlie use?

  9. Practice #2: Julie lifts a crate by exerting a 150-N force. The crate moves at a constant speed. The crate weighs 125 N. A) How much work has to be done to raise the crate 6.5 m?B) How efficient was Julie in doing her work?

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