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Power and Efficiency Review: Understanding Work, Joules, Watts and Efficiency

Explore the concepts of power and efficiency in this quick review session. Learn how to calculate power using work and time, measure efficiency, and apply the Law of Conservation of Energy. Practice problems included for hands-on learning.

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Power and Efficiency Review: Understanding Work, Joules, Watts and Efficiency

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Presentation Transcript

  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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