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 …

…the rate at which work is done.

Calculating Power

Power (W) = Work(J) / Time(s)

SI unit = Watts (W), or J/s

t

Fd

t

WP

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

Efficiency

Efficiency = (work out / work in) * 100 expressed as a percent (no units)

Actual efficiency will ALWAYS be less than 100%

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?

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?