today
DESCRIPTION
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 …. - PowerPoint PPT PresentationTRANSCRIPT
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?