PowerPower is the rate of which work is

done.No matter how fast we get up the

stairs, our work is the same.When we run upstairs, power demands

on our body are high.When we walk upstairs, power

demands on our body are lower.

PowerThe rate at which work is

done.Pave = W / t

P = dW/dtP = F • v = Fvcosθ

Units of Power

Watt = J/sft lb / shorsepower

•550 ft lb / s

•746 Watts

Power Problem

Develop an expression for the power output of an airplane cruising at constant speed v in level flight. Assume that the aerodynamic drag force is given by FD = bv2. By what factor must the power be increased to increase airspeed by 25%?

How We Buy Energy…The kilowatt-hour is a commonly used unit

by the electrical power company.Power companies charge you by the

kilowatt-hour (kWh), but this not power, it is really energy consumed.

1 kW = 1000 W1 h = 3600 s

1 kWh = 1000J/s • 3600s = 3.6 x 106J

More about force types

Conservative forces:– Work in moving an object is path independent.– Work in moving an object along a closed path is zero.– Work may be related to a change in potential energy

or used in the work-energy theorem.– Ex: gravity, electrostatic, magnetostatic, springs

Non-conservative forces:– Work is path dependent.– Work along a closed path is NOT zero.– Work may be related to a change in total energy

(including thermal energy).– Ex: friction, drag, magnetodynamic