how quickly work is done Power how quickly work is done § 6.7

Power Rate of doing work Power = DE Dt = W Dt DE = change in energy ( = work) Dt = time interval

Units of Power P = DE Dt Energy time = J/s = W = watt W = kg m2 s2 s

Power P = W Dt = Dt F·Ds = F·v F = force Ds = displacement A different but equivalent formula P = W Dt = Dt F·Ds = F·v F = force Ds = displacement v = velocity

Power P = F·v = ma·v = a·mv = a·p a = acceleration p = momentum You can also do P = F·v = ma·v = a·mv = a·p a = acceleration p = momentum but it isn’t as generally useful.

Question Dragging a box across a level floor against friction at 1 m/s requires a power of 20 W. How much power is required to drag the same box at 2 m/s? 10 W. 20 W. 40 W. 80 W.