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

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

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

4. 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

5. 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.

6. 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.