What is Power?
Common Units
of
Power
SI:
Watts
(W)
1000 W = 1 kW
Kilowatt
(kW)
1 kW = 1.341 hp
English
Horsepower
(hp)
1 hp = 0.746 kW
|
Power is a measure of how
quickly work can be done. Using a lever, you may be able to
generate 200 ft-lb of torque. But could you spin that lever
3,000 times per minute? That is exactly what your car engine
does.
The SI unit for power is the watt. A watt breaks
down into other units that we have already talked about. One
watt is equal to 1 Newton-meter per second (Nm/s). You can
multiply the amount of torque in Newton-meters by the
rotational speed in order to find the power in watts. Another
way to look at power is as a unit of speed (m/s) combined with
a unit of force (N). If you were pushing on something with a
force of 1 N, and it moved at a speed of 1 m/s, your power
output would be 1 watt.
An interesting way to figure out how much power you can
output is to see how quickly you can run up a flight of
stairs.
- Measure the height of a set of stairs that takes you up
about three stories.
- Time yourself while you run up the stairs as quickly as
possible.
- Divide the height of the stairs by the time it took you
to ascend them. This will give you your speed.
For
instance, if it took you 15 seconds to run up 10 meters, then
your speed was 0.66 m/s (only your speed in the vertical
direction is important). Now you need to figure out how much
force you exerted over those 10 meters, and since the only
thing you hauled up the stairs was yourself, this force is
equal to your weight. To get the amount of power you output,
multiply your weight by your speed.
Power (W) = (height of stairs (m) /
Time to climb (s) ) * weight (N)
Power (hp) = [(height of stairs (ft) / Time to climb (s) )
* weight (lb)] / 550
