Power is the rate at which work is done.

P

=

dW dt

The MKS unit for power is the watt (W).     1 W = 1 J/s

For constant force, work done W = F • d , and the power becomes P = F • v

Note that 1 hp = 746 W

What is the average power output of a human?

Estimate the food input: Most of us eat around 2500 "calories" (actually kilocalories) daily. We use this number as our daily energy output.

Convert 2.5 x 10⁶ cal to J (1 cal = 4.186 J): 1.05 x 10⁷ J

To find our average power output, divide the energy by the number of seconds in a day, 86400, which gives approximately 120 W.

What if you're not a couch potato? Example: climb Mt. Monadnock in 1 hour

U = mgh ≈ 70 x 10 x 1000 = 700,000 J.
But efficiency of human motion is approximately 20%.

So energy expended in climbing ≈ 3.5 x 10⁶ J ≈ 800 "calories".

Power output = 3,500,000/3,600 ≈ 1000 W, of which ≈ 200 W goes into motion.