To estimate the energy in wind, let’s imagine holding up a hoop with area
A, facing the wind whose speed is v. Consider the mass of air that passes
through that hoop in one second. Here’s a picture of that mass of air just
before it passes through the hoop:
And here’s a picture of the same mass of air one second later:
The mass of this piece of air is the product of its density ρ, its area A, and
its length, which is v times t, where t is one second.
The kinetic energy of this piece of air is
So the power of the wind, for an area A – that is, the kinetic energy passing
across that area per unit time – is
This formula may look familiar – we derived an identical expression on
p255 when we were discussing the power requirement of a moving car.
What’s a typical wind speed? On a windy day, a cyclist really notices
the wind direction; if the wind is behind you, you can go much faster than