% 1m^3 is 1 Kg; Want 1/2 m v^2 = 1kJ -> v^2 = 2000 -> v = 44 m/s
% 44 (meters per second) = 98.4251969 miles per hour
\subsection{Kinetic energy: air}
A device that takes eight cubic metre of stationary air per second and
sets it swirling around at a speed of 30 miles per hour
is pumping energy into
% delivering energy to
the air at a rate of 1\,kW.
An example of such a device is a motorcycle travelling at 30 miles per hour.
%% should I interrupt the flow by saying this? ...
(Swirling air is one of the main things that motorcycles and cars do with
their energy, as we'll see in chapter \ref{ch.cars}.)
% If you hold a four-square-metre
When the wind blows at 10 miles per hour through an area of
16 square metres,
% 4 metres by 4 metres,
the power of that passing kinetic energy is 1\,kW.
\subsection{Kinetic energy: metal boxes}
%%invalid carriages}
%%
Imagine a one-ton metal box on wheels that keeps on
speeding up from zero to 30\,mph in 5 seconds, then decelerating
back to zero in the next 5 seconds.
%% check distance gone:
%% x = v_max * t_up = 15m/s * 5s = 75 m
Such a suburban invalid carriage (or car, for short)
uses about 10\kW, if it is perfectly efficient.
[When cars go faster than 30\,mph,
they use more power; we'll discuss this in part I.]