Except for a few trifling practical details, we could easily get
around without the consumption of hydrocarbons and the
production of greenhouse gases, simply making use of the force
of gravity that is constant and inexhaustible. For example,
suppose we must journey to a location 30 miles away and at the
same elevation. We keep our vehicle at the top of a ramp 100 ft.
high. It only has to be hoisted there once. When we wish to
travel, we shove off and when we reach the foot of the ramp we
are going at about 55 mph. The speed attained is v = √(2gh),
where g is the acceleration of gravity (32 fpsps or 980 cm/s2).
Our 30-mile trip will require about 33 minutes, quite comparable
to the capability of an ordinary motorcar, but now with
effortless quietness and unsurpassable economy. At the
destination, we slow the vehicle by letting it climb a ramp, and
it stops when it reaches an elevation of 100 ft. To return, we
simply turn the vehicle around and push off. The only problem is
friction, but many proposals we hear