Natural Science Forum / Physics / General Physics / June 2007

Sunlight evaporates water, water falls as rain, rain fills streams and
rivers,
rivers fill lakes behind dams, kinetic energy of falling water drives
turbines
which drive generators which do useful work. The work eventually comes
heat which radiates back into space.

The dam did no work, the energy came from the sun.
Dams cannot do work because dams do not pump water uphill.

Force is not energy. You can compress a spring, and the spring
will then exert a force which allows work to be done, but only until
the spring is once again relaxed. Then you must apply more energy to
compress it again. Whilst the spring is compressed and not allowed
to relax, it exerts a force but does no work. Force is not energy.

All energy we use on Earth (saving minute amounts of starlight
and more recently tidal energy) comes or came directly from the sun.

Energy can be stored and it can be converted from one form to another,
that's what technology is all about.
Sunlight -> photo-chemical reaction with CO2 releasing oxygen -> tree growth
building carbon molecules -> wood -> coal/vegetable oil->
petroleum/plastics-> cars/car fuel -> chemical reaction with oxygen
(burning) -> heat -> radiate to space.

Efficiency is a measure of how much energy is converted. Power stations
run at about 34% efficient, the remaining 66% of heat from the coal/oil
is converted to steam and radiated into space, leaving 34% electricity.
Cars and trucks run at around 18% efficiency, electrical transformers
and motors at around 99% efficient, transferring electrical energy to
motive power with very little heat loss.

Gravity is a force and doesn't relax. You lift a brick onto a shelf, you've
given it sufficient energy to fall and break your toe. Stand above the shelf
and the brick no longer has energy to break your toe but you've given
yourself sufficient energy to fall and crack your skull. Thus energy is
relative. Gravity does no work, the work was done when you lifted
the brick. We say the raised brick has "potential" energy, PE, and we
say the falling brick has "kinetic" (motion) energy, KE. It was first lifted
against gravity in order for it to have energy.
No work is done by the falling brick until it suddenly decelerates upon
collision with your toe, when the energy is converted in breaking the
electrical bonds between the molecules of your toe bones and tearing
the flesh apart as you get a laceration at the same time.
                    ****** FORCE IS NOT ENERGY. ********

Power.
One horsepower was a useful measure to compare how well
a steam engine could perform against a horse. It is 550 ft
lbs per second, meaning that a horse can lift 550 lbs to a height
of 1 foot or 1 lb to a height of 550 feet in one second and
keep it up all day long. A man can generate one horsepower
but he cannot maintain it all day, he will collapse with the exertion.
You can prove this by running upstairs and timing yourself
with a stopwatch. It is your own weight you are lifting.
A horse can easily generate 10 horsepower, it can run faster than
a man but will not maintain it all day.
Thus a man is not as powerful as a horse which is not as powerful
as a steam engine. As a rough guide, a domestic vacuum cleaner
or washing machine has about a horsepower (746 watts).
The watt was named after James Watt who did much theoretical
work on steam engines.
This is basic schoolboy physics which you should have been
taught in your early teenage years.
We are now a heck of a long way from trying to discover the
nature of action-at-a-distance which I cannot debate with you
because you haven't understood the basics and do not know
the language of physics.
No! A dam cannot do work.