Energy is a fundamental physical concept
which can be used to interpret many everyday phenomena.
Physicists claim that they can understand almost anything if
they know what happens to the energy. We will discuss how fluids
flow in terms of energy, too. First, though, we have to remind
ourselves of the basic facts.
There are many forms of energy. Here are some examples:
An object has more energy when it is moving ("kinetic
energy"). Kinetic energy also depends on mass, so that
heavy objects have more kinetic energy than
light ones do, if they are moving at the same speed..
A book on a high shelf has more energy than when it is
on the floor ("gravitational potential energy"). Gravity energy also
depends on the weight of the book.
A stretched spring has more energy than the unstretched
spring ("elastic energy").
A hot object has more energy than when it is cold
("thermal energy").
Sound carries energy. Sound is air in motion, and
motion means kinetic energy.
Light delivers energy
("radiant energy").
A chemical fuel (such as coal or gasoline) releases
energy when it is burned ("chemical energy").
Energy can be converted from one form to another; however,
the total amount of energy in the universe does not change.
We say:"Energy is conserved," and refer to
this idea as the Principle of Conservation of Energy,
or even The First Law of Thermodynamics
(with these two grandiose names it must be important!).
To understand this idea, we need to be able to
recognize the various forms of energy, and to notice
the conversions taking place.
As examples of the many conversions that can happen, consider a rock being
shot into the air by a slingshot.
At first there is elastic
energy in the stretched rubber.
This is then converted into
the kinetic energy of the moving rock.
As the rock rises, its gravitational energy
is increasing. This energy comes from the kinetic energy:
the rock slows down.
As the rock falls back to earth the gravitational energy
is converted back into kinetic energy,
and the rock speeds up.
When the rock hits the ground, the energy is converted into other
forms, that may be harder to detect -- the sound it makes, vibrations
in the ground, and a rise in temperature of the rock and whatever it hit.
Conservation of energy relates
the initial energy of the slingshot (determined by how
far back it was pulled) to the initial speed of the
rock (related to its kinetic energy) and to the height
to which the rock can go.