Energy is what you have to add to make cold things warmer, slowly moving objects go faster, or to lift something up. It
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.