ENERGY: The Basic Concepts - Energy: What Everyone Needs to Know - page 2

What is energy?
If one wanted to lift the same object to the height from which it originally fell, the
person would need to spend a given amount of energy exactly equal to W. Energy
may be defined as the capacity to produce work.
The energy may be kinetic (for example, the force deriving from waves and
winds), gravitational (from waterfalls), electric (from turbines and batteries),
chemical (obtained from exothermic reactions, such as diesel and gasoline combus-
tion), thermal (from burning charcoal or wood), radiant (from sunlight), and nucle-
ar (obtained from the fission of uranium atoms or the fusion of hydrogen nuclei).
Some forms are more useful than others; several can be transformed. For example,
the energy obtained from a nuclear reaction may be used to heat water and produce
high pressure steam, which, in turn, can produce work to move a turbine to produce
electricity.
The ability to move objects is essential to our survival, and the amount of work
needed for that depends very much on how much we do and the energy we expend.
Which are the common forces in nature?
There are three types of forces that are considered fundamental: gravitational, elec-
tromagnetic, and nuclear.
Gravitational forces exist between bodies, owing to their mass. It is part of our
everyday experience that all bodies fall downward when set free. Since ancient
times, scientists have studied the movement of bodies when falling, but Isaac New-
ton, who studied gravitational forces in 17th-century England, was the first to fully
understand them. What Newton did was to realize that one could understand why
bodies fall to the ground, why the Moon rotates around the Earth, and why the
Earth circles around the Sun. He introduced the idea that there is an attractive force
between any two bodies, with masses m 1 and m 2 , and that the force is proportion-
al to the mass of these bodies; this force decreases as the distance between them
increases, proportionally to the inverse square of the distance. This is the Law of
Universal Gravitation. It is possible to show that all the Earth exerts an attraction
on a body as if its whole mass were concentrated at its center. Electromagnetic
(electric and magnetic) forces exist due to electric charges. Electric forces are at-
tractive when they have different charges (positive and negative) or repulsive when
having the same charge. Magnetism has been understood since the 5th century be-
fore the Christian era. The Greeks were familiar with the attractive (and repulsive)