Environmental Engineering Reference
In-Depth Information
blocks used to manufacture cannons. In 1843 James Prescott Joule established the
mechanical equivalent of heat.
More recently, the kinetic theory of gases led to a full understanding of the in-
timate connection between mechanical work and heat. According to this theory,
gases are formed when molecules collide with each other and with the walls of the
object holding them. Heat, therefore, is nothing other than mechanical energy: the
higher the temperature, the higher the average velocity of the molecules.
Friction is not a fundamental force like gravitational or electromagnetic forces. It
originates in surface irregularities and/or forces between objects that come into
contact with each other. Its characteristics are entirely empiric and depend on the
nature of the surfaces in contact.
For example, two very clean glass plates, once put into contact, even in a vacu-
um, will be difficult to separate. It is as if there were “tentacles” that, starting from
one surface, retain the other, making it necessary to break them to separate them.
Friction has a very important role in the performance of all kinds of machines,
because overcoming it requires work that otherwise would be used for other pur-
poses. Friction is thus sometimes referred to as a “dissipative force,” which is irre-
versible. If the movement is reversed, such as in the sliding of two surfaces against
each other, energy would have to be spent again.
In the absence of friction, the continuous movement of a system that periodic-
ally returns to its original position, such as an oscillating pendulum, would be pos-
sible. But in reality air causes friction, so the amplitude of an oscillation is reduced
gradually until it stops. In a vacuum a pendulum would oscillate much longer be-
fore slowing down and stopping, since the only existing friction is at the point of
suspension.
One joule is defined as the work performed by a force of 1 newton (N) in a dis-
placement of 1 m. The force of gravity on 1 kg of matter is 9.8 N; therefore, 1 J is
the amount of energy needed to lift a body of 102 g to a height of 1 m.
That amount of work (or the energy needed to produce it) can also be measured
in calories. One calorie is the quantity of energy needed to increase the temperature
