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encountered on the job may involve rapidly revolving wheels or flywheels. If the wheel is revolving
fast enough, and if the molecular structure of the wheel is not strong enough to overcome the cen-
trifugal force, it may fracture and pieces of the wheel would fly off tangent to the arc described by
the wheel. The safety implications are obvious. Any worker using such a device, or near it may be
severely injured when the rotating member ruptures. This is what happens when a grinding wheel
on a pedestal grinder “bursts.” Rim speed determines the centrifugal force, and rim speed involves
both the speed (rpm) of the wheel and the diameter of the wheel.
11.5.5 s tress and s train
In materials, stress is a measure of the deforming force applied to a body. Strain (which is often erro-
neously used as a synonym for stress) is really the resulting change in its shape (deformation). For
perfectly elastic material, stress is proportional to strain. This relationship is explained by Hooke's
law, which states that the deformation of a body is proportional to the magnitude of the deforming
force, provided that the body's elastic limit is not exceeded. If the elastic limit is not reached, the
body will return to its original size once the force is removed. For example, if a spring is stretched 2
cm by a weight of 1 N, it will be stretched 4 cm by a weight of 2 N, and so on; however, once the load
exceeds the elastic limit for the spring, Hooke's law will no longer be obeyed, and each successive
increase in weight will result in a greater extension until the spring finally breaks.
Stress forces are categorized in three ways:
1. Tension (or tensile stress), in which equal and opposite forces that act away from each other
are applied to a body; tends to elongate a body.
2. Compression stress, in which equal and opposite forces that act toward each other are
applied to a body; tends to shorten a body.
3. Shear stress, in which equal and opposite forces that do not act along the same line of
action or plane are applied to a body; tends to change the shape of a body without changing
its volume.
11.6 MATERIALS AND PRINCIPLES OF MECHANICS
To be able to recognize hazards and to select and implement appropriate controls, environmental
engineers must have a good understanding of the properties of materials and principles of mechan-
ics. In this section, we start with the properties of materials, and then cover the wide spectrum that
is comprised of mechanics and soil mechanics. Our intent is to clearly illustrate the wide scope of
knowledge required in areas germane to the properties of materials and the principles of mechanics, as
well as those topics on the periphery, all of which are blended in the mix—the safety knowledge mix
that helps to produce the well-rounded, knowledgeable environmental engineer. Mechanics is the cor-
nerstone of physics and is of primary interest because the world is full of many kinds of motions that
are often used for practical purposes such as the motions of falling objects, of cars, of boats, of planes,
of rolling wheels, of flowing liquids, of moving air masses in meteorology, and so forth (Reif, 1996).
In this section, we discuss the mechanical principles of statics, dynamics, moments, beams, columns,
floors, industrial noise, and radiation. The field environmental engineer should have at least some
familiarity with all of these. (Note that the environmental engineer whose function is to verify design
specifications, with safety in mind, should have more than just a passing familiarity with these topics.)
11.6.1 s tatiCs
Statics is the branch of mechanics concerned with the behavior of bodies at rest and forces in equilib-
rium and is distinguished from dynamics (concerned with the behavior of bodies in motion). Forces
acting on statics do not create motion. Static applications are bolts, welds, rivets, load-carrying
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