In this chapter, we will explain fundamental engineering dimensions, such as length
and time, and their units, such as meter and second, and their role in engineering
analysis and design. As an engineering student, and later as practicing engineer,
when performing an analysis, you will find a need to convert from one system of
units to another. We will explain the steps necessary to convert information from one
system of units to another correctly.
In this chapter, we will also emphasize the fact that you must always show the
appropriate units that go with your calculations. Finally, we will explain what is
meant by an engineering system and an engineering component.
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The evolution of the human intellect has taken shape over a period of thousands of years. Men
and women all over the world observed and learned from their surroundings. They used the
knowledge gained from their observations of nature to design, develop, test, and fabricate tools,
shelter, weapons, water transportation, and means to cultivate and produce more food. More-
over, they realized that they needed only a few physical quantities to fully describe natural events
and their surroundings. For example, the length dimension was needed to describe how tall or
how long or how wide something was. They also learned that some things are heavier than
other things, so there was a need for another physical quantity to describe that observation: the
concept of mass and weight. Early humans did not fully understand the concept of gravity;
consequently, the correct distinction between mass and weight was made later.
Time was another physical dimension that humans needed to understand in order to be able
to explain their surroundings and to be able to answer questions such as: “How old are you?” “How
long does it take to go from here to there?” “How long does it take to cook this food over fire?” The
response to these questions in those early days may have been something like this: “I am many
many Moons old,” or “It takes a couple of Moons to go from our village to the other village on the
other side of mountain.” Moreover, to describe how cold or hot something was, humans needed
yet another physical quantity, or physical dimension, that we now refer to as temperature.
By now, you understand why we need to formally define physical variables. The other impor-
tant fact you need to realize is that early humans needed not only physical dimensions to describe
their surroundings but also some way to scale or divide these physical dimensions. This realiza-
tion led to the concept of units. For example, time is considered a physical dimension, but it can
be divided into both small and large portions, such as seconds, minutes, hours, days, years, decades,
centuries, millennia, and so on. Today, when someone asks you how old you are, you reply by say-
ing, “I am 19 years old.” You don’t say that you are approximately 170,000 hours old or
612,000,000 seconds old, even though these statements may very well be true at that instant! Or
to describe the distance between two cities, we may say that they are 2000 kilometers apart; we
don’t say the cities are 2,000,000,000 millimeters apart. The point of these examples is that we
use appropriate divisions of physical dimensions to keep numbers manageable. We have learned
to create an appropriate scale for these fundamental dimensions and divide them properly so that
we can describe particular events, the size of an object, the thermal state of an object, or its inter-
action with the surroundings correctly, and do so without much difficulty.6.1 Engineering Problems and Fundamental Dimensions 131
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