Engineers are also good bookkeepers. What do we mean by this? Any of us with a check-
ing account knows the importance of accurate record keeping. In order to avoid problems, most
of us keep track of the transactions in terms of payments (debits) and deposits (credits). Good
bookkeepers can tell you instantly what the balance in their account is. They know they need
to add to the recorded balance whenever they deposit some money and subtract from the bal-
ance with every withdrawal from the account. Engineers, like everyone else, need to keep track
of their accounts. Moreover, similar to bookkeeping a checking account, engineers keep track
of (bookkeep) physical quantities when analyzing an engineering problem.
To better understand this concept, consider the air inside a car tire. If there are no leaks,
the mass of air inside the tire remains constant. This is a statement expressingconservation of
mass, which is based on our observations. If the tire develops a leak, then you know from your
experience that the amount of air within the tire will decrease until you have a flat tire. Fur-
thermore, you know the air that escaped from the tire was not destroyed; it simply became part
of the surrounding atmosphere. The conservation of mass statement is similar to a bookkeep-
ing method that allows us to account for what happens to the mass in an engineering problem.
What happens if we try to pump some air into the tire that has a hole? Well, it all depends on
the size of the hole and the pressure and flow rate of the pressurized air available to us. If the
hole is small, we may be able to inflate the tire temporarily. Or the hole may be so large that
the same amount of air that we put into the tire comes right back out. To completely describe
all possible situations pertaining to this tire problem, we can express the conservation of mass
asthe rate at which air enters the tire minus the rate at which the air leaves the tire should be equal
to the rate of accumulation or depletion of air inside the tire.Of course, we will use the physical
quantity mass along with mathematics to express this statement. We will discuss the conserva-
tion of mass in more detail in Chapter 9.
There are other physical laws based on our observations that we use to analyze engineering
problems.Conservation of energyis another good example. It is again similar to a bookkeeping
method that allows us to keep track of various forms of energy and how they may change from one
form to another. We will spend more time discussing the conservation of energy in Chapter 13.
Another important law that all of you have heard about isNewton’s second law of motion.
If you place a book on a smooth table and push it hard enough, it will move. This is simply the
way things work. Newton observed this and formulated his observation into what we call
Newton’s second law of motion. This is not to say that other people had not made this simple
observation before, but Newton took it a few steps further. He noticed that as he increased the
mass of the object being pushed, while keeping the magnitude of the force constant (pushing
with the same effort), the object did not move as quickly. Moreover, he noticed that there was
a direct relationship between the push, the mass of the object being pushed, and the accelera-
tion of the object. He also noticed that there was a direct relationship between the direction of
the force and the direction of the acceleration. Newton expressed his observations using math-
ematics, but simply expressed, this law states that unbalanced force is equal to mass times accel-
eration. You will have the opportunity to take physics classes that will allow you to study and
explore Newton’s second law of motion further. Some of you may even take a dynamics class
that will focus in greater detail on motion and forces and their relationship. Don’t lose sight of
the main idea: Physical laws are based on observations.
Another important idea to keep in mind is that a physical law may not fully describe all pos-
sible situations. Statements of physical laws have limitations because we may not fully understand
how nature works, and thus we may fail to account for all variables that can affect the behavior of150 Chapter 6 Fundamental Dimensions and Units
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