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Table 1: SI Units (Himmelblau & Riggs).


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Table 2: AE Units (Himmelblau & Riggs).


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Conversion of Units:

An engineer has to be able to convert units from one

system to another. The procedure is done by

multiplying or dividing by conversion factors. Theprocedure described here is done by multiplying by

1.

For example:

cm542in1 .=

(1)

Therefore:

1cm542

in1=

.(2)

So, that 10 cm are:

in943cm542

in1cm10 .

.= (3)

Example 1.2 (H&R): Convert 400 in3 /day to

cm3/min.

/mincm554min60

hr1

hr24

day1

in1

cm542

day1

in400 333

..

=

(4)


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The gc Conversion Factor:

In the past you used to write Newtons second law of

motion in the following wrong way:

amF= (5)

Where Fis force, m is mass and a is the acceleration.

However: N kgm/s2.

In order to make the equation dimensionallyconsistent, we introduce a conversion factor:

amg

Fc

1= (6)

The force of 1 N is required to accelerate a mass of 1

kg at a rate of 1 m/s2. Therefore, in SI units the

conversion factor is gc = 1 kgm/Ns2. In the AE

system gc = 32.2 lbmft/ lbfs2.

Most of the books do not place the gc in the

equations. However, you will find out that this is

essential, especially when working with the AEsystem.


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Significant Figures:

A measurement should include the following: (a) the

magnitude of the variable being measured, (b) its

units and (c) an estimate of its uncertainty.

When we have no idea of the accuracy of

measurement we can do the following. For example,

1.43 indicates 1.43 0.005, so the value can bebetween 1.425 and 1.435. Another interpretation is

1.43 0.01.

The numbers 81, 81.0 and 81.00 are different. The

first one has 2 significant figures, the second one has

3 significant figures and the third one has 4

significant figures. The number 2300 has 2

significant figures and the number 23,040 has 4

significant figures.

When we multiply or divide numbers, we should

retain in the final answer the lowest number of

significant figures among all the numbers involved.

For example:

55454612409263471 .... == (7)


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Consider the following example:

110536510193

98..

.== (8)

If we follow the rule, the answer is a distortion of the

true error. Since 98 1 has an error of about 1%while the result 1.1 0.1 has an error of about 10%.This time 1.05 may reflect a better answer. Avoid

increasing the precision of your answer very muchover the precision in your measurements. Also, use

some common sense!

When adding or subtracting, the number with the

lowest decimal places will determine the number of

digits in the solution. For example:

311033811003803110 .... ==+ (9)

Finally, a rule of thumb for rounding off numbers in

which the digit to be dropped is 5 is always to make

the last digit an even number. For example:

41351 .. (10)

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