Page 15 - Thomson, William Tyrrell-Theory of Vibration with Applications-Taylor _ Francis (2010)

2 The SI System of Units

Because the meter is a large unit of length, it will be more convenient to
express it as the number of millimeters multiplied by 10“^. Vibration instruments,
such as accelerometers, are in general calibrated in terms of g = 9.81 m/s^, and
hence expressed in nondimensional units. It is advisable to use nondimensional
presentation whenever possible.
In the English system, the weight of an object is generally specified. In the SI
system, it is more common to specify the mass, a quantity of matter that remains
unchanged with location.
In working with the SI system, it is advisable to think directly in SI units. This
will require some time, but the following rounded numbers will help to develop a
feeling of confidence in the use of SI units.
The newton is a smaller unit of force than the pound. One pound of force is
equal to 4.4482 newtons, or approximately four and a half times the value for the
pound. (An apple weighs approximately ^ lb, or approximately I newton.)
One inch is 2.54 cm, or 0.0254 meter. Thus, the acceleration of gravity, which
is 386 in./s^ in the English system, becomes 386 x 0.0254 = 9.81 m/s^, or
approximately 10 m/s^.
TABLE OF APPROXIMATE EQUIVALENTS
1 lb = 4.5 N
Acceleration of gravity g = 10 m/s^
Mass of I slug = 15 kg
1 ft = I m

SI conversion. A simple procedure to convert from one set of units to
another follows: Write the desired SI units equal to the English units, and put in
canceling unit factors. For example, if we wish to convert torque in English units
into SI units, we proceed as follows:
Example 1
[Torque SI] = [Torque English] X [multiplying factors]

[ Nm] =
= [lb • in.](4.448)(0.0254)
= [lb • in.](0.1129)
Example 2

[Moment of inertia SI] = [Moment of inertia English] X [multiplying factors]
[kg • m‘ -2 = N • m • s^] = [)b • jti.- s 2 ](^ •

= [lb • in. - s2](4.448 X 0.0254)
= [lb • in. - s2](0.1129)