SM0486
of gravity acting on that body. The difference between mass and weight can be seen
when we compare a 5-pound weight with a 10-pound weight.
If both weights are
dropped from heights in the same locality, their accelerations are the same (32
ft/sec/sec). However, the inertia (force resisting any change in the velocity of a
body) of the 10-pound weight is twice that of the 5-pound weight because of the
inertia of a body is directly proportional to its weight. Since the gravitational
pull on a body varies inversely with the square of the distance from the earth, the
weight of a body is not constant.
d. The force which tends to pull a body toward the center of the earth is
known as gravity.
The intensity of force varies inversely as the square of the
distance that a body is moved away from the center of the earth. This means that
the "pull" of gravity on a body which is situated at sea level is greater than it
would be on the same body at a point 5000 feet above sea level.
e. There are several terms which are more directly related to the pressure
measuring instruments used in your laboratory.
These terms and the corresponding
definitions are included in the list which follows:
Buoyancy--The "upward" force that pushes against a body which is submerged
wholly or partially in a liquid.
The force with which the body is buoyed up is
equal to the weight to the liquid displaced by the body.
Resolution--The sensitivity of an instrument. The smallest alteration in the
quantity to be measured which produces any change whatever (response) in the
indication of the instrument.
Sensitivity--The degree of responsiveness.
The rate of displacement of the
indicating element with respect to changes in the measured quantity.
Repeatability--Performance relative to the instrument itself. A measure of the
consistency of performance.
The quality of repeatability is usually expressed in
terms of percentage variation of reading.
Hydrostatic Head--Sometimes referred to as hydraulic head, oil head, or head.
The height of a column of body of fluid above a given point considered as causing,
counteracting, or measuring pressure.
In determining the quality of pressure
caused by a certain head, multiply the height of the column of fluid by the fluid
density.
Tolerance--A specified allowance for error or variation from standard operation
or measurement.
f. The terms included in the preceding list are used in technical documents
which provide information concerning equipment changes and calibration procedures.
When you are sure that you understand all of the terms listed, you should have very
little trouble understanding the material which follows in this chapter. Let's see
how these terms (some of which represent theories and laws of physics) are related
to other physi-
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