ELEMENTARY PHYSICS RELATING TO FRICTION AND LUBRICATION.
~15.~ Most of those who may read this work, are no doubt familiar with the
laws of elementary physics; but as _all_ may not be, for a better
understanding of that which follows, it may be well to treat briefly of
some of the physical laws bearing on the subject.
~16. The Molecule.~[5] _Every visible body of matter is composed of
exceedingly small particles called molecules._ This is the basis of the
theory of the constitution of matter which physicists have usually
adopted. It is estimated that if we should attempt to count the number
of molecules in a pin's head, counting at the rate of 10,000,000 per
second, we should require 250,000 years.
~17. Porosity.~ The term _pore_ in physics is restricted to the invisible
space that separates molecules. All matter is porous; thus dense gold
will absorb (24) liquid mercury, much as chalk will water; but the
cavities to be seen in a sponge are not pores.
~18. Gravitation.~ _That attraction which is exerted on all matter, at all
distances, is called gravitation._ Gravitation is universal, that is,
every molecule of matter attracts every other molecule of matter in the
universe. The whole force with which two bodies attract one another is
the sum of the attraction of their molecules, and depends upon the
number of molecules the two bodies collectively contain, and the mass of
each molecule. Hence, all bodies attract, and are attracted by, all
other bodies.
In a ball suspended from the ceiling by a thread an attraction exists
between the ball and the ceiling, but on account of a greater attraction
existing between the ball and the earth, if we cut the thread the ball
will move toward the earth, or in the direction of the greater
attraction.
~19. The Effect of Distance.~ _Gravitation varies inversely with the
distance by which two bodies are separated._
As the sun is many times greater than the earth, the attraction between
the ball (18) and the sun would cause the ball to leave the earth and
move toward the sun were it not for the fact that the ball is so much
_nearer_ to the earth than to the sun.
~20. Cohesion.~ _The attraction which holds the molecules of the same
substance together so as to form larger bodies is called cohesion._
It acts only at insensible distances and is strictly a molecular force.
It is that force which prevents solid bodies from falling apart. Liquids
like molasses and honey possess more cohesive force among the molecules
of which they are composed than limpid liquids like water and alcohol.
The former are said to be viscous, or to possess _viscosity_.
~21. Adhesion.~ _That force which causes unlike substances to cling
together is called adhesion._ It is that force which keeps nails,
driven into wood, in their places. You can climb a pole because of the
adhesion between your hands and the pole. We could not pick anything up
if it were not for adhesion. Glue, when dry, possesses both cohesion and
adhesion to a great degree.
~22. Capillarity.~ Examine the surface of water in a vessel. You find the
surface level, except around the edge next the glass, as at A (Fig. 7.)
[Illustration: Fig. 7.]
[Illustration: Fig. 8.]
1. Thrust vertically into water three glass tubes, A, B and C (Fig. 8),
open at both ends. You notice the water ascends in each to a different
height, and that _the ascension varies inversely as the diameter of the
bore_; i. e., the smaller the bore, the higher the water ascends.
2. Seal one of the tubes at its upper end. The water enters but little,
as shown at D (Fig. 8), on account of the resistance of the air pressure
within the tube.
3. Thrust vertically two plates of glass into water, and gradually bring
the surfaces near to each other. Soon the water rises between the
plates, and rises higher as the plates are brought nearer. If their
surfaces be mutually parallel and vertical, the water rises to the same
height at all points between the plates, as shown at A (Fig. 9.)
[Illustration: Fig. 9.]
4. If the plates be united by a hinge, and form an angle, the height to
which the water ascends increases as the _distance_ between the plates
decreases up to their line of junction, where it attains a maximum, as
shown at B (Fig. 9.)
5. Decrease the _angle_ between the plates, and the water ascends
higher, as shown at C (Fig. 9.) Thus it is seen that _the ascension
varies inversely with the angle between the plates_; i. e., the smaller
the angle, the higher the water ascends.
6. When a drop of oil is placed between two glass plates arranged as
shown at A (Fig. 10), if the surfaces are not too far distant, and if
the oil touches both surfaces, it will be seen to work its way to the
junction of the plates; showing that _oil between surfaces has a
tendency to flow towards the apex of the angle_.
[Illustration: Fig. 10.]
7. Place a drop of oil on a taper piece of metal, as shown at B (Fig.
10). The oil will gradually recede from the point to a place where there
is more metal, showing that _oil on surfaces has a tendency to flow
towards the largest part_.
[Illustration: Fig. 11.]
8. When a drop of oil is placed between two watch glasses arranged with
flat and convex sides adjacent, as at A (Fig. 11), or with convex sides
adjacent, as at B (Fig. 11), if the glasses are rigidly fixed in their
relative positions the drop of oil can be shaken from its location only
with great difficulty; the oil at C holding its place with greater
tenacity than the oil at D.
The foregoing phenomena are called _capillary action_, or _capillarity_.
Capillary action is due to the forces of cohesion (20), and to the
forces of adhesion (21.)
~23. Centrifugal Force.~--_The tendency of a body rotating round a point
to escape from that point is called centrifugal force._
Place a small quantity of oil on the arm of a balance, near the arbor.
Rotate the wheel rapidly. The oil is seen to flow towards the rim of the
wheel.
~24. Absorption of Gases by Liquids~ depends on molecular attraction and
motion. Water at a temperature of 0 deg. cen. (32 deg. f.), is capable of
condensing in its _pores_ (17) six hundred times its own bulk of ammonia
gas. The absorption of oxygen from the air causes some oils to become
more viscous, to eventually become solid, without losing in weight, in
fact sometimes gaining. Other oils dry up, or _evaporate_, leaving
little or no residue.
~25. Force.~--_Force is that which can produce, change or destroy motion._
We see a body move; we know there must be a cause; that cause we call
force. We see a body in motion come to rest; this effect must have had a
cause; that cause we attribute to force. The forces acting in machines
are distinguished into _driving_ and _resisting_ forces. That component
of the force which does the work is called the "_effort_."
~26. Friction~ is usually a resisting (29) force, tending to destroy
motion; but it is sometimes the means of the transmission of motion.
~27. Work~ is the result of force acting through space. When force
produces motion, the result is work. _Work is measured by the product of
the resistance into the space through which it is overcome._
~28. Energy~, which is defined[6] as the capacity for doing work, is
either _actual_ or _potential_. _Actual_ or _kinetic energy_ is the
energy of an actually moving body, and is measured by the work which it
is capable of performing while being brought to rest under the action of
a retarding force.
_Potential Energy_ is the capacity for doing work possessed by a body in
virtue of its position, of its condition, or of its intrinsic
properties. A bent bow or a coiled spring has potential energy, which
becomes actual in the impulsion of the arrow or is expended in the work
of the mechanism driven by the machine. A clock weight, condensed air
and gunpowder are examples.
This form of energy appears in every moving part of every machine and
its variations often seriously affect the working of machinery. (84.)
FOOTNOTES:
[5] This and some of the definitions that follow are adapted from
"Elements of Physics" by A. P. Gage.
[6] Thurston. "Friction and Lost Work in Machinery," from which
excellent work much of the next chapter is adapted.