12 Characteristics of Force
12–1 What is a force?
Although it is interesting and worth while to study the physical laws
simply because they help us to understand and to use nature, one ought
to stop every once in a while and think, “What do they really mean?”
The meaning of any statement is a subject that has interested and
troubled philosophers from time immemorial, and the meaning of
physical laws is even more interesting, because it is generally
believed that these laws represent some kind of real knowledge. The
meaning of knowledge is a deep problem in philosophy, and it is always
important to ask, “What does it mean?”
Let us ask, “What is the meaning of the physical laws of
Newton, which we write
as F=m a? What is the meaning of force, mass, and acceleration?” Well,
we can intuitively sense the meaning of mass, and we can define
acceleration if we know the meaning of position and time. We shall not
discuss those meanings, but shall concentrate on the new concept of
force. The answer is equally simple: “If a body is accelerating,
then there is a force on it.” That is what Newton’s
laws say, so the most precise and beautiful
definition of force imaginable might simply be to say that force is the
mass of an object times the acceleration. Suppose we have a law which
says that the conservation of momentum is valid if the sum of all the
external forces is zero; then the question arises, “What does it
mean, that the sum of all the external forces is zero?” A
pleasant way to define that statement would be: “When the total
momentum is a constant, then the sum of the external forces is zero.”
There must be something wrong with that, because it is just not saying
anything new. If we have discovered a fundamental law, which asserts
that the force is equal to the mass times the acceleration, and then
define the force to be the mass times the acceleration, we have
found out nothing. We could also define force to mean that a moving
object with no force acting on it continues to move with constant
velocity in a straight line. If we then observe an object not
moving in a straight line with a constant velocity, we might say that
there is a force on it. Now such things certainly cannot be the content
of physics, because they are definitions going in a circle. The
Newtonian statement above, however, seems to be a most precise
definition of force, and one that appeals to the mathematician;
nevertheless, it is completely useless, because no prediction whatsoever
can be made from a definition. One might sit in an armchair all day long
and define words at will, but to find out what happens when two balls
push against each other, or when a weight is hung on a spring, is
another matter altogether, because the way the bodies behave is
something completely outside any choice of definitions.
For example, if we were to choose to say that an object left to itself
keeps its position and does not move, then when we see something
drifting, we could say that must be due to a “gorce”—a gorce is the
rate of change of position. Now we have a wonderful new law, everything
stands still except when a gorce is acting. You see, that would be
analogous to the above definition of force, and it would contain no
information. The real content of Newton’s laws is this: that the
force is supposed to have some independent properties, in
addition to the law F=m a; but the specific independent
properties that the force has were not completely described by
Newton or by anybody else, and
therefore the physical law F=m a is an incomplete law. It implies that
if we study the mass times the acceleration and call the product the
force, i.e., if we study the characteristics of force as a program of
interest, then we shall find that forces have some simplicity; the law
is a good program for analyzing nature, it is a suggestion that the
forces will be simple.
Now the first example of such forces was the complete law of
gravitation, which was given by Newton, and in stating the law he answered the question, “What
is the force?” If there were nothing but
gravitation, then the combination of this law and the
force law (second law of motion) would be a complete theory, but there
is much more than gravitation, and we want to use Newton’s
laws in many different situations.
Therefore in order to proceed we have to tell something about the
properties of force.
For example, in dealing with force the tacit assumption is always made
that the force is equal to zero unless some physical body is present,
that if we find a force that is not equal to zero we also find
something in the neighborhood that is a source of the force. This
assumption is entirely different from the case of the “gorce” that
we introduced above. One of the most important characteristics of
force is that it has a material origin, and this is not just a
definition.
Newton also gave one rule about the
force: that the forces between interacting bodies are equal and
opposite—action equals reaction; that rule, it turns out, is not
exactly true. In fact, the law F=m a is not exactly true; if it were a
definition we should have to say that it is always exactly true;
but it is not.
The student may object, “I do not like this imprecision, I should
like to have everything defined exactly; in fact, it says in some
books that any science is an exact subject, in which everything
is defined.” If you insist upon a precise definition of force, you
will never get it! First, because Newton’s Second Law is not exact,
and second, because in order to understand physical laws you must
understand that they are all some kind of approximation.
Any simple idea is approximate; as an illustration, consider an
object, … what is an object? Philosophers are always saying,
“Well, just take a chair for example.” The moment they say that, you
know that they do not know what they are talking about any more. What
is a chair? Well, a chair is a certain thing over there
… certain?, how certain? The atoms are evaporating from it from
time to time—not many atoms, but a few—dirt falls on it and gets
dissolved in the paint; so to define a chair precisely, to say exactly
which atoms are chair, and which atoms are air, or which atoms are
dirt, or which atoms are paint that belongs to the chair is
impossible. So the mass of a chair can be defined only
approximately. In the same way, to define the mass of a single object
is impossible, because there are not any single, left-alone objects in
the world—every object is a mixture of a lot of things, so we can
deal with it only as a series of approximations and idealizations.
The trick is the idealizations. To an excellent approximation of
perhaps one part in 1010, the number of atoms in the chair does
not change in a minute, and if we are not too precise we may idealize
the chair as a definite thing; in the same way we shall learn about
the characteristics of force, in an ideal fashion, if we are not too
precise. One may be dissatisfied with the approximate view of nature
that physics tries to obtain (the attempt is always to increase the
accuracy of the approximation), and may prefer a mathematical
definition; but mathematical definitions can never work in the real
world. A mathematical definition will be good for mathematics, in
which all the logic can be followed out completely, but the physical
world is complex, as we have indicated in a number of examples, such
as those of the ocean waves and a glass of wine. When we try to
isolate pieces of it, to talk about one mass, the wine and the glass,
how can we know which is which, when one dissolves in the other? The
forces on a single thing already involve approximation, and if we have
a system of discourse about the real world, then that system, at least
for the present day, must involve approximations of some kind.
This system is quite unlike the case of mathematics, in which everything
can be defined, and then we do not know what we are talking
about. In fact, the glory of mathematics is that we do not have to
say what we are talking about. The glory is that the laws, the
arguments, and the logic are independent of what “it” is. If we have
any other set of objects that obey the same system of axioms as Euclid’s
geometry, then if we make new definitions and
follow them out with correct logic, all the consequences will be
correct, and it makes no difference what the subject was. In nature,
however, when we draw a line or establish a line by using a light beam
and a theodolite, as we do in surveying, are we measuring a line in the
sense of Euclid? No, we are making an
approximation; the cross hair has some width, but a geometrical line has
no width, and so, whether Euclidean geometry can be used for
surveying or not is a physical question, not a mathematical question.
However, from an experimental standpoint, not a mathematical standpoint,
we need to know whether the laws of Euclid apply to the kind of geometry that we use in measuring land; so we
make a hypothesis that it does, and it works pretty well; but it is not
precise, because our surveying lines are not really geometrical lines.
Whether or not those lines of Euclid,
which are really abstract, apply to the lines of experience is a
question for experience; it is not a question that can be answered by
sheer reason.
In the same way, we cannot just call F=m a a definition, deduce
everything purely mathematically, and make mechanics a mathematical
theory, when mechanics is a description of nature. By establishing
suitable postulates it is always possible to make a system of
mathematics, just as Euclid did, but we
cannot make a mathematics of the world, because sooner or later we have
to find out whether the axioms are valid for the objects of nature. Thus
we immediately get involved with these complicated and “dirty” objects
of nature, but with approximations ever increasing in accuracy.
12–2 Friction
The foregoing considerations show that a true understanding of
Newton’s laws requires a discussion
of forces, and it is the purpose of this chapter to introduce such a
discussion, as a kind of completion of Newton’s
laws. We have already studied the
definitions of acceleration and related ideas, but now we have to study
the properties of force, and this chapter, unlike the previous chapters,
will not be very precise, because forces are quite complicated.
To begin with a particular force, let us consider the drag on an
airplane flying through the air. What is the law for that force?
(Surely there is a law for every force, we must have a law!)
One can hardly think that the law for that force will be simple. Try
to imagine what makes a drag on an airplane flying through the
air—the air rushing over the wings, the swirling in the back, the
changes going on around the fuselage, and many other complications,
and you see that there is not going to be a simple law. On the other
hand, it is a remarkable fact that the drag force on an airplane is
approximately a constant times the square of the velocity, or F≈c v2.
Now what is the status of such a law, is it analogous to F=m a? Not at
all, because in the first place this law is an empirical thing that is
obtained roughly by tests in a wind tunnel. You say, “Well F=m a might
be empirical too.” That is not the reason that there is a difference.
The difference is not that it is empirical, but that, as we understand
nature, this law is the result of an enormous complexity of events and
is not, fundamentally, a simple thing. If we continue to study it more
and more, measuring more and more accurately, the law will continue to
become more complicated, not less. In other words, as we
study this law of the drag on an airplane more and more closely, we find
out that it is “falser” and “falser,” and the more deeply we study
it, and the more accurately we measure, the more complicated the truth
becomes; so in that sense we consider it not to result from a simple,
fundamental process, which agrees with our original surmise. For
example, if the velocity is extremely low, so low that an ordinary
airplane is not flying, as when the airplane is dragged slowly through
the air, then the law changes, and the drag friction depends more nearly
linearly on the velocity. To take another example, the frictional drag
on a ball or a bubble or anything that is moving slowly through a
viscous liquid like honey, is proportional to the velocity, but for
motion so fast that the fluid swirls around (honey does not but water
and air do) then the drag becomes more nearly proportional to the square
of the velocity (F=c v2), and if the velocity continues to increase,
then even this law begins to fail. People who say, “Well the
coefficient changes slightly,” are dodging the issue. Second, there are
other great complications: can this force on the airplane be divided or
analyzed as a force on the wings, a force on the front, and so on?
Indeed, this can be done, if we are concerned about the torques here and
there, but then we have to get special laws for the force on the wings,
and so on. It is an amazing fact that the force on a wing depends upon
the other wing: in other words, if we take the airplane apart and put
just one wing in the air, then the force is not the same as if the rest
of the plane were there. The reason, of course, is that some of the wind
that hits the front goes around to the wings and changes the force on
the wings. It seems a miracle that there is such a simple, rough,
empirical law that can be used in the design of airplanes, but this law
is not in the same class as the basic laws of physics, and
further study of it will only make it more and more complicated. A study
of how the coefficient c depends on the shape of the front of the
airplane is, to put it mildly, frustrating. There just is no simple law
for determining the coefficient in terms of the shape of the airplane.
In contrast, the law of gravitation is simple, and further study only
indicates its greater simplicity.
We have just discussed two cases of friction, resulting from fast
movement in air and slow movement in honey. There is another kind of
friction, called dry friction or sliding friction, which occurs when
one solid body slides on another. In this case a force is needed to
maintain motion. This is called a frictional force, and its origin,
also, is a very complicated matter. Both surfaces of contact are
irregular, on an atomic level. There are many points of contact where
the atoms seem to cling together, and then, as the sliding body is
pulled along, the atoms snap apart and vibration ensues; something
like that has to happen. Formerly the mechanism of this friction was
thought to be very simple, that the surfaces were merely full of
irregularities and the friction originated in lifting the slider over
the bumps; but this cannot be, for there is no loss of energy in that
process, whereas power is in fact consumed. The mechanism of power
loss is that as the slider snaps over the bumps, the bumps deform and
then generate waves and atomic motions and, after a while, heat, in
the two bodies. Now it is very remarkable that again, empirically,
this friction can be described approximately by a simple law. This law
is that the force needed to overcome friction and to drag one object
over another depends upon the normal force (i.e., perpendicular to the
surface) between the two surfaces that are in contact. Actually, to a
fairly good approximation, the frictional force is proportional to
this normal force, and has a more or less constant coefficient; that
is,
(12.1)
where μ is called the coefficient of
friction (Fig. 12–1). Although this coefficient is not
exactly constant, the formula is a good empirical rule for judging
approximately the amount of force that will be needed in certain
practical or engineering circumstances. If the normal force or the speed
of motion gets too big, the law fails because of the excessive heat
generated. It is important to realize that each of these empirical laws
has its limitations, beyond which it does not really work.
Fig. 12–1. The relation between frictional force and the normal force
for sliding contact.
That the formula F=μ N is approximately correct can be
demonstrated by a simple experiment. We set up a plane, inclined at a
small angle θ, and place a block of weight W on the plane. We
then tilt the plane at a steeper angle, until the block just begins to
slide from its own weight. The component of the weight downward along
the plane is W sinθ, and this must equal the frictional
force F when the block is sliding uniformly. The component of the weight
normal to the plane is W cosθ, and this is the normal
force N. With these values, the formula becomes W sinθ=μ W cosθ, from which we get
μ=sinθ/cosθ=tanθ. If this law were exactly true,
an object would start to slide at some definite inclination. If the
same block is loaded by putting extra weight on it, then, although W
is increased, all the forces in the formula are increased in the same
proportion, and W cancels out. If μ stays constant, the loaded
block will slide again at the same slope. When the angle θ is
determined by trial with the original weight, it is found that with
the greater weight the block will slide at about the same angle. This
will be true even when one weight is many times as great as the other,
and so we conclude that the coefficient of friction is independent of
the weight.
In performing this experiment it is noticeable that when the plane is
tilted at about the correct angle θ, the block does not slide
steadily but in a halting fashion. At one place it may stop, at
another it may move with acceleration. This behavior indicates that
the coefficient of friction is only roughly a constant, and varies
from place to place along the plane. The same erratic behavior is
observed whether the block is loaded or not. Such variations are
caused by different degrees of smoothness or hardness of the plane,
and perhaps dirt, oxides, or other foreign matter. The tables that
list purported values of μ for “steel on steel,” “copper on
copper,” and the like, are all false, because they ignore the factors
mentioned above, which really determine μ. The friction is never
due to “copper on copper,” etc., but to the impurities clinging to
the copper.
In experiments of the type described above, the friction is nearly
independent of the velocity. Many people believe that the friction to
be overcome to get something started (static friction) exceeds the
force required to keep it sliding (sliding friction), but with dry
metals it is very hard to show any difference. The opinion probably
arises from experiences where small bits of oil or lubricant are
present, or where blocks, for example, are supported by springs or
other flexible supports so that they appear to bind.
It is quite difficult to do accurate quantitative experiments in
friction, and the laws of friction are still not analyzed very well,
in spite of the enormous engineering value of an accurate
analysis. Although the law F=μ N is fairly accurate once the
surfaces are standardized, the reason for this form of the law is not
really understood. To show that the coefficient μ is nearly
independent of velocity requires some delicate experimentation,
because the apparent friction is much reduced if the lower surface
vibrates very fast. When the experiment is done at very high speed,
care must be taken that the objects do not vibrate relative to one
another, since apparent decreases of the friction at high speed are
often due to vibrations. At any rate, this friction law is another of
those semiempirical laws that are not thoroughly understood, and in
view of all the work that has been done it is surprising that more
understanding of this phenomenon has not come about. At the present
time, in fact, it is impossible even to estimate the coefficient of
friction between two substances.
It was pointed out above that attempts to measure μ by sliding
pure substances such as copper on copper will lead to spurious
results, because the surfaces in contact are not pure copper, but are
mixtures of oxides and other impurities. If we try to get absolutely
pure copper, if we clean and polish the surfaces, outgas the materials
in a vacuum, and take every conceivable precaution, we still do not
get μ. For if we tilt the apparatus even to a vertical position,
the slider will not fall off—the two pieces of copper stick
together! The coefficient μ, which is ordinarily less than unity
for reasonably hard surfaces, becomes several times unity! The reason
for this unexpected behavior is that when the atoms in contact are all
of the same kind, there is no way for the atoms to “know” that they
are in different pieces of copper. When there are other atoms, in the
oxides and greases and more complicated thin surface layers of
contaminants in between, the atoms “know” when they are not on the
same part. When we consider that it is forces between atoms that hold
the copper together as a solid, it should become clear that it is
impossible to get the right coefficient of friction for pure metals.
The same phenomenon can be observed in a simple home-made experiment
with a flat glass plate and a glass tumbler. If the tumbler is placed
on the plate and pulled along with a loop of string, it slides fairly
well and one can feel the coefficient of friction; it is a little
irregular, but it is a coefficient. If we now wet the glass plate and
the bottom of the tumbler and pull again, we find that it binds, and
if we look closely we shall find scratches, because the water is able
to lift the grease and the other contaminants off the surface, and
then we really have a glass-to-glass contact; this contact is so good
that it holds tight and resists separation so much that the glass is
torn apart; that is, it makes scratches.
12–3 Molecular forces
Fig. 12–2. The force between two atoms as a function of their distance
of separation.
We shall next discuss the characteristics of molecular forces. These
are forces between the atoms, and are the ultimate origin of
friction. Molecular forces have never
been satisfactorily explained on a basis of classical physics; it takes
quantum mechanics to understand them fully. Empirically, however, the
force between atoms is illustrated schematically in
Fig. 12–2, where the force F between two atoms is plotted
as a function of the distance r between them. There are different
cases: in the water molecule, for example, the negative charges sit more
on the oxygen, and the mean positions of the negative charges and of the
positive charges are not at the same point; consequently, another
molecule nearby feels a relatively large force, which is called a
dipole-dipole force. However, for many systems the charges are very much
better balanced, in particular for oxygen gas, which is perfectly
symmetrical. In this case, although the minus charges and the plus
charges are dispersed over the molecule, the distribution is such that
the center of the minus charges and the center of the plus charges
coincide. A molecule where the centers do not coincide is called a polar
molecule, and charge times the separation between centers is called the
dipole moment. A
nonpolar molecule is one in which the centers of the charges coincide.
For all nonpolar molecules, in which all the electrical forces are
neutralized, it nevertheless turns out that the force at very large
distances is an attraction and varies inversely as the seventh power of
the distance, or F=k/r7, where k is a constant that depends on the
molecules. Why this is we shall learn only when we learn quantum
mechanics. When there are dipoles the forces are greater. When atoms or
molecules get too close they repel with a very large repulsion; that is
what keeps us from falling through the floor!
These molecular forces can be demonstrated in a fairly direct way: one
of these is the friction experiment with a sliding glass tumbler;
another is to take two very carefully ground and lapped surfaces which
are very accurately flat, so that the surfaces can be brought very
close together. An example of such surfaces is the Johansson blocks
that are used in machine shops as standards for making accurate length
measurements. If one such block is slid over another very carefully
and the upper one is lifted, the other one will adhere and also be
lifted by the molecular forces, exemplifying the direct attraction
between the atoms on one block for the atoms on the other block.
Nevertheless these molecular forces of attraction are still not
fundamental in the sense that gravitation is fundamental; they are due
to the vastly complex interactions of all the electrons and nuclei in
one molecule with all the electrons and nuclei in another. Any
simple-looking formula we get represents a summation of complications,
so we still have not got the fundamental phenomena.
Since the molecular forces attract at large distances and repel at
short distances, as shown in Fig. 12–2, we can make up
solids in which all the atoms are held together by their attractions
and held apart by the repulsion that sets in when they are too close
together. At a certain distance d (where the graph in
Fig. 12–2 crosses the axis) the forces are zero, which means
that they are all balanced, so that the molecules stay that distance
apart from one another. If the molecules are pushed closer together than
the distance d they all show a repulsion, represented by the portion
of the graph above the r-axis. To push the molecules only slightly
closer together requires a great force, because the molecular repulsion
rapidly becomes very great at distances less than d. If the molecules
are pulled slightly apart there is a slight attraction, which increases
as the separation increases. If they are pulled sufficiently hard, they
will separate permanently—the bond is broken.
If the molecules are pushed only a very small distance closer, or
pulled only a very small distance farther than d, the
corresponding distance along the curve of Fig. 12–2 is also
very small, and can then be approximated by a straight line. Therefore,
in many circumstances, if the displacement is not too great the
force is proportional to the displacement. This principle is known as
Hooke’s law, or the law of elasticity,
which says that the force in a body which tries to restore the body to
its original condition when it is distorted is proportional to the
distortion. This law, of course, holds true only if the distortion is
relatively small; when it gets too large the body will be torn apart or
crushed, depending on the kind of distortion. The amount of force for
which Hooke’s law is valid depends
upon the material; for instance, for dough or putty the force is very
small, but for steel it is relatively large. Hooke’s
law can be nicely demonstrated with a long
coil spring, made of steel and suspended vertically. A suitable weight
hung on the lower end of the spring produces a tiny twist throughout the
length of the wire, which results in a small vertical deflection in each
turn and adds up to a large displacement if there are many turns. If the
total elongation produced, say, by a 100-gram weight, is measured, it
is found that additional weights of 100 grams will each produce an
additional elongation that is very nearly equal to the stretch that was
measured for the first 100 grams. This constant ratio of force to
displacement begins to change when the spring is overloaded, i.e.,
Hooke’s law no longer holds.
12–4 Fundamental forces. Fields
We shall now discuss the only remaining forces that are
fundamental. We call them fundamental in the sense that their laws are
fundamentally simple. We shall first discuss electrical force. Objects
carry electrical charges which consist simply of electrons or
protons. If any two bodies are electrically charged, there is an
electrical force between them, and if the magnitudes of the charges
are q1 and q2, respectively, the force varies inversely as the
square of the distance between the charges, or F=(const) q1 q2/r2. For unlike charges, this law is like the law of
gravitation, but for like charges the force is repulsive and
the sign (direction) is reversed. The charges q1 and q2 can be
intrinsically either positive or negative, and in any specific
application of the formula the direction of the force will come out
right if the q’s are given the proper plus or minus sign; the force
is directed along the line between the two charges. The constant in
the formula depends, of course, upon what units are used for the
force, the charge, and the distance. In current practice the charge is
measured in coulombs, the distance in meters, and the force in
newtons. Then, in order to get the force to come out properly in
newtons, the constant (which for historical reasons is
written 1/4 π ϵ0) takes the numerical value
or
Thus the force law for static charges is
(12.2)
In nature, the most important charge of all is the charge on a single
electron,
which is 1.60×10−19 coulomb. In working with electrical forces
between fundamental particles rather than with large charges, many
people prefer the combination 
, in which
qel is defined as the charge on an electron. This
combination occurs frequently, and to simplify calculations it has been
defined by the symbol e2; its numerical value in the mks system of
units turns out to be (1.52×10−14)2. The advantage of using
the constant in this form is that the force between two electrons in
newtons can then be written simply as e2/r2, with r in meters,
without all the individual constants. Electrical forces are much more
complicated than this simple formula indicates, since the formula gives
the force between two objects only when the objects are standing still.
We shall consider the more general case shortly.
, in which
qel is defined as the charge on an electron. This
combination occurs frequently, and to simplify calculations it has been
defined by the symbol e2; its numerical value in the mks system of
units turns out to be (1.52×10−14)2. The advantage of using
the constant in this form is that the force between two electrons in
newtons can then be written simply as e2/r2, with r in meters,
without all the individual constants. Electrical forces are much more
complicated than this simple formula indicates, since the formula gives
the force between two objects only when the objects are standing still.
We shall consider the more general case shortly.
In the analysis of forces of the more fundamental kinds (not such forces
as friction, but the electrical force or the gravitational force), an
interesting and very important concept has been developed. Since at
first sight the forces are very much more complicated than is indicated
by the inverse-square laws and these laws hold true only when the
interacting bodies are standing still, an improved method is needed to
deal with the very complex forces that ensue when the bodies start to
move in a complicated way. Experience has shown that an approach known
as the concept of a “field” is of great utility for the analysis of
forces of this type. To illustrate the idea for, say, electrical force,
suppose we have two electrical charges, q1 and q2, located at
points P and R respectively. Then the force between the charges is
given by
(12.3)
To analyze this force by means of the field concept, we say that the
charge q1 at P produces a “condition” at R, such that when
the charge q2 is placed at R it “feels” the force. This is one
way, strange perhaps, of describing it; we say that the force F
on q2 at R can be written in two parts. It is q2 multiplied by
a quantity E that would be there whether q2 were there or not
(provided we keep all the other charges in their right
places). E is the “condition” produced by q1, we say,
and F is the response of q2 to E. E is called an
electric field, and it is a vector. The formula for the
electric field E that is produced at R by a charge q1
at P is the charge q1 times the constant 1/4 π ϵ0 divided
by r2 (r is the distance from P to R), and it is acting in the
direction of the radius vector (the radius vector r divided by
its own length). The expression for E is thus
(12.4)
We then write
(12.5)
which expresses the force, the field, and the charge in the
field. What is the point of all this? The point is to divide the
analysis into two parts. One part says that something produces
a field. The other part says that something is acted on by the
field. By allowing us to look at the two parts independently, this
separation of the analysis simplifies the calculation of a problem in
many situations. If many charges are present, we first work out the
total electric field produced at R by all the charges, and then,
knowing the charge that is placed at R, we find the force on it.
In the case of gravitation, we can do exactly the same thing. In this
case, where the force F=−G m1 m2 r/r3, we can make an
analogous analysis, as follows: the force on a body in a gravitational
field is the mass of that body times the field C. The force
on m2 is the mass m2 times the field C produced by m1;
that is, F=m2 C. Then the field C produced by a body
of mass m1 is C=−G m1 r/r3 and it is directed radially,
as in the electrical case.
In spite of how it might at first seem, this separation of one part
from another is not a triviality. It would be trivial, just another
way of writing the same thing, if the laws of force were simple, but
the laws of force are so complicated that it turns out that the fields
have a reality that is almost independent of the objects which create
them. One can do something like shake a charge and produce an effect,
a field, at a distance; if one then stops moving the charge, the field
keeps track of all the past, because the interaction between two
particles is not instantaneous. It is desirable to have some way to
remember what happened previously. If the force upon some charge
depends upon where another charge was yesterday, which it does, then
we need machinery to keep track of what went on yesterday, and that is
the character of a field. So when the forces get more complicated, the
field becomes more and more real, and this technique becomes less and
less of an artificial separation.
In analyzing forces by the use of fields, we need two kinds of laws
pertaining to fields. The first is the response to a field, and that
gives the equations of motion. For example, the law of response of a
mass to a gravitational field is that the force is equal to the mass
times the gravitational field; or, if there is also a charge on the
body, the response of the charge to the electric field equals the
charge times the electric field. The second part of the analysis of
nature in these situations is to formulate the laws which determine
the strength of the field and how it is produced. These laws are
sometimes called the field equations. We shall learn more about
them in due time, but shall write down a few things about them now.
First, the most remarkable fact of all, which is true exactly and
which can be easily understood, is that the total electric field
produced by a number of sources is the vector sum of the electric
fields produced by the first source, the second source, and so on. In
other words, if we have numerous charges making a field, and if all by
itself one of them would make the field E1, another would make
the field E2, and so on, then we merely add the vectors to get
the total field. This principle can be expressed as
(12.6)
or, in view of the definition given above,
(12.7)
Can the same methods be applied to gravitation? The force between two
masses m1 and m2 was expressed by
Newton
as F=−G m1 m2 r/r3. But according to the field concept, we may
say that m1 creates a field C in all the surrounding space,
such that the force on m2 is given by
(12.8)
By complete analogy with the electrical case,
(12.9)
and the gravitational field produced by several masses is
(12.10)
Equations (12.6) and (12.10) express what is known
as the principle of superposition of fields. This principle
states that the total field due to all the sources is the sum of the
fields due to each source. So far as we know today, for electricity this
is an absolutely guaranteed law, which is true even when the force law
is complicated because of the motions of the charges. There are apparent
violations, but more careful analysis has always shown these to be due
to the overlooking of certain moving charges. However, although the
principle of superposition applies exactly for electrical forces, it is
not exact for gravity if the field is too strong, and Newton’s
equation (12.10) is only approximate, according to
Einstein’s gravitational
theory.
Closely related to electrical force is another kind, called magnetic
force, and this too is analyzed in terms of a field. Some of the
qualitative relations between electrical and magnetic forces can be
illustrated by an experiment with an electron-ray tube
(Fig. 12–3). At one end of such a tube is a source that
emits a stream of electrons. Within the tube are arrangements for
accelerating the electrons to a high speed and sending some of them in a
narrow beam to a fluorescent screen at the other end of the tube. A spot
of light glows in the center of the screen where the electrons strike,
and this enables us to trace the electron path. On the way to the screen
the electron beam passes through a narrow space between a pair of
parallel metal plates, which are arranged, say, horizontally. A voltage
can be applied across the plates, so that either plate can be made
negative at will. When such a voltage is present, there is an electric
field between the plates.
Fig. 12–3. An electron-beam tube.
The first part of the experiment is to apply a negative voltage to the
lower plate, which means that extra electrons have been placed on the
lower plate. Since like charges repel, the light spot on the screen
instantly shifts upward. (We could also say this in another way—that
the electrons “felt” the field, and responded by deflecting upward.)
We next reverse the voltage, making the upper plate
negative. The light spot on the screen now jumps below the center,
showing that the electrons in the beam were repelled by those in the
plate above them. (Or we could say again that the electrons had
“responded” to the field, which is now in the reverse direction.)
The second part of the experiment is to disconnect the voltage from
the plates and test the effect of a magnetic field on the electron
beam. This is done by means of a horseshoe magnet, whose poles are far
enough apart to more or less straddle the tube. Suppose we hold the
magnet below the tube in the same orientation as the letter U, with
its poles up and part of the tube in between. We note that the light
spot is deflected, say, upward, as the magnet approaches the tube from
below. So it appears that the magnet repels the electron
beam. However, it is not that simple, for if we invert the magnet
without reversing the poles side-for-side, and now approach the tube
from above, the spot still moves upward, so the electron beam
is not repelled; instead, it appears to be attracted this
time. Now we start again, restoring the magnet to its original U
orientation and holding it below the tube, as before. Yes, the spot is
still deflected upward; but now turn the magnet 180 degrees around a
vertical axis, so that it is still in the U position but the poles are
reversed side-for-side. Behold, the spot now jumps downward, and stays
down, even if we invert the magnet and approach from above, as before.
To understand this peculiar behavior, we have to have a new
combination of forces. We explain it thus: Across the magnet from one
pole to the other there is a magnetic field. This field has a
direction which is always away from one particular pole (which we
could mark) and toward the other. Inverting the magnet did not change
the direction of the field, but reversing the poles side-for-side did
reverse its direction. For example, if the electron velocity were
horizontal in the x-direction and the magnetic field were also
horizontal but in the y-direction, the magnetic force on the
moving electrons would be in the z-direction, i.e., up or down,
depending on whether the field was in the positive or negative
y-direction.
Although we shall not at the present time give the correct law of
force between charges moving in an arbitrary manner, one relative to
the other, because it is too complicated, we shall give one aspect of
it: the complete law of the forces if the fields are known. The
force on a charged object depends upon its motion; if, when the object
is standing still at a given place, there is some force, this is taken
to be proportional to the charge, the coefficient being what we call
the electric field. When the object moves the force may be
different, and the correction, the new “piece” of force, turns out
to be dependent exactly linearly on the velocity, but at
right angles to v and to another vector quantity which we
call the magnetic induction B. If
the components of the electric field E and the magnetic
induction B are, respectively, (Ex,Ey,Ez)
and (Bx,By,Bz), and if the velocity v has the
components (vx,vy,vz), then the total electric and magnetic force
on a moving charge q has the components
(12.11)
If, for instance, the only component of the magnetic field were By
and the only component of the velocity were vx, then the only term
left in the magnetic force would be a force in the z-direction, at
right angles to both B and v.
12–5 Pseudo forces
The next kind of force we shall discuss might be called a pseudo
force. In Chapter 11 we discussed the relationship between
two people, Joe and Moe, who use different coordinate systems. Let us
suppose that the positions of a particle as measured by Joe are x
and by Moe are x′; then the laws are as follows:
where s is the displacement of Moe’s system relative to Joe’s. If we
suppose that the laws of motion are correct for Joe, how do they look
for Moe? We find first, that
Previously, we considered the case where s was constant, and we
found that s made no difference in the laws of motion, since d s/d t=0; ultimately, therefore, the laws of physics were the same in both
systems. But another case we can take is that s=u t, where u is a
uniform velocity in a straight line. Then s is not constant, and
d s/d t is not zero, but is u, a constant. However, the
acceleration d2 x/d t2 is still the same as d2 x′/d t2, because d u/d t=0. This proves the law that we used in Chapter 10,
namely, that if we move in a straight line with uniform velocity the
laws of physics will look the same to us as when we are standing
still. That is the Galilean
transformation. But we wish to
discuss the interesting case where s is still more complicated, say s=a t2/2. Then d s/d t=a t and d2 s/d t2=a, a uniform
acceleration; or in a still more complicated case, the acceleration
might be a function of time. This means that although the laws of motion
from the point of view of Joe would look like
the laws of motion as looked upon by Moe would appear as
That is, since Moe’s coordinate system is accelerating with respect to
Joe’s, the extra term m a comes in, and Moe will have to correct his
forces by that amount in order to get Newton’s laws to work. In other
words, here is an apparent, mysterious new force of unknown origin
which arises, of course, because Moe has the wrong coordinate
system. This is an example of a pseudo force; other examples occur in
coordinate systems that are rotating.
Another example of pseudo force is what is often called “centrifugal
force.”
An observer in a rotating coordinate system, e.g., in a
rotating box, will find mysterious forces, not accounted for by any
known origin of force, throwing things outward toward the walls. These
forces are due merely to the fact that the observer does not have
Newton’s coordinate system, which is the simplest coordinate system.
Pseudo force can be illustrated by an interesting experiment in which
we push a jar of water along a table, with acceleration. Gravity, of
course, acts downward on the water, but because of the horizontal
acceleration there is also a pseudo force acting horizontally and in a
direction opposite to the acceleration. The resultant of gravity and
pseudo force makes an angle with the vertical, and during the
acceleration the surface of the water will be perpendicular to the
resultant force, i.e., inclined at an angle with the table, with the
water standing higher in the rearward side of the jar. When the push
on the jar stops and the jar decelerates because of friction, the
pseudo force is reversed, and the water stands higher in the forward
side of the jar (Fig. 12–4).
Fig. 12–4. Illustration of a pseudo force.
One very important feature of pseudo forces is that they are always
proportional to the masses; the same is true of gravity. The
possibility exists, therefore, that gravity itself is a pseudo
force. Is it not possible that perhaps gravitation is due simply to
the fact that we do not have the right coordinate system? After all,
we can always get a force proportional to the mass if we imagine that
a body is accelerating. For instance, a man shut up in a box that is
standing still on the earth finds himself held to the floor of the box
with a certain force that is proportional to his mass. But if there
were no earth at all and the box were standing still, the man inside
would float in space. On the other hand, if there were no earth at all
and something were pulling the box along with an
acceleration g, then the man in the box, analyzing physics, would find a pseudo
force which would pull him to the floor, just as gravity does.
Einstein put forward the famous hypothesis that accelerations give an
imitation of gravitation, that the forces of acceleration (the pseudo
forces) cannot be distinguished from those of gravity; it is not
possible to tell how much of a given force is gravity and how much is
pseudo force.
It might seem all right to consider gravity to be a pseudo force, to say
that we are all held down because we are accelerating upward, but how
about the people in Madagascar, on the other side of the earth—are
they accelerating too? Einstein found that gravity could be considered a
pseudo force only at one point at a time, and was led by his
considerations to suggest that the geometry of the world is more
complicated than ordinary Euclidean geometry. The present
discussion is only qualitative, and does not pretend to convey anything
more than the general idea. To give a rough idea of how gravitation
could be the result of pseudo forces, we present an illustration which
is purely geometrical and does not represent the real situation. Suppose
that we all lived in two dimensions, and knew nothing of a third. We
think we are on a plane, but suppose we are really on the surface of a
sphere. And suppose that we shoot an object along the ground, with no
forces on it. Where will it go? It will appear to go in a straight line,
but it has to remain on the surface of a sphere, where the shortest
distance between two points is along a great circle; so it goes along a
great circle. If we shoot another object similarly, but in another
direction, it goes along another great circle. Because we think we are
on a plane, we expect that these two bodies will continue to diverge
linearly with time, but careful observation will show that if they go
far enough they move closer together again, as though they were
attracting each other. But they are not attracting each
other—there is just something “weird” about this geometry. This
particular illustration does not describe correctly the way in which
Einstein’s geometry is “weird,” but it illustrates that if we distort
the geometry sufficiently it is possible that all gravitation is related
in some way to pseudo forces; that is the general idea of the
Einsteinian theory of gravitation.
12–6 Nuclear forces
We conclude this chapter with a brief discussion of the only other
known forces, which are called nuclear forces. These forces are
within the nuclei of atoms, and although they are much discussed, no
one has ever calculated the force between two nuclei, and indeed at
present there is no known law for nuclear forces. These forces have a
very tiny range which is just about the same as the size of the
nucleus, perhaps 10−13 centimeter. With particles so small and at
such a tiny distance, only the quantum-mechanical laws are valid, not
the Newtonian laws. In nuclear analysis we no
longer think in terms of forces, and in fact we can replace the force
concept with a concept of the energy of interaction of two particles, a
subject that will be discussed later. Any formula that can be written
for nuclear forces is a rather crude approximation which omits many
complications; one might be somewhat as follows: forces within a nucleus
do not vary inversely as the square of the distance, but die off
exponentially over a certain distance r, as expressed
by F=(1/r2) exp(−r/r0), where the distance r0 is of the order of
10−13 centimeter. In other words, the forces disappear as soon as
the particles are any great distance apart, although they are very
strong within the 10−13 centimeter range. So far as they are
understood today, the laws of nuclear force are very complex; we do not
understand them in any simple way, and the whole problem of analyzing
the fundamental machinery behind nuclear forces is unsolved. Attempts at
a solution have led to the discovery of numerous strange particles, the
π-mesons, for example, but the origin of these forces remains
obscure.