Vol.3, No.7, 633-639 (2011) Natural Science
http://dx.doi.org/10.4236/ns.2011.37086
Centrifugal force: an appreciation
Kern E. Kenyon
4632 North Lane, Del Mar, USA; *Corresponding Author:
Received 19 April, 2011; revised 2 May, 2011; accepted 8 May, 2011.
ABSTRACT
The centrifugal force is used to increase the
physical understanding of five examples taken
from fluid dynamics, geophysics and the solar
system, as well as four hypothetical orbital
problems. Each example involves a balance of
forces between the centrifugal force and one or
two other forces, such as a pressure gradient
and a component of the force of gravity. Among
the examples chosen for examination are: the
orbital motion of fluid particles in surface grav-
ity waves, the boundary layer character of
steady flow next to a curved rigid surface, the
tornado, the rotating self-gravitating mass and
the three-body problem.
Keywords: Centrifugal Force
1. INTRODUCTION
It may be easier for a trained dynamical oceanogra-
pher (the author) or meteorologist to take the centrifugal
force seriously than for a practitioner of classical physics.
In the field of physics there has existed for a long time a
considerable amount of confusion surrounding the cen-
trifugal force: is it real or fictitious? For example, see the
descriptions of the centrifugal force in the various avail-
able dictionaries of physics, of which there are at least
three. Here are two quotes from one such dictionary [1].
“Centrifugal force is said to be ‘fictitious’ force, and the
use is best avoided.” Also, “It is sometimes said that the
centrifugal force is ‘reaction’ to the centripetal force—
this is not true.” Both quotes illustrate the present state
of confusion with respect to the centrifugal force.
Throughout the discussions that follow support can be
found for the second quote but none for the first one.
Strangely, use of the centrifugal force in physics appears
to have been restricted to purely circular motion. Why
can’t it be applied also to elliptical orbital motion? Ex-
actly when the dissatisfaction with the centrifugal force
began is not known; apparently no single event or person
started it, unless it was Newton. He, on the one hand,
pointed out that the flattening of the poles of the earth is
direct evidence for the working of the centrifugal force,
but, on the other hand, he used “centrifugal force” very
sparingly in the Principia.
However, the physical oceanographer, operating in a
relatively new field and not being hampered by all the
physics history, learns in his first year of graduate work
to balance the Coriolis force against a horizontal pres-
sure gradient in order to help him understand the dy-
namics of the major permanent ocean currents. I believe
the same thing happens when studying the large-scale
wind systems in graduate courses in meteorology. Then
it is only a small step to seek to balance the centrifugal
force against another force, such as a pressure force, in
order to increase understanding of some smaller scale
motions. But there has never been much of a motivation
to consider the centrifugal force in either ocean or atmo-
spheric science. One reason for this is that the spatial
scales of most weather features, even a hurricane, and
ocean circulations are large enough to cause the Coriolis
force to dominate over the centrifugal force.
In about 1982, a question was asked by me of myself
regarding understanding the centrifugal force in surfaced
gravity waves, since the fluid particles have an orbital
motion, as indicated by observing small neutrally buoy-
ant particles. This is discussed further in Section 2. By
attempting to answer the question a chain of similar qu-
estions was triggered resulting in several totally different
papers, all connected by a single thread (or ‘most’ com-
mon denominator): the centrifugal force. This chain of
events was not planned or organized as such and the
time sequence of the related publications did not always
follow the order that is most convenient to put down
here. But it all started with the surface gravity wave, a
special subject of interest to me, before I found out from
the dictionaries that I was not supposed to use the cen-
trifugal force.
2. SURFACE GRAVITY WAVES
Thirty years ago, I began to think about the motion of
the fluid particles in a surface gravity wave. Imagine the
fluid particles in the fixed frame (fixed to the solid bot-

K. E. Kenyon et al. / Natural Science 3 (2011) 633-639 634
tom of the body of water), which go round and round in
circles while the sinusoidal wave shape propagates past
the observer. During circular motion each fluid particle
must experience a centrifugal force acting radially out-
ward from the orbit’s center. For the organized circular
motion to be maintained and not break down into chaos
or turbulence, there has to be an equal but opposite force
to balance the centrifugal force. What is this other force?
From looking into the literature it appears that nobody
had ever asked this question before. The answer is a
pressure force related to the different elevations of the
sea surface and to the vertical fluid accelerations [2].
Immediately popping out of the force balance equation is
the usual theoretical dispersion relation of these waves,
derived on the basis of potential theory. There was a
name already waiting in the oceanographic literature for
the pressure/centrifugal force balance, the cyclostrophic
balance, but it had never been applied to this example in
the past.
At the time I asked the question of myself about how
the centrifugal force on the orbiting fluid particles is
balanced, my mind was not exactly prepared to do an-
swer it. Ever since graduate school (in the 1960s), when
I discovered a little known paper on water waves by Ein-
stein [3], I had been under the spell of his elementary
model that explained both the existence of the waves and
their propagation speed with simple physics. He used a
steady state model by assuming that the observer moves
at the speed of the wave so that the wave shape remains
steady and fluid under the wavy surfaces flows past the
observer. This point of view had a pretty firm grip on my
mind because of its usefulness for understanding some
features of the waves. However, the observed orbital
motion of the particles in the fixed frame does not come
naturally out of the Einstein model. In other words, for
all these years I had not been thinking much at all about
the orbital movements of the particles. In another sense
the question was ripening and just waiting to occur to me
(or to somebody).
An opportunity came up, as it does every four years,
to present a paper at an international meeting (IUGG),
which happened at that time to be in Vienna. The timing
was right to take into the public arena my thoughts on
the centrifugal force as it applies to the surface gravity
wave. My oral presentation (I also presented a poster)
was scheduled for the largest lecture hall I had ever been
in. Only a handful of people were there to hear the talk
including my wife and our two kids. There I was up on a
platform behind a podium far removed from my small
audience in this giant room. One of the many amusing
experiences of my academic career to think back on lat-
er.
After seeing this project through to publication, a re-
lated question came up. According to the standard irrota-
tional wave theory the particle orbits transform from cir-
cles to ellipses when the mean total depth of water be-
comes comparable to or less than a wave length and the
long axes of the ellipses are horizontal, parallel to the
mean free surface. The centers of rotation of the particles
are the centers of the ellipses. So how does the cyclos-
trophic balance work in that case, since in general the
centrifugal force cannot be anti-parallel to the central
attractive (restoring) force? The restoring force, what-
ever its true nature is, must always point to the ellipse’s
center due to the inherent symmetry whereas the cen-
trifugal force is always normal to the orbit. For guidance
I turned to an undergraduate mechanics text I had used at
MIT, which has a brief section on celestial mechanics.
Of special interest to me then were the elliptical paths of
the planets around the sun. I had gotten an “A” in this
physics course many years before, but on revisiting the
book I plunged into a really deep confusion, lasting for
about two years. Sometimes in the book it seemed as if
the author was dancing around the centrifugal force and
other times not. For example, for purely circular motion
the centrifugal force was occasionally mentioned whe-
reas for elliptical orbital paths it was never used. A reso-
lution of this confusion is described in Section 4.
In a way it was not completely logical for me look for
help in astronomy for my problem with surface gravity
waves in the ocean. Elliptical orbits are similar in both
cases but there is a fundamental difference in the con-
figurations with respect to the central attractive forces.
For the planets, the gravitational force is centered at one
focus of the ellipse whereas the restoring force center for
the fluid particles in the surface gravity wave must be at
the center of the ellipse, although the nature of the cen-
tral attractive force was not yet identified at this point.
At any rate, that is the path I chose to take, and I am now
living with some of its consequences.
3. TORNADOES
Wouldn’t a person think that among all observable na-
tural geophysical phenomena the one with the fastest
circular speed and relatively small radius of curvature-
should be a perfect subject for the application of the cen-
trifugal force concept? However, there is little evidence
of this in the literature on tornadoes (of course Newton
never knew about tornadoes). Circular winds parallel to
and near the ground have been measured to sometimes
exceed 200 mph by aiming Doppler radar beams at solid
objects caught up in the tornado. What holds these or-
biting winds together and keeps them from flying apart
due to the outward acting centrifugal force? Evidently
the core pressure must be a lot lower than the ambient
pressure outside. Then how is such low pressure gene-

K. E. Kenyon et al. / Natural Science 3 (2011) 633-639 635
rated?
Very few direct or indirect measurements have ever
been made of the characteristics of a tornado, which is
understandable in view of the high wind velocities and
the dangers of getting too close to them. A man can stand
against a 100 mph wind, at a rather large angle from the
vertical to be sure, but very few things, including mete-
orological instruments, can stand up to a 200 mph wind.
A lack of direct observations may have led to some ra-
ther mysterious ideas about tornadoes that have taken
root in the minds of scientists.
Independent of any of my particular research topics
that were occupying my time, my son took up the tor-
nado as a project in grade school. He built a model tor-
nado and used smoke to make the vortex motion visible.
He got some assistance from a colleague of mine who
knew some meteorology but he got little help from me
(who didn’t know any). Summers spent in Iowa, where
his mother was born and raised and where his grandpar-
ents still lived, may have put this notion into his head. I
could not help noticing from the sidelines, though, that
his model tornado went from the ground up! After parts
of many summers spent in Iowa, I have yet to see a tor-
nado, except in still photographs and news videos. There
is a strong suggestion in these visual aids, though, that
the tornado comes down from the clouds to the ground.
In other words, the vertical motion in the tornado is
downward. For example, the funnel of the tornado is
made visible because some cloud material is brought
down to the ground with it. If one adopts this starting
point, it makes the explanations of a few observed fea-
tures easier, such as the funnel shape, the very large cir-
cular winds at the ground and the extremely low pres-
sure inside.
Some key observations needed for a complete under-
standingof tornadoes are missing since they are difficult
to obtain. Outside the tornado the air is always reported
to be very warm and very humid, but is it cold and dry
inside? Nobody knows for sure, except possibly one
crazy storm chaser who apparently ran and dove into one
and survived, according to a TV news report in May
2008. He came out saying that it was cool inside. Actu-
ally there is another storm chaser who probably knows
the answer to some of these questions because at least
once he successfully placed in the way of a tornado an
instrument which had the capability of measuring tem-
perature and humidity as well as pressure, and the instru-
ment was not destroyed when the tornado went right
over it on a paved road. However, so far only the record
breaking low pressure has been reported to the public for
some reason or other.
Suppose cool dry air, because it is heavier, falls down
through the warm humid air, from the cloud to the
ground. Then acceleration under gravity of the down-
ward flow plus conservation of mass qualitatively ac-
counts for the shape of the tornado’s funnel, narrowing
toward the ground. Suppose also that in or above the
cloud base there has been generated somehow a hori-
zontal circular motion which is comparatively weak. As
it falls down this circulation is magnified by conserva-
tion of angular momentum as the funnel diameter be-
comes smaller. It is not difficult in this way to go from a
relatively small horizontal velocity in the clouds to a 200
mph wind at the ground. What balances the outward
centrifugal force is a pressure gradient in which there is
low pressure at the core and relatively high pressure in
the outside air. Low pressure is consistent with Ber-
noulli’s law (pressure is least where the speed is greatest
along a streamline) as the air falls (accelerates) down-
ward at the center of the tornado. This is the gist of an
elementary model [4] of a tornado in which the centri-
fugal force is strategically embedded.
4. NEWTON’S HYPOTHETICAL ORBITS
In addition to the one “real” Kepler problem, in the
Principia Newton examined mathematically the follow-
ing four hypothetical orbits of a single particle moving
about a force center in two dimensions:
1) Elliptical orbit with the force center in the middle
of the ellipse;
2) Circular orbit with the force center on the circum-
ference;
3) Circular orbit with the force center in the middle of
the circle;
4) Spiral with the force center at the pole of the spiral.
Some years ago five books were published within a
few months of each other that all attempted to explain
Newton’s Principia to the educated public. Why this
sudden and apparently widespread burst of interest in
trying to understand the Principia, and why now after
about 300 years? Earlier I had attempted on my own to
read the Principia, but I couldn’t see the forest for the
trees, being lost in a tangle of theorems, corollaries,
lemmas, etc. The reason for my searching the Principia
was to hopefully relate my derivation of Kepler’s laws to
Newton’s derivation of these laws, which is well-known
to be the first theoretical prediction of the empirical laws.
Perhaps I had found an independent derivation. But I
could not follow Newton’s geometrical and logical de-
ductions, and I still can’t. To this day I have trouble even
locating the universal law of gravitation in the Principia!
However, one of the recent books allowed me to see
better just what it was that Newton was trying to accom-
plish in one small part of the Principia. Now, although I
still do not completely understand all the details of New-
ton’s method, I saw that I can independently check his

K. E. Kenyon et al. / Natural Science 3 (2011) 633-639 636
results on the four hypothetical orbits, plus his result of
the Kepler problem, because my force balance method is
the inverse of his method. He posed his orbital problems
in the following way. Given the shape of the orbit and
the location of the force center, find the functional form
of the central attractive force that will keep a body in
this orbit. My method is [5]: given the functional form of
the central force and the force balance, find the shape of
the orbit and its relation to the force center.
The Kepler problem is: for an elliptical orbit with the
force center at one focus of the ellipse, find the form of
the central attractive force. Newton obtained the inverse
square law for the force in this case, i.e. the force varies
as the inverse square of the distance between the focus
and the body. My force balance method puts the inverse
square law in and comes out with an elliptical orbit with
the force center at one focus.
Four hypothetical orbital problems worked out in the
Principia have not led to any practical applications so far,
with one recent exception: the elliptical orbit with the
force center at the center of the ellipse. Here Newton
proved that the attractive force varies directly as the dis-
tance, like a linear spring, which is a special case of
Hooke’s Law. To the best of my knowledge Newton
never used the term “spring” in connection with this hy-
pothetical problem in the Principia, but it is known that
Hooke and Newton were not the closest of colleagues.
Recently a practical application of this orbital problem to
surface gravity waves has been found, as discussed in
Section 6.
By putting Newton’s results for the central forces, one
at a time, into my force balance equation in polar coor-
dinates, that includes the centrifugal force, I can easily
get the differential equations to be solved for the shapes
of the orbits. In all cases the left side of the equation is
the linear harmonic oscillator equation in polar coordi-
nates where the variable of oscillation is the inverse of
the radius (which may seem a bit unusual). Depending
on the form of the force, the right side of the force bal-
ance equation can be linear or more generally nonlinear.
Then if the corresponding shapes of the orbits that New-
ton began with are inserted into the differential equations,
these equations can be solved and complete consistency
is found between the two independent methods for all
four hypothetical orbital problems in the Principia as
well as for the Kepler problem. One striking offshoot is
that analytic solutions in closed form, in terms of ele-
mentary functions, exist for two different differential
equations that are both fully nonlinear, which one would
never expect to find (one simply would not bother to
search for it). Faced with such nonlinear differential equ-
ations the contemporary graduate student would automa-
tically rush to the computer to solve them by means of
numerical techniques.
This consistency between Newton’s geometric method
and my independent force balance method further
strengthens the position that the centrifugal force is a
real and useful force and not a fictitious one to be
avoided. What is really needed, though, is a comparison
between theory and measurements. That is not likely to
happen with most of the hypothetical orbital problems.
Perhaps the elliptical problem, with the force center at
the center of the ellipse, may lead to such a comparison
someday by means of a mechanical model (i.e. a mass
rotating and vibrating on a spring, see Section 6).
Could one use the same force balance method with the
centrifugal force to attack the famous unsolved 3-body
problem of astronomy? This is a far more ambitious
theoretical project than any that occurred to me before
(or any that came after either).
5. THREE-BODY PROBLEM
At the end of the 1990s, I attempted to solve the cele-
brated 3-body problem in a plane (i.e. in two dimensions)
using my force balance method, including the centrifugal
force, and I got farther than I had expected to get on the
first try. However, the equations started to look a little
messy so I stopped. But the recent progress with New-
ton’s Principia, in which all five of the orbital problems
were found to be consistent with the force balance me-
thod, gave me the extra courage to try again. On a vaca-
tion trip to New Mexico I did not have any notes with
me, but nevertheless I sat down and drew a set of train-
gles connecting the three masses. In doing so I just hap-
pened to define two angles differently than before, and
this produced enough of a simplicity in the trigonometry
that it eventually led to an initial breakthrough, which
allowed me to proceed further than I had done previ-
ously. Without my books and papers I had to rederive
most of the necessary mathematical relations; a few I re-
membered but had to check. Four pages in the notebook
were enough to contain the derivation of the kernel of
the solution of the 3- and 4-body problems, and then the
generalization to the n-body problem was just a rela-
tively simple extension of the same procedure.
As it turns out now my original solution had errors in
it, which were pointed out by one peer reviewer in due
course, but these mistakes were corrected one by one
until the paper was finally accepted after about a two-
year review period. Of course I am very grateful to this
particular reviewer, whoever he or she is. The word “so-
lution” is not quite accurate, since what I have done so
far is to set up the differential equations to be solved for
the orbits of the gravitating masses. I have not actually
solved these equations and I am not the most likely per-
son to do so either. The equations are strongly nonlinear

K. E. Kenyon et al. / Natural Science 3 (2011) 633-639 637
and in the general case almost certainly must be addre-
ssed by numerical methods, about which I am almost
totally ignorant. Only after that work is done can a com-
parison between theory and measurement be carried out,
upon which the theory will either stand or fall.
For 300 years the 3-body problem has teased resear-
chers, mathematicians, physicists, and astronomers. It
gave Newton a headache. It gave Euler a headache. Many
mathematicians and physicists devoted a considerable
amount of time to the problem. A 1967 Encyclopedia of
Science estimates that about 1000 papers had been pub-
lished on the 3-body problem. There must be consider-
ably more papers than that now, and there may even
have been more then.
On the other hand, I never declared in the beginning
that I was working on the 3-body problem, but rather
bumped into it sideways: picked it up for a day, put it
down for three years, and then picked it up again for a
few days and thought I had finished with it (until the re-
viewer got hold of it). If I have really solved this famous
problem, which at the moment I think I have, it will be
something to wonder at briefly before hopefully going
on to write other papers. Then I predict that the physi-
cists will more readily accept the centrifugal force as
being a real useful tool.
The actual working out of the setting up of the solu-
tion of the 3-body problem took a few interesting twists
and turns through the review process. One anonymous
revie- wer thought I had made a fundamental mistake in
setting up the force balance equations for the three
masses in the first place. I was so sure of myself that I
did not come to realize that he was right for a consider-
able time, but kept trying to justify my position. How-
ever, the reviewer was unyielding. Eventually, with the
help of some friends on a particular point of the problem
(and mostly by e-mail), I saw my mistake and stayed up
all night correcting it.
Then the same reviewer had another major objection
to my suggested method of solution of the corrected
force balance equations. Again I was sure of my pro-
posed method, because it was a straight forward exten-
sion, I thought, of an earlier published paper of mine on
the 2-body or Kepler problem. I was not happy with the
result either and could not think of a way around the
obstacle. None of my friends came back with any use-
ful information when I asked them about it. Finally, the
reviewer pushed me hard enough that I found a way to
solve the difficulty which was at the same time more
mathematically correct and not physically inconsistent,
but it confused me because of my published results and
made me think I had made a mistake before. There was a
tug of war going on between my feelings of happiness
for having found a way to solve the 3-body problem and
my confusion over the way I had previously solved the
2-body problem. Gradually the feeling of happiness be-
gan to win out after a few anxious days. Naturally I am
indebted to this reviewer for his patience and persistence
in sticking to his position, and the editor was unbelieva-
bly great throughout; a real test for all three of us actually.
But this was not the end of the story either, because I
had gone out on a limb in offering an opinion about a
way that the equations could be solved numerically. One
more time the reviewer was very critical of my sugges-
tion, and probably rightly so. After taking this section
out of the paper, the editor finally accepted it for publi-
cation [6]. No solutions are contained in the paper, and
no comparisons are made between theory and measure-
ment. These important pieces of work are left for the fu-
ture. About one year and a half after the review process
was finished the paper finally saw the light of day.
6. ROTATING MASS ON A SPRING
Since Newton’s day one of his hypothetical orbital
problems, an elliptical path with the force center in the
middle of the ellipse, has come to be associated with a
practical situation: particle orbits of the surface gravity
wave [8]. In this case Newton discovered that the central
force needed to keep a mass in its orbit varied directly as
the distance the body was from the force center. In other
words the force behaved like a linear spring. Thus the
present exploration of problems incorporating the centri-
fugal force has come full circle, so to speak, starting and
ending with the surface gravity wave. However, the end
of the tale has not yet been reached because two more
sections follow this one.
Another way this particular hypothetical problem
could become a practical one is by simply attaching a
mass to a spring, anchoring the other end of the spring,
giving the mass a push while restricting the motion to
the horizontal plane, for example. But there are two ide-
alizations of the theoretical spring that may take a bit of
creative engineering in order to build such a device: the
spring is supposed to be mass less and have zero equilib-
rium length. Also friction must be eliminated (or greatly
reduced).
A noteworthy property of such a theoretical device,
apparently not noticed by Newton, is that the orbital
period of the mass is independent of the size, shape and
orientation of the elliptical orbit. Such a machine would
be a very good clock in the sense that in spite of several
types of disturbances of the mass, within its plane of
motion, it will keep perfect time!
The period of the mass rotating on a spring is directly
related to the spring constant and to the mass of the ro-
tating body but totally independent of the major and mi-
nor axes of the elliptical trajectory as well as its orienta-

K. E. Kenyon et al. / Natural Science 3 (2011) 633-639 638
tion. This is in stark contrast to Kepler’s third law by
which the elliptical orbital period of a planet around the
sun does depend on the major axis of the planet. [It
seems a bit strange, when one thinks about it, that of the
two length-scales of the elliptical orbit, the period only
depends on one of them (the major axis).]
Identifying the surface gravity wave with Newton’s
hypothetical orbital problem of an elliptical orbit due to
a force emanating at the center of the orbit did not come
easily or by a straight path although it very well could
have. Circles are special cases of ellipses and the fluid
particles in propagating surface gravity waves move in
circles for infinitesimal amplitudes in deep water. Also
in the cyclostrophic balance of forces on each particle
the outward centrifugal force is equal and opposite to the
pressure force, and each of these two forces varies line-
arly with the distance of the particle from the center of
its orbit, the attractive pressure force acting like a spring.
From another point of view, the radii of the circular
particle orbits are known observationally to decrease
with increasing mean depth. Therefore, all radii from
zero to the wave amplitude exist simultaneously in the
water column. And each particle makes one complete
revolution around its orbit in the same length of time, the
wave period. In other words, the orbital period of the
particle is independent of the size of the orbit! There is
only one known orbital problem with this characteristic:
the mass rotating in a plane on a linear spring.
When the mean depth is comparable to or less than the
wavelength the particles orbit in ellipses, theoretically,
with the major axes being parallel to the flat bottom.
Both major and minor axes decrease with increasing
depth but the period of each orbit remains constant, the
wave period. This example comes closer to being an
analogue of Newton’s hypothetical orbital problem.
7. ROTATING SELF-GRAVITATING
MASS
An extension of the simple model of the mass rotating
and vibrating on a spring in two dimensions is as follows:
a rotating self-gravitating mass in three dimensions, or a
model of a planet made of either solid or gaseous mate-
rial. Here, although counter-intuitive for a solid, compre-
ssibility is the key characteristic for solving the problem.
Obviously each particle completes its orbit in the same
period of time, the rotational period of the planet, inde-
pendent of the size of the orbit. This extension might
seem a bit farfetched until it is realized what the con-
necting link is. At an interior point of a spherical mass of
uniform density the gravitational attraction acting on it is
known to be a linear function of the distance of the point
from the center of the body. In other words, the attrac-
tion inside the body acts like an ideal linear spring,
which is both mass less and has zero equilibrium length.
The linear spring behavior is the common feature, along
with the centrifugal force, between the two- and three-
dimensional problems.
One difference between the two and three dimensional
problems is that for the rotating self-gravitating body
there are three instead of two forces that need to be bal-
anced. In addition to the restoring spring force due to
gravitational attraction and the outward centrifugal force
there is also a pressure force inside the body. It needs to
be demonstrated that the pressure force also depends
linearly on the distance from the center of the mass.
Then all three forces are directly proportional to the ra-
dius, since the centrifugal and gravitational forces al-
ready are, and when they are balanced, the radius drops
out of the problem. The result is a formula for the rota-
tional frequency of the body that is independent of the
radius, and also independent of the latitude, as it turns
out [8].
If the mean density and the rotational periods of the
six planets that have nonzero values of their flattenings
are supplied from measurements, the model accounts for
35% - 75% of the observed flattenings. Increasing den-
sity with decreasing radius inside the planets accounts
qualitatively for the discrepancies. Therefore, what ap-
pears to be a rather rough kind of agreement between
data and a theory involving the centrifugal force has
been obtained. Hopefully in the future a closer corre-
spondence can be established.
The balance of forces leads both to the equilibrium
form of the rotating body as well as the stability of cer-
tain perturbations of it. For the equilibrium form the
small excess of the equatorial radius over the polar ra-
dius, due to the stretching of the compressible solid ma-
terial under the centrifugal force, trails off like the
square of the cosine of the latitude. Therefore the equi-
librium form is not strictly an ellipsoid. Superimposed
on this equilibrium form is a perturbation that is taken to
be in planes perpendicular to the rotation axis of the
body. Such a perturbation might be caused, for example,
by the gravitational attraction of a sufficiently distant
object, like a moon or a sun. Before the perturbation the
planes normal to the rotation axis are circles, but after
the perturbation they are more than likely concentric
ellipses with centers on the axis of rotation. Thus there
are two bulges on opposite sides of the mass. Conse-
quently, in one rotational period there are two high and
two low bodily tides. The model in its present form has
nothing to say about the expected amplitude or phase of
the bulges in relation to the given perturbing force. In
any case the seeds have been planted for a possible al-
ternative explanation for why two tides should occur per

K. E. Kenyon et al. / Natural Science 3 (2011) 633-639 639
day, at least in the solid earth.
8. FLOWS ADJACENT TO A CURVED
SOLID BOUNDARY
A relatively new type of boundary layer, called the
curvature boundary layer [9], that involves the centrifu-
gal force but no friction, comes last in this list of topics
that was initiated by wondering about the orbital motion
of fluid particles in a surface gravity wave. Consider
fluid flowing steadily next to a smooth flat rigid bound-
ary with one bump protruding into the flow. Observa-
tions will show that if the fluid speed is not too great and
the slopes of the bump are small, then the fluid will fol-
low the bump’s contours without separation or eddies
being shed off the back face. Also flow visualization
techniques will illustrate that the perturbed velocity
caused by the bump only penetrates a finite distance
above the bump. Within a few lengths of the bump,
marked off in the vertical direction, the curved stream-
lines have degenerated into straight lines. This is an in-
viscid boundary layer due to the working of the centri-
fugal force on the curved flow. When measurements in-
dicate that flow near a curved solid boundary is steady,
there must be an equal but opposite force to counter ba-
lance the centrifugal force wherever it exists. This force
can only be a pressure gradient under the given circum-
stances.
Bernoulli’s law states that where the speed is greatest
the pressure is least along a streamline in steady friction-
less flow. For the streamline just going over the top of
the bump the pressure is least at the top of the bump
because the speed is largest there by conservation of
mass. Through each cross-section equal amounts of mass
must flow in equal times. Above the top of the bump the
cross-section is least due to the fact that the perturbed
velocity dies off upward away from the bump. Conser-
vation of mass only works in this way if the complete
trailing off of the perturbed velocity takes place within a
finite distance of the bump. Assuming incompressibility
of the fluid is fine but not essential.
To see why it is that the disturbed pressure also decays
upward over the bump, look at the force balance in the
cross-stream direction: the pressure gradient equals the
centrifugal force. By convention the radius of curvature
of the flow over the bump is positive and the square of
the flow speed is positive so the centrifugal force is posi-
tive. That means the counterbalancing pressure gradient
must be positive. In other words the pressure, which has
a deficit at the top of the bump, increases upward and
will equal the ambient pressure some finite distance
away from the bump.
One of the consequences of the “vertical” gradients in
pressure, normal to the bump, is that there are “horizon-
tal” variations in pressure parallel to the bump. Since the
bump is assumed rigid, these pressure variations will not
distort its shape. Relatively low pressure at the bump is
the origin of the well-known lift force on the bump
As the pressure increases upward above the bump the
speed of the horizontal flow must decrease in accordance
with Bernoilli’s law. For a smaller flow speed the up-
ward centrifugal force is smaller requiring a smaller pre-
ssure gradient to balance it. So not only does the pres-
sure increase upward but the pressure gradient decreases
upward. Therefore, pressure and velocity perturbations,
caused by the presence of the bump, are confined to the
neighborhood of the bump in a boundary layer configu-
ration. For flow past a cylinder it turns out that the pres-
sure perturbation decays at a faster rate than the velocity
perturbation does over the top (and bottom) of the cyl-
inder. This appears to be a new result in theoretical fluid
dynamics [10].
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Facts on File, New York.
[2] Kenyon, K.E. (1991) Cyclostrophic balance in surface
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[3] Einstein, A. (1916) Elementare theorie der wasserwellen
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[4] Kenyon, K.E. and Kenyon, D.C. (1989) An elementary
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