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`SCIENCE: Always question, Always wonder.
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`V 2015(7)
`V January (7)
`10 Science Principles You
`See in Action Every Day
`10 Science Principles You
`See in Action Every Day
`10 Science Principles You
`See in Action Every Day
`10 Science Principles You
`See in Action Every Day
`10 Science Principles You
`See in Action Every Day
`# 9Bemou||i's Principle:
`Bemoul|i's principle...
`THE ORIGIN OF LIFE IS
`0 N E O F TH E
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`# 9
`BERNOULLl'S PRINCIPLE:
`
`T
`Hlgher Pressure
`Lower Speed
`
` 1‘
`
`Lower Pressure
`Higher Speed
`
`T
`Higher Pressure
`Lower Speed
`
`Bernoulli's principle, physical principle formulated by Daniel Bernoulli that states that as the
`speed of a moving fluid (liquid or gas) increases, the pressure within the fluid decreases.
`The phenomenon described by Bernoulli's principle has many practical applications;
`it
`is
`employed in the carburetor and the atomizer,
`in which air is the moving fluid, and in the
`aspirator, in which water is the moving fluid.
`In the first two devices air moving through a
`tube passes through a constriction, which causes an increase in speed and a corresponding
`reduction in pressure. As a result, liquid is forced up into the air stream (through a narrow
`tube that leads from the body of the liquid to the constriction) by the greater atmospheric
`pressure on the surface of the liquid. In the aspirator air is drawn into a stream of water as
`the water flows through a constriction. Bernoulli's principle can be explained in terms of the
`law of conservation of energy. As a fluid moves from a wider pipe into a narrower pipe or a
`constriction, a corresponding volume must move a greater distance forward in the narrower
`pipe and thus have a greater speed. At the same time, the work done by corresponding
`volumes in the wider and narrower pipes will be expressed by the product of the pressure
`and the volume. Since the speed is greater in the narrower pipe, the kinetic energy of that
`volume is greater. Then, by the law of conservation of energy, this increase in kinetic energy
`must be balanced by a decrease in the pressure-volume product, or, since the volumes are
`equal, by a decrease in pressure.
`The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University
`Press. All rights reserved.
`
`How It Works:
`(1700-1782) discovered the
`The Swiss mathematician and physicist Daniel Bernoulli
`principle that bears his name while conducting experiments concerning an even more
`fundamental concept: the conservation of energy. This is a law of physics that holds that a
`system isolated from all outside factors maintains the same total amount of energy, though
`energy transformations from one form to another take place.
`
`For instance, if you were standing at the top of a building holding a baseball over the side,
`the ball would have a certain quantity of potential energy—the energy that an object
`possesses by virtue of its position. Once the ball
`is dropped,
`it immediately begins losing
`potential energy and gaining kinetic energy—the energy that an object possesses by virtue
`of its motion. Since the total energy must remain constant, potential and kinetic energy have
`an inverse relationship: as the value of one variable decreases, that of the other increases in
`exact proportion.
`
`In
`The ball cannot keep falling forever, losing potential energy and gaining kinetic energy.
`fact,
`it can never gain an amount of kinetic energy greater than the potential energy it
`possessed in the first place. At the moment before the ball hits the ground, its kinetic energy
`is equal to the potential energy it possessed at the top of the building. Correspondingly, its
`potential energy is zero—the same amount of kinetic energy it possessed before it was
`dropped.
`
`Then, as the ball hits the ground, the energy is dispersed. Most of it goes into the ground,
`and depending on the rigidity of the ball and the ground, this energy may cause the ball to
`bounce. Some of the energy may appear in the form of sound, produced as the ball hits
`
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`bottom, and some will manifest as heat. The total energy, however, will not be lost: it will
`simply have changed form.
`
`Bernoulli was one of the first scientists to propose what is known as the kinetic theory of
`gases: that gas, like all matter,
`is composed of tiny molecules in constant motion.
`In the
`1730s, he conducted experiments in the conservation of energy using liquids, observing how
`water flows through pipes of varying diameter. In a segment of pipe with a relatively large
`diameter, he observed, water flowed slowly, but as it entered a segment of smaller diameter,
`its speed increased.
`
`It was clear that some force had to be acting on the water to increase its speed. Earlier,
`Robert Boyle (1627-1691) had demonstrated that pressure and volume have an inverse
`relationship, and Bernoulli seems to have applied Boyle's findings to the present situation.
`Clearly the volume of water flowing through the narrower pipe at any given moment was
`less than that flowing through the wider one. This suggested, according to Boyle's law, that
`the pressure in the wider pipe must be greater.
`
`As fluid moves from a wider pipe to a narrower one, the volume of that fluid that moves a
`given distance in a given time period does not change. But since the width of the narrower
`pipe is smaller, the fluid must move faster in order to achieve that result. One way to
`illustrate this is to observe the behavior of a river:
`in a wide, unconstricted region,
`it flows
`slowly, but
`if
`its flow is narrowed by canyon walls (for
`instance),
`then it speeds up
`dramatically.
`
`The above is a result of the fact that water is a fluid, and having the characteristics of a fluid,
`it adjusts its shape to fit that of its container or other solid objects it encounters on its path.
`Since the volume passing through a given length of pipe during a given period of time will be
`the same, there must be a decrease in pressure. Hence Bernoulli's conclusion: the slower
`the rate of flow, the higher the pressure, and the faster the rate of flow, the lower the
`pressure.
`
`Bernoulli published the results of his work in Hydrodynamica (1738), but did not present his
`ideas or their implications clearly. Later, his friend the German mathematician Leonhard
`Euler (1707-1783) generalized his findings in the statement known today as Bernoulli's
`principle.
`
`The Venturi Tube:
`
`Upweam pressure ta)
`
`\ P-
`n,,,,..,m.°..
`D.‘
`
`Also significant was the work of the
`1822), who is credited with developing
`Italian physicist Giovanni Venturi (1746-
`the Venturi
`tube, an instrument
`for
`measuring the drop in pressure that
`takes place as the velocity of a fluid
`increases.
`It consists of a glass tube with an inward-sloping area in the middle, and
`manometers, devices for measuring pressure, at three places: the entrance, the point of
`constriction, and the exit. The Venturi meter provided a consistent means of demonstrating
`Bernoulli's principle.
`
`Downsliearn unessune tan
`P”/ Ap;P__R
`,,,,,,,,,
`
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`""""“"“
`
`Like many propositions in physics, Bernoulli's principle describes an ideal situation in the
`absence of other forces. One such force is viscosity, the internal friction in a fluid that makes
`it resistant
`to flow.
`In 1904,
`the German physicist Ludwig Prandtl
`(1875-1953) was
`conducting experiments in liquid flow, the first effort in well over a century to advance the
`findings of Bernoulli and others. Observing the flow of liquid in a tube, Prandtl found that a
`tiny portion of the liquid adheres to the surface of the tube in the form of a thin film, and does
`not continue to move. This he called the viscous boundary layer.
`
`itself, Prandt|'s findings would play a significant part
`Like Bernoulli's principle
`aerodynamics, or the study of airflow and its principles. They are also significant
`hydrodynamics, or the study of water flow and its principles, a discipline Bernoulli founded.
`
`in
`in
`
`Laminar vs. Turbulent Flow:
`
`Air and water are both examples of fluids,
`substances which—whether gas or liquid-
`conform to the shape of their container. The
`flow patterns of all fluids may be described in
`terms either of laminar flow, or of its opposite,
`turbulent flow.
`
`
`
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`Laminar flow is smooth and regular, always moving at the same speed and in the same
`direction. Also known as streamlined flow,
`it is characterized by a situation in which every
`particle of fluid that passes a particular point follows a path identical to all particles that
`passed that point earlier. A good illustration of laminar flow is what occurs when a stream
`flows around a twig.
`
`By contrast, in turbulent flow, the fluid is subject to continual changes in speed and direction
`—as, for instance, when a stream flows over shoals of rocks. Whereas the mathematical
`model of laminar flow is rather straightfonrvard, conditions are much more complex in
`turbulent flow, which typically occurs in the presence of obstacles or high speeds.
`
`Turbulent flow makes it more difficult for two streams of air, separated after hitting a barrier,
`to rejoin on the other side of the barrier; yet that is their natural tendency. In fact, if a single
`air current hits an airfoi|—the design of an airp|ane's wing when seen from the end, a
`streamlined shape intended to maximize the aircraft's response to ain‘low—the air that flows
`over the top will ''try' to reach the back end of the airfoil at the same time as the air that flows
`over the bottom.
`In order to do so,
`it will need to speed up—and this, as will be shown
`below, is the basis for what makes an airplane fly.
`
`When viscosity is absent, conditions of perfect laminar flow exist: an object behaves in
`complete alignment with Bernoulli's principle. Of course, though ideal conditions seldom
`occur in the real world, Bernoulli's principle provides a guide for the behavior of planes in
`flight, as well as a host of everyday things.
`
`Real-Life Applications:
`Flying Machines:
`For thousands of years, human beings vainly sought to fly ''like a bird," not realizing that this
`is literally impossible, due to differences in physiognomy between birds and homo sapiens.
`No man has ever been born (or ever will be) who possesses enough strength in his chest
`that he could flap a set of attached wings and lift his body off the ground. Yet the bird's
`physical structure proved highly useful to designers of practical flying machines.
`Rrdmcd nlr prwsull
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`
`trnulnnlulr puma»:
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`"7"
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`
`
`
`A bird's wing is curved along the top, so
`that when air passes over the wing and
`divides, the curve forces the air on top
`to travel a greater distance than the air
`on the bottom. The tendency of airflow,
`as noted earlier,
`is to correct for the
`presence of solid objects and to return
`to its original pattern as quickly as
`
`possible. Hence, when the air hits the
`
`front of the wing, the rate of flow at the
`top increases to compensate for the
`greater distance it has to travel than the
`air below the wing. And as shown by
`Bernoulli, fast-moving fluid exerts less pressure than slow-moving fluid; therefore, there is a
`difference in pressure between the air below and the air above, and this keeps the wing
`aloft.
`
`co-~samatrn'iv-«arr
`
`-\W-=-
`
`Only in 1853 did Sir George Cayley (1773-1857) incorporate the avian airfoil to create
`history's first workable (though engine-less) flying machine, a glider. Much, much older than
`Cay|ey's glider, however, was the first manmade flying machine built "according to
`Bernoulli's princip|e"—on|y it first appeared in about 12,000 b.c., and the people who created
`it had little contact with the outside world until the late eighteenth century a.d. This was the
`boomerang, one of the most ingenious devices ever created by a stone-age society—in this
`case, the Aborigines of Australia.
`
`Contrary to the popular image, a boomerang flies through the air on a plane perpendicular to
`the ground, rather than parallel. Hence, any thrower who properly knows how tosses the
`boomerang not with a side-arm throw, but overhand. As it flies, the boomerang becomes
`both a gyroscope and an airfoil, and this dual role gives it aerodynamic lift.
`
`Like the gyroscope, the boomerang imitates a top; spinning keeps it stable. It spins through
`the air, its leading wing (the forward or upward wing) creating more lift than the other wing.
`As an airfoil, the boomerang is designed so that the air below exerts more pressure than the
`air above, which keeps it airborne.
`
`Another very early example of a flying machine using Bernoulli's principles is the kite, which
`first appeared in China in about 1000 b.c. The kite's design, particularly its use of lightweight
`fabric stretched over two crossed strips of very light wood, makes it well-suited for flight, but
`what keeps it in the air is a difference in air pressure. At the best possible angle of attack,
`
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`
`the kite experiences an ideal ratio of pressure from the slower-moving air below versus the
`faster-moving air above, and this gives it lift.
`
`Later Cayley studied the operation of the kite, and recognized that it—rather than the
`balloon, which at first seemed the most promising apparatus for flight—was an appropriate
`model for the type of heavier-than-air flying machine he intended to build. Due to the lack of
`a motor, however, Cay|ey's prototypical airplane could never be more than a glider: a steam
`engine, then state-of-the-art technology, would have been much too heavy.
`
`it was only with the invention of the internal-combustion engine that the modern
`Hence,
`airplane came into being. On December 17, 1903, at Kitty Hawk, North Carolina, Orville
`(1871-1948) and Wilbur (1867-1912) Wright tested a craft that used a 25-horsepower
`engine they had developed at their bicycle shop in Ohio. By maximizing the ratio of power to
`weight, the engine helped them overcome the obstacles that had dogged recent attempts at
`flight, and by the time the day was over, they had achieved a dream that had eluded men for
`more than four millennia.
`
`Within fifty years, airplanes would increasingly obtain their power from jet rather than
`internal-combustion engines. But the principle that gave them flight, and the principle that
`kept them aloft once they were airborne, reflected back to Bernoulli's findings of more than
`160 years before their time. This is the concept of the airfoil.
`
`Its shape is rather like that of an
`As noted earlier, an airfoil has a streamlined design.
`elongated, asymmetrical teardrop lying on its side, with the large end toward the direction of
`airflow, and the narrow tip pointing toward the rear. The greater curvature of its upper
`surface in comparison to the lower side is referred to as the airp|ane's camber. The front end
`of the airfoil is also curved, and the chord line is an imaginary straight line connecting the
`spot where the air hits the front—known as the stagnation point—to the rear, or trailing
`edge, of the wing.
`
`Again, in accordance with Bernoulli's principle, the shape of the airflow facilitates the spread
`of laminar flow around it. The slower-moving currents beneath the airfoil exert greater
`pressure than the faster currents above it, giving lift to the aircraft. Of course, the aircraft has
`to be moving at speeds sufficient to gain momentum for its leap from the ground into the air,
`and here again, Bernoulli's principle plays a part.
`
`Thrust comes from the engines, which run the prope||ers—whose blades in turn are
`designed as miniature airfoils to maximize their power by harnessing airflow. Like the aircraft
`wings, the blades’ angle of attack—the angle at which airflow hits it. In stable flight, the pilot
`greatly increases the angle of attack (also called pitched), whereas at takeoff and landing,
`the pitch is dramatically reduced.
`Drawing Fluids Upward: Atomizers and Chimneys
`
`A number of everyday objects use Bernoulli's principle to draw fluids upward, and though in
`terms of their purposes, they might seem very different—for instance, a perfume atomizer
`vs. a chimney—they are closely related in their application of pressure differences. In fact,
`the idea behind an atomizer for a perfume spray bottle can also be found in certain garden-
`hose attachments, such as those used to provide a high-pres-sure car wash.
`
`therefore, according to
`inside the perfume bottle is moving relatively slowly;
`The air
`Bernoulli's principle, its pressure is relatively high, and it exerts a strong downward force on
`the perfume itself. In an atomizer there is a narrow tube running from near the bottom of the
`bottle to the top. At the top of the perfume bottle,
`it opens inside another tube, this one
`perpendicular to the first tube. At one end of the horizontal tube is a simple squeeze-pump
`which causes air to flow quickly through it. As a result, the pressure toward the top of the
`bottle is reduced, and the perfume flows upward along the vertical tube, drawn from the
`area of higher pressure at the bottom. Once it is in the upper tube, the squeeze-pump helps
`to eject it from the spray nozzle.
`
`A carburetor works on a similar principle, though in that case the lower pressure at the top
`draws air rather than liquid. Likewise a chimney draws air upward, and this explains why a
`windy day outside makes for a better fire inside. With wind blowing over the top of the
`chimney, the air pressure at the top is reduced, and tends to draw higher-pressure air from
`down below.
`
`The upward pull of air according to the Bernoulli principle can also be illustrated by what is
`sometimes called the "Hoover bug|e"—a name perhaps dating from the Great Depression,
`when anything cheap or contrived bore the appellation "Hoover" as a reflection of popular
`dissatisfaction with President Herbert Hoover. In any case, the Hoover bugle is simply a long
`corrugated tube that, when swung overhead, produces musical notes.
`
`Spin, Curve, and Pull: The Counterintuitive Principle:
`
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`
`interesting illustrations-
`There are several other
`sometimes fun and in one case potentially tragic—of
`Bernoulli's principle. For instance, there is the reason
`why a shower curtain billows inward once the shower
`is turned on.
`It would seem logical at first that the
`pressure created by the water would push the curtain
`outward, securing it to the side of the bathtub.
`
`Loiurlir pruuufl
`
`Instead, of course, the fast-moving air generated by
`the flow of water from the shower creates a center of
`lower pressure, and this causes the curtain to move away from the slower-moving air
`outside. This is just one example of the ways in which Bernoulli's principle creates results
`that, on first glance at least, seem oounterintuitive—that is, the opposite of what common
`sense would dictate.
`
`Another fascinating illustration involves placing
`two empty soft drink cans parallel to one another
`on a table, with a couple of inches or a few
`centimeters between them. At that point, the air
`on all sides has the same slow speed. If you were
`to blow directly between the cans, however, this
`would create an area of low pressure between
`them. As a result, the cans push together. For
`ships in a harbor,
`this can be a frightening
`prospect: hence, if two crafts are parallel to one another and a strong wind blows between
`them, there is a possibility that they may behave like the cans.
`
`
`
`Then there is one of the most illusory uses of Bernoulli's principle, that infamous baseball
`pitcher's trick called the curve ball. As the ball moves through the air toward the plate,
`its
`velocity creates an air stream moving against the trajectory of the ball itself. Imagine it as two
`lines, one curving over the ball and one curving under, as the ball moves in the opposite
`direction.
`
`In an ordinary throw, the effects of the airflow would not be particularly intriguing, but in this
`case, the pitcher has deliberately placed a "spin" on the ball by the manner in which he has
`thrown it. How pitchers actually produce spin is a complex subject unto itself, involving grip,
`wrist movement, and other factors, and in any case, the fact of the spin is more important
`than the way in which it was achieved.
`
`If the direction of airflow is from right to left, the ball, as it moves into the airflow, is spinning
`clockwise. This means that the air flowing over the ball is moving in a direction opposite to
`the spin, whereas that flowing under it is moving in the same direction. The opposite forces
`produce a drag on the top of the ball, and this cuts down on the velocity at the top compared
`to that at the bottom of the ball, where spin and airflow are moving in the same direction.
`
`Thus the air pressure is higher at the top of the ball, and as per Bernoulli's principle, this
`tends to pull the ball downward. The curve ball—of which there are numerous variations,
`such as the fade and the s|ider—creates an unpredictable situation for the batter, who sees
`the ball leave the pitcher's hand at one altitude, but finds to his dismay that it has dropped
`dramatically by the time it crosses the plate.
`
`A final illustration of Bernoulli's often counterintuitive principle neatly sums up its effects on
`the behavior of objects. To perform the experiment, you need only an index card and a flat
`surface. The index card should be folded at the ends so that when the card is parallel to the
`surface, the ends are perpendicular to it. These folds should be placed about half an inch
`from the ends.
`
`it would be handy to have an unsuspecting person—someone who has not
`At this point,
`studied Bernoulli's princip|e—on the scene, and challenge him or her to raise the card by
`blowing under it. Nothing could seem easier, of course: by blowing under the card, any
`person would naturally assume, the air will
`lift
`it. But of course this is completely wrong
`according to Bernoulli's principle. Blowing under the card, as illustrated, will create an area
`of high velocity and low pressure. This will do nothing to lift the card: in fact, it only pushes
`the card more firmly down on the table.
`
`WHERE TO LEARN MORE
`
`Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991.
`
`"Bernou||i's Principle: Explanations and Demos." (Web site).
`<http://207.10.97.102/physicszone/lesson/02forces/bernoull/bernoull.html> (February 22,
`2001).
`
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`Cockpit Physics (Department of Physics, United States Air Force Academy. Web site.).
`<http://www.usafa.af.mil/dfp/cockpit-phys/> (February 19, 2001).
`
`K8A|T Principles of Aeronautics Advanced Text. (Web site).
`<http://wings.ucdavis.edu/Book/advanoed.html> (February 19, 2001 ).
`
`Schrier, Eric and William F. Allman. Newton at the Bat: The Science in Sports. New York:
`Charles Scribner's Sons, 1984.
`
`Smith, H. C. The Illustrated Guide to Aerodynamics. Blue Ridge Summit, PA: Tab Books,
`1992.
`
`Stever, H. Guyford, James J. Haggerty, and the Editors of Time-Life Books. Flight. New
`York: Time-Life Books, 1965.
`
`|nfop|ease.com
`Read more: Bernoulli's principle |
`http://www.infoplease.com/encyclopedia/science/bernoulli-principle.htm|#ixzz3O FJfzq1s
`Posted by HAMID AZIZ at 09:54
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