```Pressure in FluidsPorter W Johnson               Illinois Institute of Technology                               Department of Physics                               Chicago IL 60616-3793                               (312) 567-3375Objectives:We shall study the properties of fluids under a variety of circumstances to understand the role or pressure in their behavior, and to relate to everyday circumstances. Materials Needed:Several two liter "pop bottles" and assorted plastic bottles, several "push pins" for making holes, a medicine dropper for the cartesian diver, a large clear funnel with a stand for holding it in place, several plastic straws and a supply of strong plastic tape or Duck Tape, and at least one "middle range" water pistol, a "battery operated" model being preferable. Strategy:The first phase is a demonstration of the flow of water out of holes punched with push pins and some enlarged with nails in plastic bottles of various shapes and sizes.  Let the students report their observations, and write them on the board.  Be sure to record every correct observation, whether it is directly relevant for the points you intend to make.  Here is a non-inclusive list of observations that might be made.      1.  If the hole is punched straight into the bottle, the water         initially moves horizontally, and gradually drops as it gets         further away from the vessel.        2.  As the water drains from one of the bottles and the level         drops, the water comes out with lesser speed, and falls on         an arc that does not extend as far.      3.  The stream of water initially gets smaller as it leaves         the hole,  and it will break into droplets if it falls far         enough.  Try this with a hole punched in the bottom of the
bottle.     4.  If the cap is put back on the bottle the flow slows and         eventually stops.  If you blow hard into the bottle the rate         of flow is increased.     5.  With two holes of the same size, the one that is deeper in         relation to the "water level" in the bottle will have a         greater exit speed.     6.  The size of the hole has relatively little effect on the         speed of water that exits, although, a bigger stream of         water flows out of the larger hole.Next go outside with the water pistol and shoot it from shoulder level.  Have the students divide into groups of three or four, with each group estimating its maximum distance of travel in meters by stepping or pacing the distance.  Be sure to take along an extra supply of water for reloading the pistol.  Go back inside, and record the various answers.  You should measure the distance in advance, so that the various answers obtained can be compared with the standard.  For the Drencher AdvantageTM power pistol, a range of about 9 meters was obtained.  Fill a two liter pop bottle with water.  Take a medicine dropper and fill it partially, so that the dropper just sinks in the water inside the bottle.  It will take some experimentation to find the right amount of water in the dropper.  Then put the lid tightly onto the bottle.  Squeeze the sides of the bottle.  Notice that, as the water pressure inside the bottle is increased thereby, the air in the dropper is compressed, and the dropper falls.  You might enjoy repeating the experiment with an oval-shaped bottle.  The device is called a Cartesian Diver. Fill a bottle completely with water, and insert a plastic straw.  Use tape to hold the straw a few millimeters above the top of the bottle and to seal the bottle.  Blow gently over the straw, either directly or by means of another straw.  If possible, use an air pump to force air over the top of the straw. You will notice that water comes out of the bottle.  The water is forced up and out of the straw because of the Bernoulli Principle.  This mechanism is used aerosol
cans, in that a liquid is ejected from the reservoir by passing a gas over a tube at high speed. Many many people believe that the water in a flushing toilet or a draining bathtub will circulate in one sense in the Northern Hemisphere, and in another sense in the Southern Hemisphere, because of the Coriolis Force, which is responsible for clockwise circulation around high pressure regions and counter-
clockwise circulation around low pressure regions in the atmosphere.  Actually, it is difficult to build a toilet or tub that does not have some directional bias and it is simpler to study the effect in a large clear funnel.  Fill the funnel with water and add dye to enhance visibility while holding your finger on the bottom of the funnel.  You will have to pour the water into the funnel symmetrically and wait for several minutes to avoid any residual circulation of the water.  Do you see the effect of the Coriolis Force? Performance Assessment:                                Laboratory Exercise:Punch a hole on the side of a 2 liter pop bottle near the bottom, and fill it with water.  Place the bottle on the edge of a table.  When the level of water is at a depth d above the hole, determine the horizontal distance s that the fluid stream lies when it has fallen to a fixed distance of, say, 20 cm below the hole.  Make measurements of h for various values of d, and make a graph of d
versus h.  Does the graph correspond to a straight line?                                 Written question:How does a water pistol work?                               Satisfactory Answer:A pump [mechanical or electrical] builds up pressure inside the vessel partially filled with water, and water is forced out of the hole in the barrel.  The operation is exactly the same as that of blowing into the partially filled bottle with a hole in it below the water level. Conclusions:The consequences of Pascal's Principle [the pressure in a liquid increases with depth and its extension the Bernoulli Principle [lower pressure corresponds to higher speed along the path of flow of a fluid] have been explored, and their implications in everyday experiences are examined. Multi-cultural Components:A story by Hans Christian Anderson describes the little Dutch boy who put his finger into the dike to block whole North Sea.  The boy is often depicted wearing wooden shoes.      Q:  Why are wooden shoes popular in Holland and other lowland countries?     A:  They are very effective in walking through mud, and it is quite easy to            remove the mud afterward without damaging the shoes. You may know that there is a difference in ocean levels of the locks at the two ends of the Panama Canal.  Part of this difference occurs because the Pacific
Ocean is more salty and therefore more dense, so that its level should be lower.  Incidentally, the locks on the Atlantic side of the canal are further West than those on its Pacific side! References:      Jearl Walker      The Flying Circus of Physics With Answers
Wiley 1975
ISBN 0 - 471 - 02984 - X

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