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Back to the Past What is it that moves from a musical instrument to our brain? Sorry this is late in posting Remember Sound --- Wave Hits Pressure Wave Tympanic Membrane The outer ear? Some Questions: What is pressure? Where does it come from? How is this stuff related to a weather nap?? Throw a ball at the wall F Force on the “Wall” One Ball: Average Force Time Force on the “Wall” More Balls Average Force Time Multiple throwing “channels” Bigger the AREA the bigger the FORCE Conclusions The more balls per unit time, the more the force on the wall. The faster the balls, the more the force on the wall. The heavier the balls, the more force on the wall. For an “extended” stream of balls, the larger the WALL, the more the total force on the wall. More on Balls Consider a ball of mass m with a “velocity” v m v IMPORTANT DEFINITION momentum = (mass) x (velocity) or p=mv Remember?? F ma or Force mass acceleration (final velocity - initial velocity) acceleration time Momentum and Impulse velocity final velocityinitial F m time F time m(velocity final velocityinitial) Ft m(velocity final ) m(velocityinitial) Ft momentumfinal momentiminitial DEFINITION IMPULSE = (Force x time it acts)=F x t IMPULSE = CHANGE IN MOMENTUM Force on the “Wall” REMEMBER??? Average Force Time Impulse = Change in Momentum Initial Final NEWTON’S THIRD LAW Force on Wall Finial – (Initial) Directional Impulse = Change in Momentum Final Initial Becomes Force on Wall (- above because of N3) Consider a box of air Air Box MAGNIFIED Magnify Some More ● ● ● MOLECULES ● Are Molecules Something Like Tennis Balls??? F F HOW BIG IS THE FORCE ON THE WALL?? The Force on the Wall INCREASES when …. The number of molecules in the box that can strike the wall increases. The AREA of the wall increases. The speed of the molecules increases. This happens when the temperature of the gas increases. The “heavier” the molecules The math Force = (constant) x (Area) x (number of molecules) x (speed) F=k n A T Define PRESSURE = Force per unit area = P P~nT An Example The pressure on a 5 square foot area wall is 10 pounds per square inch (psi). What is the TOTAL FORCE on the wall? P= F/A so F=PA F=(10 lbs/ft2)(5 ft2)= 50 lbs Air Pressure Normal Atmospheric pressure 14.7 lbs/in2 Also called a BAR A 2005 storm had a pressure at its center listed as 0.920 bars or 920 millibars. Milli = one one thousandth Question – What is one atmosphere in pounds per square FOOT? 1 sq. ft. = 12 in x 12 in = 144 in2 pounds in pounds P 14.7 x 144 2 2116 2 2 in ft ft 2 To Ponder Imagine a pane of glass in your house that is one foot square. The force on it is 2116 pounds. Balance! Molecules Move Around and collide with each other! WALL Collisions Ft momentumfinal momentuminitial Is there a connection? Map Application The Musical Tube Reflection Movement PRESSURE: HIGH LOW CLOSED Musical Tube reflection Open Musical Tube Open Tube Oh where, oh where has my sound wave gone … Oh, Oh, another wall!!! Wall is a velocity node but a Pressure Anti-node due to the wave itself. Lots of collisions! Average velocity at wall is zero Wave is reflected Open End REFLECTION From Textbook pg. 185 So now you know what pressure is!