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Back to the Past. Music is organized sound ( one definition anyway ) but WHAT THE HECK IS SOUND??. Sound ---. Force on the “Wall”. Time. Throw a ball at the wall. F. One Ball:. Average Force. Force on the “Wall”. Time. More Balls. Average Force. Multiple throwing “channels”.
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Back to the Past Music is organized sound (one definition anyway) but WHAT THE HECK IS SOUND??
Force on the “Wall” Time Throw a ball at the wall F One Ball: Average Force
Force on the “Wall” Time More Balls Average Force
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.
Back to the Balls NEW CONCEPT: MOMENTUM
Consider a mass m with a “velocity” v v m IMPORTANT DEFINITION momentum = (mass) x (velocity) or p=mv
DEFINITION • IMPULSE = (Force x time it acts)=F x t • IMPULSE = CHANGE IN MOMENTUM
Force on the “Wall” Time REMEMBER??? Average Force
Initial Final Impulse = Change in Momentum NEWTON’S THIRD LAW Force on Wall Finial – (Initial) Directional
Consider a box of air Air MAGNIFIED Box
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
To Ponder • Imagine a pane of glass in your house that is one foot square. • The force on it is 2116 pounds. Why doesn't it break??
Balance! That's Why!
Map Why?? H
Movement PRESSURE: HIGH LOW
CLOSED Musical Tube reflection Reflection Easily Understood.
Open Tube An open ended tube can reflect a sound wave!
And Reflection + Incident + ….. = Standing Wave
Open Tube NODE NODE ANTI-NODE
Clarinet OPEN END