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Back to the Past

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

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  1. Back to the Past Music is organized sound (one definition anyway) but WHAT THE HECK IS SOUND??

  2. Sound ---

  3. Force on the “Wall” Time Throw a ball at the wall F One Ball: Average Force

  4. Force on the “Wall” Time More Balls Average Force

  5. Multiple throwing “channels” Bigger the AREA the bigger the FORCE

  6. 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.

  7. Back to the Balls NEW CONCEPT: MOMENTUM

  8. Consider a mass m with a “velocity” v v m IMPORTANT DEFINITION momentum = (mass) x (velocity) or p=mv

  9. Remember??

  10. Momentum and Impulse

  11. DEFINITION • IMPULSE = (Force x time it acts)=F x t • IMPULSE = CHANGE IN MOMENTUM

  12. Force on the “Wall” Time REMEMBER??? Average Force

  13. Initial Final Impulse = Change in Momentum NEWTON’S THIRD LAW Force on Wall Finial – (Initial) Directional

  14. Consider a box of air Air MAGNIFIED Box

  15. Magnify Some More ● ● ● MOLECULES ●

  16. Are Molecules Something Like Tennis Balls??? F F HOW BIG IS THE FORCE ON THE WALL??

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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??

  22. Balance! That's Why!

  23. Molecules Move Around and collide with each other! WALL

  24. Collisions

  25. Link

  26. Is there a connection?

  27. Map Why?? H

  28. Movement PRESSURE: HIGH LOW

  29. CLOSED Musical Tube reflection Reflection Easily Understood.

  30. Open Musical Tube

  31. Open Tube An open ended tube can reflect a sound wave!

  32. Strings

  33. String

  34. And Reflection + Incident + ….. = Standing Wave

  35. Open Tube NODE NODE ANTI-NODE

  36. Clarinet OPEN END

  37. So now you know what pressure is!

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