Transcript 5.3 Elements of Pressure

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:
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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
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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
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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 ….
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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.
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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
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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
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Air Pressure
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Normal Atmospheric pressure
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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.

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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
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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!