Transcript Sound Intro Notes

Wave Equations
Definitions
Day 1
• Amplitude
– Maximum deflection (the biggest x value)
– Measured in meters
Click for amplitude example
• Period (T)
– Time for one event
– Measured in seconds
• Frequency (f)
– Events per second
– Measured in Hertz
• f = 1/T
f = #events/second
Hand out purple definition wksh
Do pendulum lab
POD
• A pendulum is released from 1.0m above
the table. swings from 1 to 3 and back ten
times in 5 seconds.
• Identify the amplitude
• Calculate the period
• Calculate the frequency
0.4m
What changes the frequency for a
pendulum?
• Amplitude? (height released)
No
• Mass?
No
• Length of string
Yes
Periodic Motion
• Any motion that repeats so it has a definite period
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Pendulum
Spring
Swing
Bungee cord
waves
Sound
• All periodic motion
– makes waves
– Has amplitude
– Has period and frequency
Now think about a
bouncing spring
• Where will the displacement be zero?
• Where will the displacement be at a
maximum?
• What about in between?
a) Equilibrium point
b) Both ends of the spring (top and
bottom)
c) PE and KE changes
For an oscillating spring. . .
• Where is the net force the greatest?
– The largest x position (F = -kx)
– At the peaks where it changes direction
• Where is the net force zero?
– When displacement is zero
– At the equilibrium – the middle
For an oscillating spring. . .
• Where is the velocity the greatest?
– At the equilibrium – the middle
– There is no net force to slow it’s motion here
• Where is the velocity zero?
– At the peaks because direction changes
– There is a maximum force, but zero velocity
Do spring lab
What changes the frequency for a
spring?
• Amplitude? (height released)
No
• Mass?
Yes - more mass less motion (inertia
resists change)
• Spring constant (k)?
Yes - strong spring “jumps” back, weak
spring slowly changes
So frequency
• For a spring ƒ = 1/(2)√(k/m)
– Where k is the spring constant
– M is the mass in kg
• For a pendulum ƒ = 1/(2)√(g/l)
– Where g is the acceleration due to gravity
which is 9.8m/s/s
– L is the length of the string in meters
A 60kg person bungee jumps from a
bridge . He bounces 10 times in 15 s.
• What is the length of time for each bounce?
• What is the frequency of bouncing?
a) 1.5 sec
b) f=0.66Hz