physics 101: lecture 27 sound

UB, Phy101: Chapter 16, Pg 1

Physics 101: Physics 101: Lecture 27Lecture 27SoundSound

Today’s lecture will cover Textbook Sections 16.1 - 16.10


What is a wave ?What is a wave ?

According to our text: A wave is a traveling disturbance that transports energy

Examples: Sound waves (air moves back & forth) Stadium waves (people move up & down) Water waves (water moves up & down) Light waves (what moves ??)

DEMO


Types of WavesTypes of Waves

Longitudinal: The medium oscillates in the same direction as the wave is moving

Sound Slinky

Transverse: The medium oscillates perpendicular to the direction the wave is moving. Water (more or less) Slinky


Wave PropertiesWave Properties

Wavelength

Wavelength: The distance between identical points on the wave.

Amplitude A

Amplitude: The maximum displacement A of a point on the wave.

A


Wave Properties...Wave Properties...

Period: The time T for a point on the wave to undergo one complete oscillation.

Speed: The wave moves one wavelength in one period T so its speed is v = / T.

Tv


Wave Properties...Wave Properties...

The speed of a wave is a constant that depends only on the medium, not on amplitude, wavelength or period (similar to SHM)

and T are related !

= v T or = 2 v / (sinceT = 2 /

or v / f (since T = 1/ f )

Recall f = cycles/sec or revolutions/sec

2f

v = / T


Preflight 1Preflight 1

Suppose a periodic wave moves through some medium. If the period of the wave is increased, what happens to the wavelength of the wave assuming the speed of the wave remains the same? 1. The wavelength increases 2. The wavelength remains the same 3. The wavelength decreases

correct


Preflight 2Preflight 2

The speed of sound in air is a bit over 300 m/s, and the speed of light in air is about 300,000,000 m/s. Suppose we make a sound wave and a light wave that both have a wavelength of 3 meters. What is the ratio of the frequency of the light wave to that of the sound wave? 1. About 1,000,000. 2. About 1,000. 2. About .000,001

correct

f = v/

fL/fS = vL/vS = 1,000,000

fS = 100 Hz (~ really low G)

fL = 100 MHz (FM radio)


Preflight 3 & 4Preflight 3 & 4

Suppose that a longitudinal wave moves along a Slinky at a speed of 5 m/s. Does one coil of the slinky move through a distance of five meters in one second? 1. Yes2. No

no single coil on the slinky will move anywhere near 5 meters. Rather many coils will move many smaller distances in shorter times to create the wave that has a speed of 5 meters per sec.

correct

5m


The wavelength of microwaves generated by a microwave oven is about 3 cm. At what frequency do these waves cause the water molecules in your burrito to vibrate ?

(a) 1 GHz (b) 10 GHz (c) 100 GHz

1 GHz = 109 cycles/sec

The speed of light is c = 3x108 m/s

Another QuestionAnother Question


Recall that v = f.

1 GHz = 109 cycles/sec

The speed of light is c = 3x108 m/s

fv 3 10 m s

.03m10 Hz 10GHz

810

H H

O

Makes water molecules wiggleMakes water molecules wiggle

Another question, ans.Another question, ans.


Absorption coefficientof water as a functionof frequency.

f = 10 GHz

Visible

“water hole”


Waves on a StringWaves on a String

T

m/L

T v


Preflights 5 & 6Preflights 5 & 6

A rope of mass M and length L hangs from the ceiling with nothing attached to the bottom (see picture). Suppose you start a transverse wave at the bottom end of the rope by giggling (sic) it a bit. As this wave travels up the rope its speed will:1. Increase 2. Decrease 3. Stay the same

the tension gets greater as you go up

v

correct


Preflight Preflight

A sound wave having frequency f0, speed v0 and wavelength 0, is traveling through air when in encounters a large helium-filled balloon. Inside the balloon the frequency of the wave is f1, its speed is v1, and its wavelength is 1

Compare the frequency of the sound wave inside and outside the balloon

1. f1 < f0

2. f1 = f0

3. f1 > f0

f1f0

correct


Preflight Preflight


Compare the speed of the sound wave inside and outside the balloon

1. v1 < v0

2. v1 = v0

3. v1 > v0

V1=965m/sV0=343m/s

correct


Preflight Preflight


Compare the wavelength of the sound wave inside and outside the balloon

1. 1 < 0

2. 1 = 0

3. 1 > 00 1

correct

= v / f


Doppler Effect DEMO


PreflightPreflightA: You are driving along the highway at 65 mph, and behind you a police car, also traveling at 65 mph, has its siren turned on.B: You and the police car have both pulled over to the side of the road, but the siren is still turned on.In which case does the frequency of the siren seem higher to you? 1. Case A 2. Case B 3. same

v

vv

v

ff

s

o

1

1'

vs

f

vo

f’vcorrect

Pg 479

NOT ON EXAM


Constructive interference Destructive interference

Interference and Superposition


Superposition & InterferenceSuperposition & Interference

Consider two harmonic waves A and B meeting at x=0. Same amplitudes, but 2 = 1.15 x 1.

The displacement versus time for each is shown below:

What does C(t) = A(t) + B(t) look like??

A(1t)

B(2t)


Superposition & InterferenceSuperposition & Interference

Consider two harmonic waves A and B meeting at x = 0. Same amplitudes, but 2 = 1.15 x 1.

The displacement versus time for each is shown below:

A(1t)

C(t) = A(t) + B(t)CONSTRUCTIVEINTERFERENCE

DESTRUCTIVEINTERFERENCE

B(2t)


BeatsBeats

tcostcosA2)tcos(A)tcos(A HL21

L 12 1 2 21H 2

1

Can we predict this pattern mathematically? Of course!

Just add two cosines and remember the identity:

where and

cos(Lt)


Standing Waves:Standing Waves:

Fixed “nodes”

HW: Airport


Standing Waves:Standing Waves:

f1 = fundamental frequency (lowest possible)

L / 2

f2 = first overtone

L

F

v f = v / tells us f if we know v and

physics 101: lecture 27 sound

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