Transcript Physics 101: Lecture 21 Waves

Exam 3
Physics 101: Lecture 21
Waves
 Today’s
lecture will cover Textbook Chapter 11
Physics 101: Lecture 21, Pg 1
Preflight 5
Which concepts did you find the most difficult when
preparing for this lecture?
Superposition and interference
Waves and wave motion in general are difficult for me to grasp…
Overtones…
Physics 101: Lecture 21, Pg 2
Waves Overview

Types

Speed

Harmonic

Superposition

Standing
Physics 101: Lecture 21, Pg 3
05
Types of Waves
“Understanding all the different kinds of waves.”

Transverse: The medium oscillates
perpendicular to the direction the wave is
moving.
Water (more or less)
Slinky demo
“I have a hard time visualizing what a period is.”

Longitudinal: The medium oscillates in the
same direction as the wave is moving
Sound
Slinky demo
Physics 101: Lecture 21, Pg 4
8
Slinky Preflight 3
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?
totally guessing here, but I LOVE
1. Yes
SLINKYS!!
2. No
correct
5 m/s isn't the distance one coil moves, its how
fast the wave moves
34%
5m
66%
0%
20%
40%
60%
80%
Physics 101: Lecture 21, Pg 5
12
Velocity of Waves Act
v 
Tension

m/L
Tension

A spring and slinky are attached and stretched. Compare the
speed of the wave pulse in the slinky with the speed of the
wave pulse in the spring.
A) vslinky > vspring
B) vslinky = vspring
C) vslinky < vspring
Slinky stretches more, so it has a smaller mass/length .
Physics 101: Lecture 21, Pg 6
17
Harmonic“theWaves
weird/new symbols get me
y(x,t) = A cos(wt –kx)
confused i never know what they
represent “
Wavelength: The distance  between identical points on the wave.
Amplitude: The maximum displacement A of a point on the wave.
Angular Frequency w: w = 2 p f
Wave Number k: k = 2 p / 
Recall: f = v / 
y
Wavelength

Amplitude A
A
x
Physics 101: Lecture 21, Pg 7
20
Period and Velocity


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.
v 

T
Physics 101: Lecture 21, Pg 8
22
Harmonic Waves Exercise
y(x,t) = A cos(wt –kx)
Label axis and tic marks if the graph
shows a snapshot of the wave
y(x,t) = 2 cos(4t –2x) at x=0. T = 2 p/w
Recall: T = 2 p /w
2p/4
1.58
+2
p/4
-2
p/2
3p/4
t
Physics 101: Lecture 21, Pg 9
25
Preflight 1+2
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
correct
2. The wavelength remains the same
3. The wavelength decreases
=vT
62%
16%
22%
0%
20%
40%
60%
80%
Physics 101: Lecture 21, Pg 10 26
ACT

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
Physics 101: Lecture 21, Pg 11 29
ACT Solution

Recall that v = f.
f 
v

H
8

3  10 m s
.03m
10
 10
Hz  10GHz
H
Makes water molecules wiggle
O
1 GHz = 109 cycles/sec
The speed of light is c = 3x108 m/s
Physics 101: Lecture 21, Pg 12 30
Visible
Absorption coefficient
of water as a function
of frequency.
f = 10 GHz
“water
hole”
Physics 101: Lecture 21, Pg 13 31
Interference and Superposition
 When
two waves overlap, the
amplitudes add.
Constructive: increases
amplitude
Destructive: decreases
amplitude
Physics 101: Lecture 21, Pg 14 34
Reflection Act
A
slinky is connected to a wall at one
end. A pulse travels to the right, hits
the wall and is reflected back to the
left. The reflected wave is
A) Inverted
B) Upright
Fixed boundary reflected wave inverted
Free boundary reflected wave upright
I don't understand the picture with free boundary versus
fixed boundary
Physics 101: Lecture 21, Pg 15 37
Standing Waves Fixed Endpoints
 Fundamental
n=1 (2 nodes)
 n = 2L/n
 fn
= n v / (2L)
I did not really understand what the point or meaning of the
fundamental frequency was.
Physics 101: Lecture 21, Pg 16
44
Standing Waves:
L  / 2
f1 = fundamental frequency
(lowest possible)
A guitar’s E-string has a length of 65 cm and is stretched to a tension of 82N. If it
vibrates with a fundamental frequency of 329.63 Hz, what is the mass of the string?
v
T

f = v / tells us v if we know f (frequency) and (wavelength)
v=f
= 2 (0.65 m) (329.63 s-1)
= 428.5 m/s
v2 = T / 
 = T / v2
m= T L / v2
= 82 (0.65) / (428.5)2
= 2.9 x 10-4 kg
Physics 101: Lecture 21, Pg 17 48
Summary
 Wave
Types
Transverse (eg pulse on string, water)
 Longitudinal (sound, slinky)
 Harmonic
y(x,t) = A cos(wt –kx) or A sin(wt – kx)
 Superposition
Just add amplitudes
 Reflection
(fixed point inverts wave)
 Standing Waves (fixed ends)
n = 2L/n
fn = n v / 2L
Physics 101: Lecture 21, Pg 18 50
Practice Problems
Chapter 11, probs 1, 9, 13, 15, 17, 21, 25,
29, 31, 33, 49, 57, 61, 65 (hard!), 77
(hard!)
Physics 101: Lecture 21, Pg 19