Physics 211 Lecture 23, Slide 1

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Physics 211 Lecture 23
Today's Concepts Harmonic Waves
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(No Transcript)
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"Everything. Especially the differential
equations. Also, how to use these concepts in
problem solving." Are we going to learn fluids?
There are fluid equations on the formula
sheet. go through the main equation and how to
use it. "y(x,t) Acos(kx-wt)" Well, the
semester's almost over, and I still haven't
gotten on the board. Thank you for forcing us to
actually watch the prelectures and pay attention
to the material before coming to class. I always
hated watching them and I still do, but I think
it really helps me learn. The one thing you could
improve is getting the scripts up earlier I
think I learn better from them than from
videos. more stadium wave stuff. it's a good way
to spend 2 or 3 minutes. Will there be a review
session at all during the weekend or extended
office hours since there is no review session
during lectures this week? I'm still hoping that
you could put up an iclicker question asking the
class if they think my ex girlfriend and I should
get back together. I visited her this weekend and
things went very well, I think she's really close
to taking me back and I just think some hard
evidence would help to persuade her. If the
majority of the smartest students at U of I all
agree that we should get back together then
there's no way she can turn me down. If you have
a few minutes before or after class I would
greatly appreciate the help.
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I can't BELIEVE we have homework due on the day
finals begin. Can we get a BREAK??? AND we have
no discussion.... Two guys approach a bar. The
first guy walks into it. The second guy
ducks. Why does velocity not depend on amplitude,
frequency, and wavelength? PLEASE ANSWER THIS
QUESTION!!!!!!! Is it possible to download the
prelectures to save them?? It would be really
convenient to keep them as reference in the
future, like an old textbook. It would be a shame
to lose them once done with this class. I think
you said something about Red Hot Chili Peppers
before class last time but I missed what you said
( This time maybe? D PLEASE?! (I'm running out
of lectures!) How does direction affect the
functional form? I'm really confused on the
pre-flight 5. While I was watching the
prelecture, I would pause in between slides to
gently shake my mug of cider and observe the
waves as they went across the surface. Thanks for
more useful real-world knowledge. P
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How do we prepare for the final exam???? Give us
the direction!!! I really don't like my roommate.
How do I get rid of him without getting sent to
jail? A LOT OF EXAMPLES WITH DETAILS ON PERIOD,
WAVE , SPEED OF WAVE. do we have to know the
wave equation or is that just a proof for the
speed on the wave on a string equation? and!! can
you give us hints on the hw again...it IS the
last one. let's all get 100 on them! D Waves
travel by way of a medium. What then is the
medium that light travels through in space which
is a vaccum? Two atoms bump into each other. One
says 'I think I lost an electron!' The other
asks, 'Are you sure?', to which the first
replies, 'I'm positive.' "Can we please do the
wave in lecture??? I think that would make up for
the fact that there is no lab this week!" "The
only waves I know of are radio waves and ocean
waves... Can you explain all the waves, like the
ones in the prelecture? Thank You!"
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What is a wave ?

• A wave is a traveling disturbance that transports
  energy but not matter.
• Examples
• Sound waves (air moves back forth)
• Stadium waves (people move up down)
• Water waves (water moves up down)
• Light waves (what moves??)

Physics 211 Lecture 23, Slide 6
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Types of Waves

• Transverse The medium oscillates perpendicular
  to the direction the wave is moving. This is what
  we study in Physics 211.

Movie (twave)
Longitudinal The medium oscillates in the same
direction as the wave is moving.
Movie (lwave)
Physics 211 Lecture 23, Slide 7
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(No Transcript)
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Wave Properties
Movie (tspeed)
Physics 211 Lecture 23, Slide 9
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Preflight
Waves A and B shown above are propagating in the
same medium. How do their frequencies
compare? A) wA lt wB B) wA wB C) wA gt wB
Physics 211 Lecture 23, Slide 10
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Waves A and B shown above are propagating in the
same medium. How do their frequencies compare?A)
wA lt wB B) wA wB C) wA gt wB
A) velocityfrequencywavelength. Both have the
same velocity, so omega a is less than omega b
because it has a longer wavelength
B) They are the same because they are moving in
the same medium
C) shorter the wavelength, higher the frequency
Physics 211 Lecture 23, Slide 11
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Act
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?
(A) About 1,000,000 (B) About 0.000001 (C)
About 1000
Physics 211 Lecture 23, Slide 12
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How to make a function move
y

• Suppose we have some function y f(x)

x
0
Physics 211 Lecture 23, Slide 13
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For a harmonic wave

• Consider a wave that is harmonic in x and has a
  wavelength of ?.

Physics 211 Lecture 23, Slide 14
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Act
y f(x - vt)
y
If a function moving to the right with speed v
is described by f(x - vt) then what describes
the same function moving to the left with speed
v ?
v
x
0
y
A) y - f(x - vt)
v
B) y f(x vt)
x
0
C) y f(-x vt)
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y
Suppose the function has its maximum at f(0).
y f(x - vt)
x
0
A) y - f(x - vt)
B) y f(x vt)
C) y f(-x vt)
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Preflight
y
A
x
We have shown that the functional form y(x,t)
Acos(kx-wt) represents a wave moving in the x
direction. Which of the following represents a
wave moving in the x direction? A) y(x,t)
Acos(wt-kx) B) y(x,t) Asin(kx-wt) C) y(x,t)
Acos(kxwt)
Physics 211 Lecture 23, Slide 17
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Energy
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Preflight

• Waves A and B shown above are propagating in the
  same medium with the same amplitude. Which one
  carries the most energy per unit length?
• A
• B) B
• C) They carry the same energy per unit length

Physics 211 Lecture 23, Slide 19
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Which one carries the most energy per unit
length? A) A B) B C) Same
A) Less frequency, more energy.
B) kinetic energy is proportional to w2A2 ...
same A, bigger w in B
C) There is no C, silly.
Physics 211 Lecture 23, Slide 20
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Look at signs in front of x and t
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Physics 211 Lecture 21, Slide 22
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q
x