PHY138

1
PHY138 Waves, Lecture 5Todays overview

• Standing Waves
• Harmonics
• Beats

2
Reading Assignment

• Please read the following from Serway and Jewett
  before class on Wednesday Chapter 24, Sections
  24.3, 24.7 and 24.9, and Context 7, pg. 896-897.
  Only the concepts of 24.3 are important, not the
  equations.
• A Web-CT quiz is due Wednesday morning, which
  tests your basic familiarity with Chapter 24 and
  25 assigned reading.

3
Written Homework

• The assignment due on Monday, November 29 at 500
  PM is now available online. It includes problems
  from Chapter 14 and one from Chapter 24.

4
Some Announcements

• Calculator hint Exp key stands for times 10
  to the power. ie, to type 10-3, type 1 Exp (-)
  3, not 10 Exp (-) 3. (the latter will be
  interpreted as 1010-310-2 !)
• Friday Tutorials Representative Assembly this
  Friday, noon-100PM, MP713.
• New medical technique to help stroke victims
  ultrasound. Sound waves can dissolve brain
  blockages. (J.P. Polak, New England Journal of
  Medicine v.351 n.21, pg. 2154, Nov. 18 2004)

5
Doppler Effect sign conventions

• Serway and Jewett say A positive value is used
  for motion of the observer or the source toward
  the other.

• Other texts say A positive value is used for
  motion to the right, where the source is located
  to the right of the observer.

6
Some results of superposition

• 14.2 Two waves, same frequency, same direction,
  diffferent phase
• Interference
• 14.3-5 Two waves, same frequency, opposite
  direction
• Standing Wave
• 14.6 Two waves, same direction, slightly
  different frequency
• Beats

7
Harmonic frequencies

• Transverse standing wave on a string clamped at
  both ends there are nodes in displacement at
  both ends.

Standing sound wave in a tube open at both ends
there are nodes in pressure both ends.
8
Quiz 1
a
b
c

• A string is clamped at both ends and plucked so
  creates a standing wave. Define upward motion to
  be positive velocities. When the string is in
  position b, the instantaneous velocity of points
  along the string
• 1. is zero everywhere
• 2. is positive everywhere
• 3. is negative everywhere
• 4. depends on location

9
Quiz 2
a
b
c

• A string is clamped at both ends and plucked so
  creates a standing wave. Define upward motion to
  be positive velocities. When the string is in
  position c, the instantaneous velocity of points
  along the string
• 1. is zero everywhere
• 2. is positive everywhere
• 3. is negative everywhere
• 4. depends on location

10
Harmonic frequencies
Standing sound wave in a tube closed at one end
there is a node in pressure at the open end, and
an anti-node at the closed end.
11
Beat frequency

• The beat frequency is the difference of the
  frequencies of the two waves that are being added