PHY138

1
PHY138 Waves, Lecture 4Todays overview

• Sound as a pressure wave
• The Doppler Effect
• The Principle of Superposition
• Constructive and Destructive Wave Interference

2
Reading Assignment

• Please read the following from Serway and Jewett
  before class on Monday Chapter 14, up to and
  including Section 14.6
• A www.masteringphysics.com assignment is due this
  Friday at 500 PM. It is based on Chapter 13
  material mechanical waves and sound.

3
Last days lecture some comments

• Regarding quiz on swings by standing up on a
  swing, you raise your centre of mass closer to
  the pivot point, thereby reducing L.
• This increases the natural angular frequency, ?.
  
• This reduces the period, T.
• Medical ultrasound imagers use sound with
  frequencies in the range f 1-20 MHz.

4
Sound Waves

• Sound is produced by an oscillating surface in a
  medium (ie a speaker in air)
• Sound is a pressure wave

• Sound is also a longitudinal displacement wave of
  individual molecules in the medium

5
Doppler Effect for sound

• Creates a change in pitch or frequency
• All speeds are measured relative to the medium
  (ie air)
• f goes up if source and observer are approaching
  each other
• f goes down if source and observer are moving
  away from each other.

6
Chapter 14 Principle of Superposition

• If two or more waves combine at a given point,
  the resulting disturbance is the sum of the
  disturbances of the individual waves.
• Two traveling waves can pass through each other
  without being destroyed or even altered.

7
Quiz

• Two traveling sinusoidal waves passing the same
  point, each of wavelength ?, result in total
  destructive interference if the waves
• 1. are in phase.
• 2. are p/2 out of phase.
• 3. are p out of phase.
• 4. are traveling in opposite directions.
• 5. None of the above

8
Wave Interference

• Two waves moving in the same direction with the
  same amplitude and same frequency form a new wave
  with amplitude 2Acos(ø/2), where ø is their phase
  difference.