Ch.11 Waves

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Ch.11 Waves

• Homework
• Concept Questions 1, 2.
• Problems 3, 9, 13, 39, 45, 49.
• Wave motion, reflection, refraction,
  interference, diffraction, standing waves.
• (sections 5, 6)

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Waves

• traveling disturbance
• transport energy (not matter)
• only longitudinal waves in fluids
• Transverse Longitudinal

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Intensity
intensity drops as the inverse square of distance
from a point source
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Ex. Intensity at 1m 1W/m2. Intensity at 0.01m
is
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Waves on Strings

• Wave Velocity depends on
• tension in string (F) and,
• the mass per unit length of string.
• Example F36N, m/L0.010kg/m

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Periodic Waves

• continuous, well defined amplitude (A), frequency
  (f), and wavelength (l)
• speed v fl.
• Example f 10 hertz, l 3 m.
• v (10/s)(3m) 30m/s.

A
A
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Principle of Superposition

• when two or more small amplitude waves overlap
  the net disturbance is the sum of the individual
  wave disturbances
• Example noise cancellation headphones
• Sound PHET
• small amplitude waves are unchanged outside the
  area of overlap

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Ex wave1 has A1 0.10m, wave2 has A2 0.30m.
Each has same f. The minimum and maximum wave
amplitudes in an area of overlap are
If they were each 0.3m, the answers would be 0.6m
and 0m.
If there were three amplitudes, 5m, 1m, and 1m,
the answers would be 7m and 3m.
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Reflection
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Refraction Diffraction

• Refraction wave direction changes due to change
  in speed, e.g. light entering water changes
  direction.
• Diffraction wave direction changes due to
  proximity to an object, e.g. waves spread out
  after passing through a narrow opening.

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Standing Waves
waves in which the amplitude at a given location
does not vary with time. Due to interference of a
confined wave.

• Nodes (places with zero amplitude)
• wavelength 2 x (Node to Node distance)
• Anti-nodes (places with maximum amplitude)
• Allowed frequencies are called harmonics
• Clip Vernier Pamp (start at 2 min)
• Waves on String PHET

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Standing Wave Both Ends Fixed
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Summary

• Intensity
• Wave Properties (f, ?, v, amplitude)
• Transverse vs. Longitudinal Waves
• Reflection, Refraction, Interference, and
  Diffraction
• Superposition of Waves
• Standing Waves on a String

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Standing Wave One End Fixed, One End Free
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Ex1. Transverse Standing Waves on 10m of 4g/m
string fixed at both ends.

• Mass?
• Stretched to 15m with force of 5N.
• m/L?
• Wave speed?
• Wavelength, frequency of 1st harmonic?

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Ex2. A String 10m long has tension 10N and
transverse wave speed of 36m/s.

• Mass of string?
• The string is stretched to 15m with 20N.
• What is increase in speed?

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Ex3 Slinky

• Stretch, measure force, estimate distance speed
  of transverse waves.
• Weigh slinky and compute theoretical value of
  speed of transverse waves for the estimated
  distance.
• How do the values compare?

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natural frequency

• lowest frequency an object vibrates with when
  struck
• also called resonant frequency
• Demo Driving Tuning Fork
• /

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resonance

• absorb max energy frequency resonant frequency
• example 256 Hz guitar string resonates when
  exposed to 256 Hz.
• Chilandi plates video

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