Resonance: Forced Vibrations 
Resonance: Forced Vibrations
by Prof. Miller
Video Lecture 33 of 46
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Date Added: February 5, 2015

Lecture Description

If a sound of a certain frequency moves through a medium - say the air -and the pulses fall upon a member FREE to vibrate - and that member has the natural frequency of the wave coming upon it - that member will "pick up" the pulses and vibrate on its own. This is resonance.

A - Air columns have certain lengths. They therefore have certain special resonant responses. If now a sounding fork is addressed to such an air column the pipe will resonate. Clearly - closed pipes will have one responsive length - open pipes another - but these lengths can be readily calculated. We show such resonance response with card¬board tubes - one sliding within another to provide various lengths. Thus it is that organs have pipes of many lengths and different di¬ameters - and some are closed - some are open.

B - We have a monochord - ONE lone string mounted on a resonating box. We fix the string at a certain length - under a certain tension. The string is a wire so fat - so thick We now excite a tuning fork and present the stem of the fork to the bridge where the string passes over. On the string is a bit of paper so we can SEE what happens. The string is put into vibration by the fork and the amplitude is enough to bounce off the paper rider. A pretty thing to see. Now the important aspect of all this is this: That the string responds to ONE fork only.. .although it MAY respond to another of multiple frequency.

C - We excite a tuning fork. We put its stem on top of the table. The table vibrates. But what we hear sounds not like the fork. Why? Consider A played on a piano and A played on a fiddle. Do they not sound unlike? They are of the same frequency but they have different QUALITY.

D - The human vocal cavity called the mouth is a most remarkable thing. If explored for resonance with forks of different frequencies we find that - in general - female voice boxes have higher frequency response than male.

E - This business of RESONANCE has interesting historical aspects. In New England there are wooden bridges still standing from the pioneer days which have signs reading: "No trotting of horses on this bridge". And soldiers crossing a bridge break their step.
And in the Old Testament we read - Joshua 6:20 - "The priests blew their trumpets and walls fell down flat". Which is very good Physics! And so I ask: Why did the ancients not fell walls of the enemy cities by having their trumpeters blow on their trumpets?

F - Cables carrying electric current cross-country often are put into oscillation by stout winds. The cables are demonstrating resonant response to wind pulses. To minimize this action the cables are loaded at strategic points with "dampers" - which you can see if you look carefully at the high-voltage transmission lines.

And finally - the human ear responds just like this. The ear-drum takes up the oscillations which fall upon it. It goes into vibration - into oscillation. And imagine the acute response to every slight change in pitch. This energy is transmitted to the bopes of the inner ear and thence to the brain — so in the last analysis we hear with our brains.

Course Index

  1. The Idea of the Center of Gravity
  2. Newton's First Law of Motion: Inertia
  3. Newton's Second Law of Motion: The Elevator Problem
  4. Newton's Third Law of Motion: Momentum
  5. Energy and Momentum
  6. Concerning Falling Bodies & Projectiles
  7. The Simple Pendulum and Other Oscillating Things
  8. Adventures with Bernoulli: Bernoulli's Principle
  9. Soap Bubbles and Soap Films
  10. Atmospheric Pressure
  11. Centrifugal Force and Other Strange Matters
  12. The Strange Behavior of Rolling Things
  13. Archimedes' Principle
  14. Pascal's Principle: The Properties of Liquids
  15. Levers, Inclines Planes, Geared-wheels and Other Machines
  16. The Ideas of Heat and Temperature
  17. Thermometric Properties and Processes
  18. How to Produce Heat Energy
  19. Thermal Expansion of Stuff: Solids
  20. Thermal Expansion of Stuff: Gases & Liquids
  21. The Strange Thermal Behavior of Ice and Water
  22. Heat Energy Transfer by Conduction
  23. Heat Energy Transfer by Convection
  24. Heat Energy Transfer by Radiation
  25. Evaporation, Boiling, Freezing: A Dramatic Adventure
  26. Miscellaneous Adventures in Heat
  27. The Drama in Real Cold Stuff: Liquid Nitrogen
  28. The Physics of Toys: Mechanical
  29. The Physics of Toys: Acoustic and Thermal
  30. Waves: Kinds of Properties
  31. Sound Waves: Sources of Sound & Pitch and Frequency
  32. Vibrating Bars and Strings: The Phenomenon of Beats
  33. Resonance: Forced Vibrations
  34. Sounding Pipes
  35. Vibrating Rods and Plates
  36. Miscellaneous Adventures in Sound
  37. Electrostatic Phenomena: Foundations of Electricity
  38. Electrostatic Toys, Part 1
  39. Electrostatic Toys, Part 2
  40. Adventures with Electric Charges
  41. Adventures in Magnetism
  42. Ways to "Produce" Electricity
  43. Properties and Effects of Electric Currents
  44. Adventures in Electromagnetism
  45. Further Adventures in Electromagnetism
  46. Miscellaneous and Wondrous Things in E&M

Course Description

Demonstrations in Physics was an educational science series produced in Australia by ABC Television in 1969. The series was hosted by American scientist Julius Sumner Miller, who demonstrated experiments involving various disciplines in the world of physics. The series was also released in the United States under the title Science Demonstrations.

This program was a series of 45 shows (approximately 15 minutes each) on various topics in physics, organized into 3 units: Mechanics; Heat and Temperature / Toys; and Waves and Sound / Electricity and Magnetism.


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