Vibrating Bars and Strings: The Phenomenon of Beats 
Vibrating Bars and Strings: The Phenomenon of Beats
by Prof. Miller
Video Lecture 32 of 46
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Date Added: February 5, 2015

Lecture Description

A - We look again more circumspectly at the two bars mounted on a reson¬ating chambers. They are fixed at two very special points. When sounded each alone they sound ALIKE - IDENTICAL. But they are not! One is 440 and the other 441. Hence we hear ONE beat per second. The "beat frequency "is the difference in the natural frequencies.

B - We explore this matter with two mounted tuning forks. They ARE identical. But now we load one with a rubber band - thus in¬creasing its inertia and lowering its pitch.
We now hear beats.

C - A long steel bar is held at a certain place along its length.. .at a point quite like the point where the A-bar is fixed to its resonating chamber. We strike the bar transversely. It vibrates with two nodes.
We ask: How far along the length of the bar is a node? The answer might well appear to be 1/4 the length - that is 0.25 L. But this is not quite right. It is 0.224 L — and this is a very difficult exercise for students of Physics. Look into this some time.

E - Two tuning forks mounted on their resonating chambers are highly responsive to each other. If they are identical we can strike ONE -damp it - that is - stop it - and low and behold - the other is heard.

F - A set of tuned sticks cut to precise lengths - of hard wood - clear -grained - can give rise to the major diatonic scale if dropped in proper sequence on a hard floor.

G - We now look at vibrating strings: A magnetically-driven steel sliver has a string fixed to it. We hold the remote end of the string. With a certain length and a certain tension in the string we can show the fundamental of the string - the first overtone - the second - and so on — called by some the harmonics. What the string does is governed by its length - its tension - its linear density - that is -how fat it is.

And so we see and hear the beautiful things that sounding devices give us. Of all our human faculties and senses that of hearing is a truly remarkable one and it is a pitiful thing for those who cannot hear.

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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