Waves: Kinds of Properties 
Waves: Kinds of Properties
by Prof. Miller
Video Lecture 30 of 46
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Date Added: February 5, 2015

Lecture Description

A wave is a strange thing. Try to answer this question: What is a wave? It is not so easy to say. The best we can do for the moment is to say this: A wave is a disturbance which transmits energy. A better attack is this: let us show some waves and seeing what a wave does will tell us better what a wave is. This we call in Physics the operational point of view.

A - We have two rubber tubes - one called "empty" - the other filled with sand. I say "called empty" because it is really NOT empty. It has air in it! One end of the tube is fixed I hold the other end. Now by a slight blow with the hand I depress a bit of the tube and we see a pulse travel along the tube to the remote fixed end. This wave is a transverse wave - where the particle disturbance is at right angles to the direction of the energy propagation. If enough energy gets to the fixed and we might have a reflection. And we see the wave travel at a certain velocity. It I pull up on the tube and increase the tension the waves travels faster. With the sand-filled tube we see the wave velocity slower. It can be shown either experimentally or mathematically the velocity of a transverse wave is given by the expression V= T/m where T is the tension and m the linear density

B- We now show a different kind of wave with a spring called a SLINKY. Here we give rise to a pulse traveling along the spring in the same direction as the energy propagation. This kind of wave we call a compressional or longitudinal wave. Sound - we shall see - is a compressional wave mechanism.

C - With a piece of blackboard chalk we show a torsional wave by twisting the cylinder of chalk. Torsional waves are important - as in of a steamship shaft which delivers energy from the driving mechanism to the propeller - or in the drive-shaft of your car - where a torque is needed.

D - A dramatic showing of the propagation of a compressional wave arises in the collision of steel spheres on a track. One sphere strikes an array at rest. The compression travels with the speed of sound in steel - about 15,000 feet per second. Pretty fast !

E - Now transverse waves possess a special character: They can be polarized -just like light can be polarized. Compressional waves cannot be polarized.

F - Water waves are especially "tricky" business. They are made up of wave motions of several types but we understand these pretty well. The expression for the velocity of a water waves comes out
w _ ug x lambda 7 2 pi T
2 pi lambda x d
Which LOOKS awfully rough but really is not!

G - Conduction of sound energy requires a medium. And the more elastic the medium the greater the conduction. Thus it is that a metal rod conducts a compressional wave better than a rubber tube. And this is why a stethoscope works as it does. Look at one closely.

H - If sound is a compressional wave it needs something to compress if it is to be transmitted. Accordingly a bell in a bottle can be heard if there is stuff in the bottle - air - say — but if the air is pumped out no sound can be heard. So on the Moon - with no atmosphere - how will we hear each other?

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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