The Idea of the Center of Gravity 
The Idea of the Center of Gravity
by Prof. Miller
Video Lecture 1 of 46
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Date Added: February 5, 2015

Lecture Description

The IDEA OF CENTER OF GRAVITY plays an enormous role in the affairs of men. In this program we show a number of enchanting and dramatic DEMONSTRATIONS bearing on the IDEA.

A - In the very first demonstration we show a double-track which serves as an inclined-plane. Things that can roll - when put at the top - should roll downhill. A uniform cylinder - like a rolling-pin - does roll DOWN. A double-cone, however, rolls UP! Or so it seems. This is not true - of course - although it looks so. What does happen is this: The center of gravity of the cylinder seeks a lower potential plane; the center of gravity of the cone seeks a lower potential plane. The energy of a system tends toward least. So they both in truth DO RO LL DOWN.

B - Although definitions are dangerous - since we can not always tell all the truth in one sentence - we can say - for our purpose here - that by CENTER OF GRAVITY we mean that point in a body or system where we may IMAGINE all the weight of the body to be. We use here the terms CENTER OF GRAVITY and CENTER OF MASS inter¬changeably although they are strictly different things.

C - We now show experimentally - and this is where all proof lies - using a meter stick - that the CENTER OF MASS of the stick is indeed that point where ALL THE STICK IS. And having said this there is NO stick any¬where else! A remarkable thing to discover.

D - We show also the CENTER OF GRAVITY or CENTER OF MASS of some simple geometric shapes using a plumb line. We learn in this adventure that a strange thing can occur: The e.g. or cm. of a body (or a system) can lie - can be - where there is no "stuff" - no material! Accordingly -we can say that the CENTER OF GRAVITY of a doughnut is in the middle of the hole. This too is remarkable to discover.

E - Using the IDEA OF CENTER OF GRAVITY we show some "systems" which look very unstable but which are indeed very stable because we make use of this~GREAT IDEA:
1 - THE CASE OF THE FORKS IN THE CORK STOPPER; THE CASE OF THE LITTLE MAN ON THE STAND; THE CASE OF THE MONKEY ON THE STAND:
All these configurations are stable because the CENTER OF GRAVITY of the system lies below the point of support.
2 - THE CASE OF THE LEANING TOWER OF PISA:
This configuration is stable if a plumb line from its CENTER OF GRAVITY falls within the base.
This great and wonderful idea of CENTER OF GRAVITY was first set forth by the genius ARCHIMEDES in the 3rd century B.C. in a classic treatise called De Aequiponderantibus. Students and teachers at all levels should read this in translation.

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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