The Idea of the Center of Gravity
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### 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 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.