Inertia ?

If someone Knows What it IS ...

I Really would like to Know !

The "nature of the beast", is well described in the college tella-course ...

The Mechanical Universe,

But, information regarding   WHAT INERTIA IS,   has not come across my desk.

(Dictionary definition — INERTIA: The tendency of matter to remain at rest if at rest,
or, if moving, to keep moving in the same direction, unless affected by some outside force.)

Problem — NOTHING IS AT REST — Energy must keep moving inorder to exist as Matter.


Science and Math Weekly

vol. 7 issue 5 & 6 — Oct. 12, 1966 & Oct. 19, 1966
© 1966, American Education Publications, Inc.


Problems of space stimulate interest in areas of basic physics we tend to take for granted ...

AN ASTRONAUT IN SPACE ASSUMES and feels he is not in a gravitational field. At least, as far as his senses go, he does not experience any pull in a specific direction. He is "weightless."

But, as physics students all know, weight is not mass, and mass is perhaps only a measure of inertia. This last concept – inertia – has fewer sentences allotted to it in a physics textbook than any other really major concept.

Yet, inertia plays a dominating role on Earth, and its importance becomes even more obvious in space. We may even, someday, use inertial principles to drive tractors over muddy fields ...

No matter where you travel in the universe you cannot escape gravitational fields. This very thing we call mass may merely be a by-product of this trap.

YOU HAVE AN OBSERVED FACT: bodies seem to resist forces which tend to change the bodies' motion. A billiard ball keeps on rolling until it hits an object, or until the tiny loss of energy via friction to the tabletop finally brings it to rest.

It is harder to get a railroad boxcar moving along a level track than it is to get a toy train moving at the same velocity. A tornado blows a straw through an oak post. A supersonic-speed jet of water cuts merrily through a thick steel rod. What do you make of that?

As man gets into space, even the rather restricted space of the recent astronauts, the problems of inertia will become more and more evident. Here on Earth we are trained from infancy to take inertia for granted. We blithely catalog objects as "light" or "heavy" and let it go at that.

Frontier of the Unknown

Inertia is one of those fascinating concepts that the books cover in a paragraph and then accept. Any further discussion is usually reserved for graduate courses in college. But with the emergence of man into space, the problems of mass and inertia show vivid signs of moving into the world of the man in the street.

No one who has watched the astronauts tumble in space-walk can deny that out in the void the problems of inertia will take on a different aspect. Here on Earth frictional forces act as a dashpot. Friction tends to drain off the energy of a moving body and eliminate the complete exhibition of Newton's First Law.

The immense gravitational field of the Earth warps our activity, The steady downward-force vector of the gravitational field obscures many patterns that will become obvious in space. Yet space itself, as far as inertia goes, will prove to be a terrific deception. One can never escape the gravitational field.

Like the textbooks, we have not defined inertia except in sterile, academic terms. This is what happens when scientists do not know what they are dealing with. We know bodies resist change in motion. We know that some bodies resist it more than others. We find that this resistance can be equated with what we call weight.

No Definition at All

We know this weight is simply a measure of the Earth's gravitational force upon something the object possesses. We might venture a guess that it is the number of atoms in the object, But then we remember we can look on the atom as merely an energy force field-an area composed of radiation, waves, pure energy. Mass suddenly goes down the drain. Maybe mass, like phlogiston (the old imaginary element that caused flame when released from chemicals), is an imaginary step on the troubled path to reality.

Some people say this is so – that what really matters is a quality we call inertia. This is the reality. Mass is just a way to measure inertia indirectly. Much of this is playing with words. We can also play with mathematical symbols to the same end, But isn't much of science a curious fumbling with words and equations?

In space where friction will be absent or at a minimum, physical behavior will be different. Newton's Laws will be strikingly obvious. If a man in space-walk pushes against a 100 - pound meteorite, he will find an opposing force. The meteorite will not move as gently as a soap bubble.

On the other hand, if the man gives a fingertip of push against the side of the space capsule he will start moving away. The capsule will move in the opposite direction by an amount proportional to the differences between the two inertials involved. it will take more force to get the capsule moving at a certain velocity than will be required to get the man moving. Without earthly friction to damp movements, the various vectors of velocity and acceleration will be painfully obvious. That is why astronauts tumble.

A spaceship is not required to obtain all this, however. Suppose the Earth were a hollow sphere with a shell 100 miles thick. You have tunneled through the shell and gently dropped into the interior. What will happen? Will you fall to the center of the earth?

A Hollow Argument

Mathematically, by pure convention and convenience, we treat such an Earth as if all the mass were concentrated in a point at the center. But this will hold only for measurements made outside the sphere. Inside this hollow sphere the gravitational accelerations on the shell will cancel out for all and any locations inside.

A man ten feet below the end of the tunnel will be as we say "weightless," and a man at the center of the hollow will experience the same situation. If you doubt this, try a paper-and-pencil test. Divide the sphere into areas and compute the force vectors from each. THEY WILL CANCEL OUT AT ANY LOCATION INSIDE THE HOLLOW. A man at rest in any spot in the hollow will stay put.

But this does not mean that gravitation has been so set up that the forces on the floating man all vanish. Now, if I push this man, I am upsetting a balanced force pattern. I am opposing a portion of the gravitational forces of the shell. I will experience a resistance to my push. I say the man has inertia.

Every object that is at rest (as we say) in a frame of reference is at rest because a situation exists where all the forces on that object cancel out. If you think of an object suspended in a ring by a large number of extended springs, the object will come to rest despite the fact that the springs are all under some tension. If you try to move the object, it is easy to see you are really opposing the springs. In this analogy, life is obvious. But when we consider invisible gravitational forces, somehow, things seem more complex.

From Model to Cosmos

So let us expand our model from a hollow Earth to space itself. The whole universe, suns, planets, stars, the most distant galaxies, all exert some trifling of gravitational pull. An object at rest (or in uniform motion) must experience a balance of all these forces. This, some say, is the explanation for inertia.

Does this prove anything? Of course not. This is merely a hypothesis that attempts to explain the facts we observed at the start of the article. Remember, Newton had no conception of either the size or the mass of the universe. His generation was not even certain that the gravitational force he extended to the moon also reached out to the stars. Newton did not have the facts to attempt today's argument.

  • Matter in space may seem "weightless," but it still has inertia and momentum. Meteorites show crystalline evidence of severe shock waves caused by collisions with other meteorites. The whole asteroidal belt is slowly being thus reduced to a powder.
  • A .22 bullet fired at a empty eggshell, suspended by a silk fiber, will not break the shell. The inertia of the eggshell is so slight, that the pressure wave from the bullet will move the shell aside before the slug touches it.   Would this be true of a slug traveling at 1 km/sec.?
  • A man in space pushing on a heavy meteorite will find considerable resistance. Both will move in proportion to their mass (inertia).
  • If the Earth were a hollow sphere, gravity would be canceled for all points in the interior. A man would experience the same sensations here as he does in a normal space-walk. What will happen if you extend this idea to the universe?

To Temp You:

  1. If the hollow Earth in our example were discus-shaped, would gravitation still be balanced out all over the hollow interior?
  2. Does explaining inertia solve the problem of what is gravitation?
  3. Explain why the eggshell is not broken by the bullet.


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