Tuesday, November 29, 2011

The simplest schoolboy is now familiar with truths for which Archimedes would have sacrificed his life. -- Ernest Renan

Motion, I thought, could be simply defined as a shift in the position of a body over time – if at all there is one. Not quite a difficult concept to grasp. But this definition is too vague. Feynman said it superbly:

“Any vague theory that is not completely absurd can be patched up by more vague talk at every point that brings up inconsistencies – and if we begin to believe in the talk rather than in the evidence, we will be in a sorry state.”

Over 2300 years ago, Aristotle tried to define motion in all its complexity, including as the increase and decrease of that which can be increased or decreased. This puzzle about motion endured until Newton came onto the scene in late 1600s. Newton’s three laws of motion even apply today when we send rockets into space or design machines.

1. A body continues at rest or in motion with a constant velocity in a straight line, unless acted upon by force.

2. The change in velocity of a body is in the direction of the force acting on it, and the magnitude of change is proportional to the force acting on the body times the time during which the force acts; and is inversely proportional to the mass of the body.

3. Whenever a first body exerts a force on a second body, the second body exerts an equal and oppositely directed force on the first body.

These laws are only special forms of the more general laws when we assume that velocities are small compared to that of light and when the scale of phenomena is large compared with the atom. Yet these laws are the building bricks of Physics.

In fact, Newton solved the problem of motion as he defined it not as Aristotle considered it to be. So Newton’s definition assumes certain things and it is only in the ambit of this assumption that his laws hold. But these laws more or less embrace the usual notion of motion that we encounter.

Similarly, our speech is adapted to our daily needs or perhaps to the needs of our ancestors. We cannot have a separate word for every distinct object and for every distinct event. If we did, we would ever be coining words. In order to have language at all, many things or many events have to be referred to a single word. It is natural to say both men and horses run and even more convenient to say a motor runs.

The unity among these concepts lies far more in our human language than in any physical similarity with which we expect science to deal easily and exactly. So, it would be foolish to seek some elegant, simple and useful scientific theory of running which would embrace runs of motors or men. It may or may not be possible, but to blame Science for not having one is tomfoolery.

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