Newton`s First Law Of Motion

The 1^st law: In the absence of a resultant force, an object will remain at rest or maintain its constant velocity.

[Alternative phrasings abound, for example: If the forces acting on an object all balance each other out the object will have no acceleration.]

(Briefly discuss and dispel the layperson s notion that motion requires a force e.g. riding a bicycle, if you stop pedalling you will come to a standstill. In empty space where no form of friction is possible, once set in motion an object will maintain it forever.)

Discussion:

At first glance there seems to be something disconcerting about this law, because it seems to suggest that if A, then either B or C , which is the sort of thing that a mathematician would scoff at, a one-to-many relationship that would not be accorded the status of a function. It seems to suggest that rest is indistinguishable from uniform motion (constant speed in a straight line).

It was Galileo who first raised this question, and who answered it in what became known as Galileo s dictum , and later called the Principle of Relativity by Pascal. He stated that a person enclosed within a chamber that is moving uniformly, who has no view or reference to the outside world, would not be able to perform any physical experiment whatsoever to determine the speed of the chamber he is in or indeed whether it was moving at all ( see quote from Penrose The Emperor s New Mind p. ...).

This principle remained in contention for centuries. Towards the end of the 19th century, it appeared to suffer a fatal blow when Thomas Young demonstrated that light was a wave phenomenon. Since light was known to travel through vacuum, and since a wave required a medium in which to propagate, it seemed to follow that there existed a very fine, all-pervading ether-like medium that had eluded humanity for too long. If this ether (as it became known like the Higgs boson, it was christened well before it was shown to exist) could be detected, then the velocity of a moving body could be determined relative to it. However just a few years later the famous Michelson Morley experiment demonstrated that the ether did not exist (unlike the Higgs boson), and the principle of relativity was resuscitated. Einstein went on to use it then, along with the observed constancy of the speed of light for all moving sources and observers, as the foundation of his theory of relativity.

It is remarkable that Galileo, who had access to but the most basic technology (Things like clocks, electricity, nuclear and particle technology and so much more, all belonged to the future), nonetheless had the physical insight to see this physical truth, and his dictum has so far withstood the test of time.

To help you visualise this principle and make real its significance, imagine yourself seated alone inside a spacecraft, deep in space, far from any star or galaxy. Looking out of the window you see nothing but darkness studded with a myriad stars. Then one day (there are no days, just one long night) you spot another craft similar to yours, passing in front of your window from left to right, and in it seated behind a window is a homo sapiens just like you... You desperately wave to your fellow being, who waves back, but the other craft soon sails off to the right and out of view, and the long good night resumes...

You lean back in your seat feeling wretched, and having all the time in the universe at your disposal, you start wondering and ask yourself this: what actually happened here? Was I stationary and he moving to the right, or was he stationary and I moving to the left, so that it appeared from my perspective that he was moving to the right? Or, were we both moving in opposite directions? Would he not appear to me to be moving to the right if we were both moving to the left, but I with greater speed? Or if we were both moving to the right, but he with greater speed? Which of all these possible scenarios has actually taken place?

You go to sleep deeply puzzled, but in the morning you suddenly recall Galileo and his Principle of Relativity. You realise that your question is a meaningless one, because all uniform motion is a relative affair. All you can say is that relative to you, he was moving to the right, or that relative to him, you were moving to the left, or that relative to some other reference system you were both moving in opposite directions, etc.

Coming back to Newton s 1^st law, here is a simpler way to regard it. It states that in the absence of a resultant force, a body will move with constant velocity. The magnitude of that velocity is immaterial it can be anything, big or small, as long as it remains constant, and v=0 is just one value of the velocity.

The essential point is that a body moving with constant velocity is in equilibrium. Equilibrium does not generally mean that the velocity is zero, but rather that the acceleration is zero. The 1^st law can be succinctly stated thus: when a body is in equilibrium (no net force), its acceleration is zero (constant velocity).

The 2^nd law, expressed in its simplest form F = ma, actually contains the 1^st law as a special case. The 2^nd law states that for a given mass, the acceleration is proportional to the net force. Again, the force may have any value, and the acceleration will vary in proportion. Thus if F = 0, then a = 0, and so with no resultant force there will be constant velocity, which is the first law. The 1^st law turns out not to be a law in its own right, but a special case of the 2^nd law.