Force  (Page 63/4)

We all have extensive experience of force in its various forms. Newton’s laws of motion provide significant insight about the nature of force and what it does. The laws of motion emphasize that application of a single force (or resultant force) on a body changes its motion (velocity) either in direction or magnitude or both.

What is force ?

What is force? This basic important question about force remains unanswered even today. We do not know : what is force and how does it come into existence ? Our best guess today is that it arises from some short of change in the characterization of vacuum around us or according to quantum field theory, force is mediated by exchange of particles called "gravitons" (gravitational force), "photons" (electromagnetic force), "gluons" (strong nuclear force) and "bosons" (weak force). One thing common to all quantum models attempting to explain existence of force is that it propagates at the speed of light. For the time being, however, it would be better if we simply ignore this question and proceed ahead with what we know about force. We shall return to this topic once we familiarize ourselves with other aspects of force.

What we know today with definite authority about force is actually its effect on interaction with a particle or a body. Newton’s laws of motion precisely provide this information. The laws tell us about the change in the motion (velocity) of the body on which force is applied. An important description of force is that it is a "push” or “pull” on a body. Importantly, this is the nature of all types of force, irrespective of their class or genesis (gravitational, physical, mechanical, chemical, nuclear, electrical, etc.).

We can measure force in exact terms as ”acceleration produced in unit mass”. Using Newton’s second law,

For m = 1 kg,

F (Newton) = ma = 1 x a = a ( $m/s^2$ )

But, this measurement of force would essentially be in terms of what it does (acceleration) rather than in terms of what it is.

The question “what is force?” is, therefore, unanswered as we actually do not know.

Context of newton’s laws of motion

Validity of Newton’s laws of motion is limited in important ways. Some of the limitations are basic. The relationship between force and acceleration as given by Newton's second law does not hold at great speeds (comparable to the speed of light) and is replaced by more general law as given by special theory of relativity. In this sense, Newton’s laws are a subset of more general laws as applicable to high speed motion. Further, Newton’s classical mechanics (so to speak) breaks down at atomic level and is substituted by quantum mechanics.

Even in the realm of classical mechanics, Newtonian mechanics is valid to motion as measured in certain type of reference system called inertial frame of reference. An inertial frame of reference is the one, in which Newton’s laws of motion are valid. The inertial frame of reference is a non-accelerated frame of reference.