A body which is at rest will remain at rest; and a body which is in motion will continue motion in a straight line as long as no unbalanced force acts on it.
This is Newton's 1st law of motion,everyone knows. But what Newton did not tell is that how the motion of the body starts. In the real world,in order to move a body,force must be given so as to overcome the frictional force. Suppose, between a body and surface,the limiting friction is $f_s$; so we have to apply a force $F$ such that $$F > f_s$$ . The extra force $ F – f_s$ does start the motion. In a frictionless world, how does a body start motion??? Obviously,to move it from rest, again some force must be given,however small it might be,right? If it is so, then there is practically no uniform motion because initially the body starts motion under the application of force,however small time it takes. Newton only told what happened after the motion starts but did not mention how the motion began. So , isn't force required to start each and every motion? And hence is there any uniform motion practically? Plz help.
[ I don't support any Aristotleian philosophy!]
Best Answer
The definition of force in Newton's second law states that in an inertial system a force F is required to give an acceleration a to a mass m: F=m*a. So how did the motion of all these stars and galaxies in the universe begin from an almost motionless cloud of gas? The answer is gravity: gas clouds are unstable under their own "weight" and they collapse. The collapse compresses the matter in some areas and leaves other ares with a very good vacuum. Because of angular momentum conservation this results in clusters of galaxies made up of hundreds of billions of stars that move at hundreds of km/s relative to each other. After the initial collapse the remaining friction of the thin gas between the stars is basically negligible for stellar and planetary motion (exceptions apply).
On the surface of Earth, however, we are seeing physics as it unfolds with friction, be that on surfaces, the water or in the air.