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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!]

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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.

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  • $\begingroup$ Gravity is the force on a unit mass. So force is always required to begin a motion. $\endgroup$ – user36790 Oct 11 '14 at 4:03
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    $\begingroup$ In an inertial system force is always required to begin a motion. $\endgroup$ – CuriousOne Oct 11 '14 at 4:08
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I think that a force will be definitely required to accelerate a body from rest to a velocity, however, once it has achieved that velocity you should be able to cease applying the force and it will continue that at that velocity in a frictionless ideal world. After you cease applying the force it should be undergoing uniform motion, but before that it will not due to the application of the force, as I understand it.

NB: I am a plebeian beginner when it comes to physics, so I may be entirely wrong.

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  • $\begingroup$ Yes, I am also arguing the same thing. To me, there is practically no uniform motion if I take that time when the force is applied under consideration. +1 . $\endgroup$ – user36790 Oct 11 '14 at 3:01
  • $\begingroup$ No, it's not even well defined whether the body is moving or not, since that depends on your frame of reference. $\endgroup$ – user4552 Oct 11 '14 at 4:46
  • $\begingroup$ Sorry, my previous comment wasn't very clear. Everything you say in your answer is correct. However, it doesn't work as an answer to the question, at least as I understand the question. If I'm understanding it correctly, the OP wants to know whether every object in motion must have been acted on by some force in the past. That's completely different from the situation you discuss in your answer, where an object is assumed to have been at rest at some initial time. $\endgroup$ – user4552 Oct 11 '14 at 5:02
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So , isn't force required to start each and every motion?

No, it's not. It's not even well-defined whether an object is in motion. That depends on your frame of reference.

Force, on the other hand, is something that all observers agree on, regardless of their frame of reference.

Suppose there were some rule or formula that said that if an object is going at some speed $v$, there must have been some force $F$ at some point in the past to start it in motion. Suppose observer A says that a certain object is moving at some speed $v$, and we calculate the past force $F$ from it. But observer B, using a different frame of reference, could say that the object's velocity was zero. The formula can't hold true for B if it also holds true for A, since they agree on $F$ but disagree on $v$.

The fundamental idea of Newtonian mechanics is that if we ask why an object is moving, the default explanation is that it was already moving before that.

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  • $\begingroup$ My teacher also said like this: Don't bother about how the motion all started;it all matters when you see it. What happened earlier is not concerned to our study. And the proof is $v-t$ graph of uniform motion having a certain intercept on the $v$-axis... $\endgroup$ – user36790 Oct 11 '14 at 5:29
  • $\begingroup$ ...So when the observer sees at the body , it has certain velocity due to the intercept at $t =0$. Motion,thus,had started already before the observer sees it but he doesn't care about the past;what he sees is important to him... $\endgroup$ – user36790 Oct 11 '14 at 5:33
  • $\begingroup$ But why did he like you ignored the event that initiated motion? Is ignorance to that fact the only answer?? You seem to be right but what about mine and the other two answers??? Still +1 for the lucid explanation. $\endgroup$ – user36790 Oct 11 '14 at 5:38

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