Newton stated his first law of motion as follows:
Every body continues in its state of rest. or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it.
The first law of motion means that if a body is protected from the influence ( forces exerted ) of other bodies, such that no force acts on it, or if balanced forces act on it such that the net force on the body is zero, then there is no change in the state of motion of the body. That is, the body will remain at rest if it is at rest or will move with the same speed with which it moves now if no unbalanced force acts on it.
But, don't we see the stars describing out large circles everyday - they are surely accelerating since they are always changing the direction of their velocity. They are definitely bodies sufficiently far from the influence of other bodies so that no major forces are acting on them. So, what makes them go round in such large circles which surely requires a large amount of force?
To this you may say - "Well, Don't you see that it's actually the earth that is rotating - the stars just "seem" to be revolving in such large circles. They aren't actually doing it." O.K. I agree. But then how do we apply Newton's Laws if things appear to be doing what is not permitted according to Newton's Laws?
The answer to this question is that we need to make observations from a system which itself is not accelerating - no net forces are acting on it. The earth in the above example is rotating, revolving, and thus accelerating, so it is not a permissible place to make observations from.
So, the First Law of Motion, leads to the idea of a special kind of reference frames in which the laws of motion hold true. Thus, in a way, it defines the way in which non-accelerating frames of reference are significant. Therefore, the first law can also be interpreted as a definition of inertial frames of reference, i.e. the frames in which without external unbalanced forces acting on them, bodies continue in their state of motion.
In the special theory of relativity too, inertial frames of reference are significant.
The examples describing the first law of motion can be easily observed in everyday experiences. When objects are set into motion with the application of force, and then the force is removed, they don't stop immediately. This shows that a force is not required to keep the bodies moving. But, we observe that the objects do change their state of motion, their speed decreases and eventually, they stop moving. This is actually because of frictional forces acting between the object and the surface on which it moves. If there were no other such forces, the objects would have never stopped.
A question is - why does the first law hold? Why is it that if observed from inertial frames of reference, bodies continue their state of motion if no net forces act upon them?
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