Newton’s first law, also known as the principle of inertia, describes the behaviour of a body left to itself, that is, free from any external influence. It is the starting point of all dynamics: before asking how forces change motion, we must establish what happens in the absence of forces.
Principle — Newton's first law
A body on which no forces act (or on which the resultant of the forces is zero) remains at rest or continues to move with uniform rectilinear motion.
The counter-intuitive aspect is that the state of rest and the state of uniform rectilinear motion are, from the point of view of physics, the same thing: both correspond to zero acceleration. No force is needed to maintain motion; a force is needed only to change it, that is, to accelerate, slow down or curve the trajectory. Everyday experience seems to suggest the opposite — an object sliding on the floor eventually stops — but this happens precisely because a force acts on it: friction. Remove the friction, and the motion would continue indefinitely.
This law, however, has an even more profound role: it defines the type of reference frame in which Newtonian mechanics is valid. An inertial reference frame is, by definition, a frame in which the first law holds, that is, a frame in which a body free from forces does not accelerate. Not all reference frames are inertial: an observer on a rotating merry-go-round, or inside a car braking sharply, sees free bodies accelerate even though no real force acts on them. In those frames the first law — and with it the whole of Newton’s mechanics in its simplest form — does not work.
Note
Newton’s three laws, and in particular the second (), hold only in inertial reference frames. The first law is what allows us to recognise them: they are the frames in which an isolated body maintains constant velocity.
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Topics: Dinamica Concepts: Primo principio della dinamica
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