Newton's laws are three principles proposed by the English mathematician and physicist Isaac Newton ( 1643-1727 ). These postulates provide an answer to most of the problems posed in classical mechanics regarding the movement of bodies .
Newton's first law is known as the principle of inertia and states that bodies do not change their state of motion or rest if a force is not applied to them, or if the resultant of the applied forces is zero .
In other words, according to the principle of inertia, a body remains in uniform rectilinear motion or at rest unless it is influenced by a force. Equilibrium occurs when the resultant of the forces acting on it is zero.
From the principle of inertia it follows that bodies oppose any modification of their state of motion or rest . Inertia, in fact, is such opposition. It should also be remembered that a force is the agent that can generate a variation in the state of the object.
In short, external forces are needed for a body to abandon its uniform rectilinear motion or rest. Therefore, without an impulse, friction , rubbing , etc., the element in question will always maintain its constant speed, even when it is zero.
The principle of inertia tells us that if we have a body at rest (not moving), it will never move unless a force is applied to it. A box, for example, will only move if someone pushes it. A body moving on a frictionless surface , on the other hand, will maintain its constant speed for an infinite time, until it is forced to stop by friction forces or another mechanism.
While all this works in theory , it is practically impossible to reproduce it in reality, and even more so to see spontaneous examples. This is because in nature we do not find perfectly smooth surfaces, without alterations that cause friction, nor environments in which there is no wind or any other element that causes forces opposing the movement of a body.
It is possible to develop a more formal definition of the principle of inertia, using derivatives . Although it is a difficult concept to understand without an application, we can define it as the rate of change of a function that is expressed according to the variation of its value when its independent variable is modified, that is, the one on which another depends.
In the field of physics, we say that to know how a given function varies, we can observe its derivative. In more precise terms, by stating that the speed of a moving body remains constant over time because there are no external forces affecting it, then it is also true that the derivative of said speed in relation to time is zero, that is, it does not vary over time.
The concept of the principle of inertia can be related to linear momentum . This is also known as quantity of movement , momentum or impetus , and it is a vector that can be used to describe the movement of a body within the field of mechanics.
Well, the movement of a body has linear momentum as one of its main characteristics, and through this concept a relationship is established between the mass of the body and its respective speed. If the mass of an isolated body remains constant throughout the process of movement, it is possible to say that its amount of movement is also constant. This is also known as the principle of conservation of linear momentum .