Newton's laws are three principles proposed by the English mathematician and physicist Isaac Newton ( 1643 – 1727 ). These postulates allow us to provide an answer to most of the problems that arise 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 appears when the resultant of the forces that affect it is zero.
From the principle of inertia it follows that bodies oppose the modification of their state of motion or rest . Inertia, in fact, is said opposition. It should be remembered, on the other hand, 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 its rest. Therefore, without an impulse, friction , friction , etc., the element in question will always maintain its constant speed, even when it is zero.
The principle of inertia reveals to us that, if we have a body at rest (without movement), 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 retain its constant velocity for infinite time, until it is forced to brake by frictional forces or another mechanism.
Although all this works in theory , it is practically impossible to reproduce it in reality, much less see examples spontaneously. This is because in nature we do not find perfectly smooth surfaces, without alterations that cause friction, nor media in which there is no wind or any other element that causes forces opposite to the movement of a body.
It is possible to develop a more formal definition of the principle of inertia, making use of the 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, when stating that the speed of a moving body remains constant in time because there are no external forces that affect it, then it is also true that the derivative of said speed in relation to time is zero, that is, does not vary over time.
The concept of the principle of inertia can be related to linear momentum . This is also known as momentum , momentum or impetus , and it is a vector that we can use to describe the movement of a body within the scope 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 with its respective speed. If the mass of an isolated body remains constant throughout the process of motion, it is possible to say that its momentum is also constant. This is also known as the principle of conservation of linear momentum .