Although Galileo is recognized as having first undertaken the rigorous analysis of the movement and the causes that determine it, the English physicist I. Newton (1642-1727) is credited with having completed and systematized these studies, reaching the mathematical formulation of the three fundamental laws of dynamics or Newton’s laws, which represent the general principles on which the science of motion is based from the dynamic point of view. The first law of dynamics (or principle of inertia) states that a body tends to maintain its state of quiet or uniform rectilinear motion until external causes intervene to urge it.
This means that, if a body is at rest, it will remain in this state until a force is applied to it, while if it moves in a uniform rectilinear motion it will continue to do so until an external force intervenes to change its speed. In everyday experience the principle of inertia is easily verifiable in the case of bodies resting on a plane at rest: a sphere remains stationary until a thrust is imparted to it; a heavy suitcase continues to remain fixed to the ground in the same place where it was placed if someone who is unable to lift it does not intervene. The case of bodies in uniform rectilinear motion is less intuitive: experience shows us, for example, that a trolley moves as long as a pushing action persists and that, moreover, this thrust force must be continuously applied if the speed of movement is to be kept constant; when the cart is abandoned, it relentlessly stops. The interruption in the movement is not, however, due to the lack of a pushing action, but rather to the presence of forces acting in the opposite direction to the movement, hindering it until its complete cancellation: thefriction forces . If the trolley moved in the void and in the total absence of friction, it would never stop its travel, continuing its motion indefinitely, with constant speed and on a rectilinear trajectory. Contrary to appearances, it is therefore not the presence of one or more applied forces that keeps the speed of a moving body constant, but rather their absence or, better, their reciprocal annihilation.
The inertia principle can therefore be reformulated by saying that a body tends to maintain its state of quiet or uniform rectilinear motion until an external force intervenes, or a sum of external forces whose resultant is different from zero