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# Dynamics

The historical introduction

A part of the mechanics in which we study motion and it's causes is called dynamics. Although all the physicians from Arhimed on have been trying to understand the causes of motion, only Galileo and Newton have succeeded. The question of the relation of the force, and motion is in fact the central question of dynamics. From the experience we know that when we move the object we must perform force on it all the time. It seems that it is the constant work of the force that is needed to obtain the constant speed. That was the belief that prevailed at the antique physicians. From this belief that without this force work bodies stay in the standstill position. Of course, this hypothesis could not explain the motion of the heavenly bodies. Two thousand years later Galileo concluded that everybody will keeps it's speed, until it appears a cause of acceleration or deceleration. This conclusion is in fact the Law of inertia.

Newtons's axioms - the basic laws of motion

According to Newton the force is: a cause of the change in the motion of the body along any quantity or direction. Each force has it's point of application, a point in which it acts, the direction of acting and the quantity. Force is in fact a vector. The forces can be recognized only according to the direction of it's action. According to Newton mass is the resistance (inertia) that body uses to resist to the change in motion. In this moment it is useful to emphasize the difference between the mass and the weight. The weight is the force by which some body presses the surface (expressed in newtons - N) and mass is the ratio of force and acceleration (expressed in kilograms - kg).

The First Newton's law - the law of inertia

Every body stays in a stand still position or uniform motion along the straight line up to the moment when it under the work of some outer forces changes the direction of the motion (law on inertia).

The Second Newton's law - the law of force

The change in motion is proportional to the force that works in the direction of the straight line in which works the force -s=1/2gt2

The Third Newton's law - the law of action and reaction

Every work has always got the opposite and equal contra work, or to say the work of the two bodies one to another are always equal and of opposite direction - m1a1=m2a2

The work and energy

In physics the concept of work is strictly defined and is related to resisting the force on the given path - work=force x passed path.

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