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The Third law of Newton

The third law of Newton says: to action always there is an equal and opposite counteraction, differently bodies operate against each other with the forces directed along one straight line, equal on the module and opposite in a direction or mathematically:approves about existence of inertial systems.

Newton has distributed action of this law to a case and collisions of bodies, and on a case of their mutual attraction. As the elementary demonstration of this law the body located on a horizontal plane on which gravity Fm and force of reaction support Fî laying on one straight line operate, equal on value and opposite directed can serve, equality of these forces allows a body to be in a condition of rest (fig. 2).

Consequences follow from three fundamental laws of movement of Newton, one of which - addition of quantity of movement by a rule of a parallelogram. Acceleration of a body depends on the sizes describing action of other bodies on the given body, and also from the sizes defining features of this body. Mechanical action on a body from other bodies which changes speed of movement of the given body, name force. It can have the different nature (a gravity, force of elasticity, etc.) . Change of speed of movement of a body depends not by nature forces, and on their size. As speed and force - vectors action of several forces develops by a rule of a parallelogram. Property of a body on which acceleration got by it depends, is the inertia measured in weight. In the classical mechanics dealing speeds, considerably smaller speeds of light, the weight is the characteristic of the body, not dependent on that, it moves or not. The weight of a body in the classical mechanics does not depend and on interaction of a body with other bodies. This property of weight has induced Newton to accept weight for a measure of a matter and to consider, that its size defines quantity of a matter in a body. Thus, the weight began to be understood as quantity of a matter.

The quantity of a matter is accessible to measurement, being proportional to weight of a body. The weight is a force from which the body acts on a support interfering its free falling. Numerically the weight is equal to product of weight of a body on acceleration of a gravity. Owing to compression of the Earth and its daily rotation the weight of a body changes with breadth and on equator on 0,5 % less, than on poles. As the weight and weight are strictly proportional, there was possible a practical measurement of weight or quantity of a matter. The understanding of that weight is variable influence on a body, has induced Newton to establish and the internal characteristic of a body - inertia which it considered as ability inherent in a body to keep the uniform rectilinear movement, proportional to weight. As a measure of inertia it is possible to measure weight by means of weights as it was done by Newton.

It is possible to measure weight in a condition of weightlessness by inertia. Measurement by inertia is the general way of measurement of weight. But inertia and weight are various physical concepts. Their proportionality is each other rather convenient in the practical attitude - for measurement of weight by means of weights. Thus, the establishment of concepts of force and weights, and also a way of their measurement has allowed Newton to formulate the second law of mechanics.

The first and second laws of mechanics concern accordingly to movement of a material point or one body. Thus action of other bodies on the given body is considered only. However any action is interaction. As in the mechanic action is characterized by force if one body acts on another with the certain force the second acts the on the first with the same force, as fixes the third law of mechanics. In the formulation of Newton the third law of mechanics is fair only for a case of direct interaction of forces or by instant transfer of action of one body on another. In case of transfer of action for a final time interval the given law is applied when time of transfer of action can be neglected.