Various practical problems concerning the interaction and motion of bodies, solve using Newton's laws. However, the forces acting on the body, is very difficult to determine.

Then use the problem solving another important physical quantity - momentum.

## What is momentum in physics

Translated from the Latin word "momentum" means"push". This physical quantity is also called the "momentum". It was introduced into a science about the same time that Newton's laws (XVII century end) opened.
Section of physics, studying the movement andthe interaction of material bodies, is a mechanic. The momentum in the mechanics - this is a vector quantity equal to the product of body mass on its velocity: p = mv. Directions momentum vectors and speed are always the same.

The SI unit of momentum take momentum of the body mass of 1 kg, which is moving at a speed of 1 m / sec. Therefore, the pulse unit to SI - it is 1 kg m / s?.

The settlement problems considering the projectionVectors speed and momentum on any axis and use the equations for these projections: for example, if you select x-axis, then consider the projection of v (x) and p (x). By definition, the momentum, these quantities are related by: p (x) = mv (x).

Depending on which axis is chosen and where it is directed, the momentum vector projection it can be both positive and negative.

## The law of conservation of momentum

The pulses of material bodies with their physicalinteraction may vary. For example, the collision of two balls suspended on strings, their impulses are mutually changed: one ball can be set in motion from a standing start or increase their speed, and the other, on the contrary, reduce speed or stop. However, in a closed system, ie when bodies interact only with each other and are not subjected to external forces, the vector sum of the pulse of the bodies remains constant in all their interactions and movements. This is the law of conservation of momentum. Mathematically, it can be derived from Newton's laws.
The law of conservation of momentum is also applicable to suchsystems where some external forces act on the body, but their vector sum is equal to zero (e.g., the force of gravity balanced by the elastic force of the surface). Conventionally, such a system can also be considered closed.
The mathematical form of the law of conservation of momentumwritten as: p1 + p2 + ... + p (n) = p1 '+ p2' + ... + p (n) '(p pulses - vectors). For a system of two bodies, this equation looks like p1 + p2 = p1 '+ p2', or m1v1 + m2v2 = m1v1 '+ m2v2'. For example, in the case considered, the total momentum of the balls with two balls before the interaction is equal to the total momentum after the interaction.