Various practical tasks concerning interaction and motion of bodies solve with application of laws of Newton. However forces operating on a body happens very difficult to define. Then in the solution of a task use one more important physical quantity – an impulse.
What is an impulse in physics
In translation from Latin "impulse" means "push". This physical quantity is called also "the number of the movement". It was entered into science approximately at the same time when Newton's laws were open (at the end of the 17th century).
The section of physics studying the movement and interaction of material bodies is the mechanics. The impulse in mechanics is the vector size equal to the work of body weight on its speed: p=mv. The directions of vectors of an impulse and speed always coincide.
In the SI system take an impulse of a body weighing 1 kg which moves with a speed of 1 m/s for unit of an impulse. Therefore impulse unit in SI is 1 kgm / page.
In settlement tasks consider projections of vectors of speed and an impulse to any axis and use the equations for these projections: for example, if axis x is chosen, then consider projections of v (x) and p (x). By definition of an impulse, these sizes are connected by a ratio: p (x) =mv(x).
Depending on what is chosen an axis and where it is directed, the impulse vector projection to it can be both positive, and negative size.
Law of conservation of momentum
Impulses of material bodies in their physical interaction can change. For example, at collision of two balls suspended on threads, their impulses mutually change: one ball can start moving from a motionless state or increase the speed, and another, on the contrary, to reduce speed or to stop. However in the closed system i.e. when bodies interact only among themselves and are not affected by external forces, the vector sum of impulses of these bodies remains to a constant in any their interactions and movements. Law of conservation of momentum consists in it. Mathematically he can be brought from Newton's laws. Law of conservation of momentum is applicable also to such systems where some external forces affect bodies, but their vector sum is equal to zero (for example, gravity is counterbalanced with surface elastic force). Conditionally such system can be considered closed too. In a mathematical form the law of conservation of momentum registers so: p1+p2+ … +p(n) =p1 ’+p2 '+ … +p(n)’ (impulses of p are vectors). For a system from two bodies this equation looks as p1+p2=p1 '+p2’, or m1v1+m2v2=m1v1 '+m2v2’. For example, with balls the total impulse of both spheres before interaction will be equal to a total impulse after interaction in the considered case.