The mass of substance is by means of the device which is called scales. It is also possible to calculate body weight if the amount of substance and its molar weight or its density and volume is known. The amount of pure substance is possible find on its mass or quantity of molecules which contains in it.

Friction – the process of interaction of two bodies causing delay of the movement at shift from each other. To find friction force – means to determine the influence size directed aside opposite to the movement because of which the body loses energy and, after all, stops.

Happens that there is such task: how to find the mass of the substance which is contained in any given volume of solution? The course of its decision depends on what basic data you have. It can be and very simple, literally in one action, and more difficult.

Irrespective of, the body moves or is in rest, it is constantly affected by physical forces. As a rule, them a little, but at the solution of tasks it is more convenient to define equally effective forces.