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.
Instruction
1. Friction force – the vector size which depends on many factors: pressure of bodies at each other, materials of which they were made, the speed of the movement. Surface area at the same time does not matter as the it is more, the more mutual pressure (force of reaction of support N) which already participates in finding of friction force.
2. These sizes are proportional each other and connected by friction coefficient μ which can be considered a constant if the big accuracy of calculations is not required. So, to find friction force, it is necessary to calculate the work: Ftr = μ\• N.
3. The given physical formula belongs to the friction caused by sliding. It can be dry and damp if between bodies there is a liquid layer. Friction force always should be taken into account when determining set of forces operating on a body at the solution of tasks.
4. Friction of swing arises at rotation of one body on a surface of another. It is present at border of contact of bodies which constantly changes. Nevertheless, friction force constantly counteracts the movement. Proceeding from it, it is equal to a ratio of the work of coefficient of friction of swing and the pressing force to the radius of the rotating body: Ftrkach = f · N/r.
5. It is necessary to distinguish coefficient of a sliding friction and swing. In the first case it is the size which does not have dimension, in the second it represents distance between the straight lines characterizing the direction of the pressing force and force of reaction of a support. Therefore, it is measured in mm.
6. The coefficient of friction of swing is, as a rule, the known size for widespread materials. For example, for iron up to gland it is equal to 0.51 mm, for iron on a tree – 5.6, a tree on a tree – 0.8-1.5, etc. It is possible to find it on a friction moment ratio formula to the pressing force.
7. Friction force of rest appears at the minimum movements of bodies or deformation. This force is always present at dry sliding. Its maximum value is equal \to U-03BC \• N. There is also internal friction, in one body between its layers or parts.