The phenomenon of friction in a person’s life plays both a positive and a negative role. On the one hand, without his presence it would be impossible to move, on the other hand, due to friction, huge losses of energy and working materials occur. In the article, we consider from the point of view of physics what constitutes friction, as well as how to find the coefficient of friction.
The phenomenon of friction
Friction is a contact phenomenon that occurs in the zone of contact of various bodies, and which counteracts any of their mutual movement.
In the mechanics of the motion of solids, three types of friction are distinguished:
- acting at rest;
- acting while sliding surfaces against each other;
- arising from rolling bodies.
Friction of rest occurs when we apply an external force tangent to the surface to the body in order to move it from its place. Vivid examples of sliding friction are ski gliding in the snow. Finally, friction during rolling manifests itself when the vehicle wheel rolls along the road.
Formula for determining the friction force
In physics, the listed types of friction are described by the same formula when calculating the acting forces. This formula is as follows:
F t = µ * N.
The friction force F t is equal to the product of the friction coefficient µ and the reaction of the support N. When considering the corresponding type of friction, only the value of the coefficient µ changes, which is a dimensionless quantity.
In the case of static and sliding friction forces, µ is of the order of tenths of a unit. Μ depends on the contacting materials, on the roughness of their surface, and does not depend on the contact area or sliding speed.
For rolling friction, the coefficient µ (it is customary to designate C R ) depends on the elasticity characteristics of the rolling body, on its hardness, on the rolling radius and some other factors. For most materials, the value of this coefficient for rolling is hundredths and thousandths of a unit.
Since there are many factors affecting the value of µ, there is no definite mathematical formula for its calculation. Answering the question of how to find the friction coefficient, it should be said that it is measured experimentally.
Determination of coefficient µ
In this section, we consider two methods for the practical determination of μ using the example of sliding friction and rest.
The first way to answer the question of how to find the coefficient of friction is to place the bar on the horizontal plane to which the dynamometer is attached. The bar and the plane are made of the studied pair of materials, for example, glass and wood. Moving the bar evenly, holding the dynamometer, you can determine the sliding force F t . Knowing the mass m of the bar, the coefficient µ is calculated as follows:
µ = F t / (m * g).
The second method is convenient for determining μ for rest friction. To do this, place a block on the horizontal plane. Then, one end of the plane should be slowly lifted, tilting it at a certain angle to the horizon. At a certain angle θ, the block starts to slide off the surface. By measuring this angle, the coefficient of friction µ can be determined from the equality:
µ = tg (θ).
The µ measurement for rolling friction involves the use of a more complex setup called an inclined pendulum. The calculation of µ in this case is performed by studying the equations of motion dynamics.