Friction is that physical process without which motion itself could not exist in our world. In physics, to calculate the absolute value of the friction force, it is necessary to know a special coefficient for the considered rubbing surfaces. How to find the friction coefficient? This article will answer this question.
Friction in Physics
Before answering the question of how to find the coefficient of friction, it is necessary to consider what friction is and by what force it is characterized.
In physics, three types of this process are distinguished, which occurs between solid objects. This is the friction of rest, sliding and rolling. Friction of rest always occurs when an external force tries to move an object from its place. Slip friction, judging by the name, occurs when one surface glides over another. Finally, friction rolling occurs when a round object (wheel, ball) rolls over a certain surface.
All types are united by the fact that they impede any movement and the point of application of their forces is in the contact area of the surfaces of two objects. Also, all these types translate mechanical energy into heat.
The causes of friction forces of sliding and rest are roughnesses of a microscopic scale on surfaces that rub. In addition, these species are due to dipole-dipole and other types of interactions between atoms and molecules that form rubbing bodies.
The reason for the rolling of friction is due to the hysteresis of the elastic deformation that appears at the contact point of the rolling object and the surface.
Friction force and coefficient of friction
All three types of solid friction forces are described by expressions having the same shape. Here is her:
F t = μ t * N.
Here N is the force acting perpendicular to the surface of the body. It is called a support reaction. The value µ t - is called the coefficient of the corresponding type of friction.
The coefficients for sliding friction and rest are dimensionless quantities. This can be understood if we look at the equality of the friction force and the friction coefficient. The left-hand side of the equation is expressed in Newtons, the right-hand side is also expressed in Newtons, since the value N is a force.
As for friction rolling, the coefficient for it will also be dimensionless, however, it is determined as the ratio of the linear characteristic of elastic deformation to the radius of the rolling object.
It should be noted that typical values of the coefficients of sliding and resting friction are tenths of a unit. For rolling friction, this coefficient corresponds to hundredths and thousandths of a unit.
How to find the coefficient of friction?
Coefficient µ t depends on a number of factors that are difficult to consider mathematically. We list some of them:
- material of rubbing surfaces;
- surface finish;
- the presence of dirt, water and so on;
- surface temperatures.
Therefore, the formula for μ t does not exist, and it has to be measured experimentally. To understand how to find the coefficient of friction, it should be expressed from the formula for F t . We have:
µ t = F t / N.
It turns out that in order to know µ t it is necessary to find friction force and reaction of the support.
The corresponding experiment is performed as follows:
- Take the body and the plane, for example, made of wood.
- They attach the dynamometer to the body and evenly move it on the surface.
In this case, the dynamometer shows some force, which is equal to F t . The reaction of the support is equal to the weight of the body on a horizontal surface.
The described method allows you to understand what is equal to the coefficient of friction of rest and sliding. In a similar way, the rolling μ t can be experimentally determined.
Another experimental method for determining μ t is given in the form of a problem in the next section.
The task of computing μt
A wooden beam is on a glass surface. Gradually tilting the surface, it was found that the sliding of the beam begins at an angle of inclination of 15 o . What is the rest friction coefficient for a tree-glass pair?
When the beam was on an inclined plane at 15 o , then the rest of the friction force for him had a maximum value. It is equal to:
F t = m * g * sin (α).
Strength N is determined by the formula:
N = m * g * cos (α).
Applying the formula for µ t , we obtain:
μ t = F t / N = m * g * sin (α) / (m * g * cos (α)) = tg (α).
Substituting the angle α, we arrive at the answer: µ t = 0.27.