Every person who is somehow connected with technology knows what the force of friction is. In our life, it can play both a positive and a negative role. This article will devote to the determination of the coefficient of friction of steel on steel.
Types of friction
Before considering the determination of the coefficient of friction of steel over steel, it is better to become familiar with friction between solids. Let's consider in more detail:
- If two solids are brought into contact, then some force will be required to effect their displacement relative to each other. It should be more than static friction, which prevents movement.
- As soon as the bodies begin to move, their surfaces rub against each other. The corresponding force resisting motion is associated with sliding friction.
- The third type is rolling friction . Based on the name it can be seen that it occurs when the bodies roll along each other, for example, a bicycle wheel on asphalt.
All three types of friction act in the contact area of hard surfaces. Appropriate forces always strive to slow down any movement.
The use of friction forces is due to the fact that they provide the possibility of the movement itself and changes in its characteristics. Harm, as a rule, is associated with energy losses during displacement and with the wear of rubbing materials.
Friction of rest and sliding: coefficient of friction of steel on steel
It's time to consider the formulas. To calculate the friction forces of sliding and rest, use the following expression in physics:
F t = µ * N
Here µ and N are the coefficient of friction and the reaction of the support, respectively. Let us explain this. The value µ for the considered types of friction mainly depends on the roughness of the contacting surfaces. The more microscopic irregularities the surfaces contain, the greater its value.
It is also determined by rubbing materials. In the case of steel surfaces, metallic bonds between iron atoms make a large contribution to the characterization of this coefficient. This is due to the tight contact of the steel sheets. This fact explains why polishing a metal surface can not only not reduce, but even increase µ.
For most types of steel, the µ value for sliding friction lies in the range 0.12–0.15, and for rest friction these limits are 0.15–0.16. Lubrication of the surfaces leads to a decrease in the indicator (to 0.1 or less).
Rolling friction and its coefficient
The formula for determining the rolling friction force has the same form as for the types considered earlier. Let's write it again:
F t = C R * N
The rolling coefficient C R depends on the hardness and elastic characteristics of the rolling body, as well as on the radius of the wheel (ball, roller).
The coefficient of friction of steel over steel C R is important to consider when moving a train. Its metal wheels roll on rails of the same material. The tabular data says that C R for train wheels is in the range 0.0002-0.001.
Note that steel is a fairly hard material; therefore, the amount of elastic deformation during rolling is small for it. The latter causes small values of C R. The above tabular data indicate that the friction force of rolling of steel on steel is 100 or more times less than the same force when sliding metal plates on each other.
The task of determining the coefficient of friction
A steel bar weighing 1 kg was attached to a dynamometer and began to be pulled evenly along a horizontal steel sheet. It is necessary to determine the coefficient of friction with such a slip, if the dynamometer with a uniform motion of the beam showed a force value of 1.2 Newton.
To determine the value of μ, we use the expression for the force F t , we have:
F t = µ * N =>
µ = F t / N
Since the experiment is carried out on a horizontal surface, the reaction of the support N will be equal to the weight of the bar. As a result, we obtain the final formula for µ:
µ = F t / (m * g)
It remains to fill in the data and calculate the value of the coefficient of slip of steel on steel: µ = 0.12.