What Is the Coefficient of Friction?

The coefficient of friction is the ratio of the friction between two surfaces to the vertical force acting on one surface. It is related to the surface roughness and has nothing to do with the size of the contact area. According to the nature of motion, it can be divided into dynamic friction coefficient and static friction coefficient. Sliding friction is caused when two objects come into contact with each other and slide relative to each other.

The coefficient of friction is the ratio of the friction between two surfaces to the vertical force acting on one surface. It is related to the surface roughness and has nothing to do with the size of the contact area. According to the nature of motion, it can be divided into dynamic friction coefficient and static friction coefficient. Sliding friction is caused when two objects come into contact with each other and slide relative to each other.

Chinese name

Coefficient of friction

Foreign name

coefficient (s) of friction

Classification

Coefficient of Dynamic and Static Friction

Symbol

k and s

Definition of friction coefficient

If the two surfaces are at rest, the contact between the two surfaces will form a strong binding force-static friction. Unless this binding force is destroyed, one surface can move to the other surface and the binding force-the force before the movement- The ratio of the vertical force to a surface is called the static friction coefficient s, and it is written as follows:

(Equation Figure 1) fs is the static friction

Or fs = s * N; N is the vertical force

This destructive force is also the maximum force to start the object. We call this force the maximum static friction force. Therefore, we should rewrite the above formula as:

(Equation Figure 2) fk is the dynamic friction

For example, after a while, the car will slowly slow down and finally stand still. This means that when an object moves, there is still friction on its surface and on the other surface. (Such as the ground) and experiments have found that this force is smaller than the friction force at rest, we define this friction force and the force perpendicular to the ground is called the dynamic friction coefficient k, written as follows:

fk = k * N

Therefore, from the above we can know that s> k ^[1]

Coefficient of friction

Material name Static friction coefficient Dynamic friction coefficient

No lubrication with lubrication

Steel-steel 0.15 0.1 0.12 0.1 0.05 0.1

Steel-mild steel 0.2 0.1 0.2

Steel-cast iron 0.3 0.2 0.05 0.15

Steel-Bronze 0.15 0.15 0.18 0.1 0.15

Mild steel-cast iron 0.2 0.18 0.05 0.15

Mild steel-bronze 0.2 0.18 0.07 0.15

Cast iron-cast iron 0.18 0.15 0.07 0.12

Cast iron-bronze 0.15 0.2 0.07 0.15

Bronze-Bronze 0.1 0.2 0.07 0.1

Leather-cast iron 0.3 0.5 0.15 0.6 0.15

Rubber-cast iron 0.8 0.5

Timber-wood 0.4 0.6 0.1 0.2 0.5 0.07 0.15

1 Friction coefficient properties: the coefficient of friction may be greater than 1

Friction coefficient from the perspective of friction mechanism

The coefficient of friction is mainly a feature of contact materials, interface stickies or surface lubricants. According to modern friction theory, friction is caused by the adhesion between atoms on the contact surface. When two objects contact each other, the surface of the convex part is the first Atoms are fairly close to form an atomic bond that is as strong as the atomic bonds that hold themselves together inside a solid. If the surface is very clean and the contacts are very close, the two surfaces that are in contact with each other will adhere very firmly, and "contact growth" will occur before obvious sliding, and the contact area will continue to increase until the entire geometric contact area becomes a huge contact point. At this time, the friction force is very large, even exceeding the positive pressure, and the friction coefficient can be equal to, greater than 1, or even greater. A table in the first volume of "Mechanics" of "Berkeley Physics Course" by C. Kittel et al. States that the static friction coefficient between copper and copper is 1.6, and the static friction coefficient between rubber and solid is 1.0 to 4.0. All the above shows that the friction coefficient can be equal to or greater than 1.

Analysis of friction coefficient from measurement method

The friction coefficient can usually be measured by the method of friction angle. The method is generally: one of the two objects to be measured is placed obliquely as an inclined surface, and the other is placed on the inclined body and slides along it, gradually reducing the inclined angle It can be found that when reaches a certain value 0, the object slides at a uniform speed.

mgsin0 = mgcos0,

Obtained: = tan 0. (1)

0 is the friction angle. After measuring 0, we can know the value of between the two objects. It is easy to see from the formula (1):

When 0 <45 °, <1,

When 0 = 45 °, = 1,

When 0> 45 °, > 1.

Since the range of the tangent function in the first quadrant is (0, ), is not necessarily less than 1. As for what is, it depends on the material properties and interface conditions. ^[2]