What Is Sliding Friction?

When an object slides on the surface of another object, the phenomenon that prevents the relative sliding between two objects on the contact surface is called "sliding friction". When there is relative sliding between objects, sliding friction is called dynamic friction. The sliding friction when there is a sliding tendency between objects without sliding is called static friction. Note that sliding friction is not dynamic friction!

When an object slides on the surface of another object, the phenomenon that prevents the relative sliding between two objects on the contact surface is called "sliding friction". When there is relative sliding between objects, sliding friction is called dynamic friction. The sliding friction when there is a sliding tendency between objects without sliding is called static friction. Note that sliding friction is not dynamic friction!

Chinese name

Sliding friction

Foreign name

sliding friction

Classification

physical

Nature

Not dynamic friction

Sliding friction concept

When an object slides on the surface of another object, the phenomenon that prevents the relative sliding between two objects on the contact surface is called "sliding friction". When there is relative sliding between objects, sliding friction is called dynamic friction. The sliding friction when there is a sliding tendency between objects without sliding is called static friction. The causes of sliding friction are complex and inconclusive. Modern friction theory believes that there are two main reasons for sliding friction. One is the theory of frictional unevenness. It is believed that the occurrence of friction is due to the rough and uneven surface of the object. When two objects are in contact, the uneven parts on the contact surface mesh with each other, which causes the object's movement to be hindered and cause friction; the second is molecular adhesion theory, which considers that the distance between the molecules when the two objects contact each other is as small as molecular gravity When it is within the range of action, the attraction of molecules is caused by the molecular gravity on the contact surface where the two objects are pressed tightly. The nature of friction needs further study.

Sliding friction is actually a moment that hinders rolling

An experiment to study the relationship between the magnitude of sliding friction and the factors: Why do you want to use a spring scale to pull a wooden block for linear motion at a constant speed? This is because the spring scale measures the magnitude of the friction, not the magnitude of the friction. When the wooden block moves in a straight line at a constant speed, the horizontal tensile force of the wooden block and the friction between the wooden block and the wooden block are a pair of balance forces. According to the condition of the balance of the two forces, the magnitude of the pulling force should be equal to the magnitude of the friction force. Therefore, when the magnitude of the tensile force is measured, the magnitude of the friction force is also measured. A large number of experiments have shown that the magnitude of sliding friction is only related to the pressure and the roughness of the contact surface. The greater the pressure, the greater the sliding friction; the rougher the contact surface, the greater the sliding friction.

Relative motion with sliding friction

Sliding friction is a force that hinders relative movement between objects in contact with each other, not necessarily a force that obstructs the movement of objects. That is, friction is not necessarily resistance, it may also be the motive force for moving objects, and it is necessary to clearly hinder the "relative movement" by using objects in contact with each other as reference objects. "Object movement" may be based on other objects. For example, in the experiment, a weight is placed on the wooden block, and the spring block is used to pull the wooden block for linear motion at a uniform speed.The weight is changed from static to moving along with the wooden block due to the static friction of the wooden block against it. . The specific situation is: When the wooden block is pulled forward by the static force, the weight slides backward relative to the wooden block, and the wooden block gives the weight a friction force that prevents it from sliding backward.The direction of this friction force is Before. Therefore, the weight does not slide relative to the wooden block, and the friction force at this time is the static friction force.

The magnitude of sliding friction has nothing to do with the speed of the object, and has nothing to do with the contact area between objects.

In the study of practical problems, in order to simplify the "ideal" approach, such as an object placed on the smooth surface of another object, this "smooth" means that if two objects move relative to each other, there is no friction.

Conditions for generating sliding friction

1. Objects are in contact with each other and under pressure;

2. There must be relative movement or the tendency of relative movement between objects;

3. The contact surface is rough. Although an absolutely smooth contact surface does not exist, it may be assumed that the contact surface is absolutely smooth in many scientific hypotheses and guesses.

Sliding friction calculation formula

The magnitude of sliding friction is proportional to the pressure, which is directly proportional to the vertical force of an object on the surface of another object. The formula for calculating sliding friction is F = Fn

Where F is equal to the sliding friction force, is the dynamic friction coefficient, and Fn is the pressure.

Here's a description of the terms in the formula

Fn is the nature of the elastic force, which is not always equal to the gravity of the object, and needs to be determined by combining the movement and the balance conditions.

The coefficient of dynamic friction is a proportional constant, and its value is closely related to the material of the contact surface and the conditions of the contact surface (such as roughness, wetness, temperature, etc.). The coefficient of dynamic friction is the ratio of two forces and therefore has no units.

The magnitude of sliding friction has nothing to do with the speed of the object's relative movement, and has nothing to do with the area of the contact surface.

The effect of sliding friction always obstructs the relative movement between objects, but it does not hinder the movement of objects. The sliding friction may be resistance or of course power.

Example of sliding friction

I. High School Section

1. Regarding friction, the following statements are correct ()

A. The larger the contact area between two objects, the larger the dynamic friction factor.

B. The dynamic friction factor is only related to the roughness of the contact surface between the two objects.

C. If there is friction between two objects, there must be elastic force.

D. Increasing the positive pressure will definitely increase the friction.

The answer is BC

2. There is a wooden box on the horizontal table. When the wooden box is pulled to the right with a pulling force of F = 200N, the wooden box moves to the right at a uniform speed, and the magnitude and direction of the sliding friction force on the wooden box are calculated.

Analysis : According to the condition that the object is balanced by the two forces, it can be known that the magnitude of the sliding friction force on the object is f = F = 200N, and the direction is opposite to the direction of F (that is, horizontal to the left).