Friction is a force that prevents objects from sliding relative to each other when they contact each other. Friction is a very common mechanical phenomenon, which is ubiquitous in human life and production. There is not only friction between solids (this kind of friction is called dry friction), but also friction between solids and liquids or between solids and gases (these two kinds of friction are called wet friction). In dry friction, friction can be divided into static friction, sliding friction and rolling friction according to its nature. Different friction forces have different factors that affect their magnitude.

First, dry friction

(1) static friction

As long as there is a tendency of relative sliding between two objects, friction will appear. If the sliding trend is not too strong, the relative sliding will not really be realized because of the friction, and the friction at this time is called static friction fS. It can be seen that the static friction is caused by the trend of relative motion between objects. The reason for the trend of relative motion is external force. Therefore, the conditions of static friction include not only unsmooth contact surface and positive pressure, but also external force. The magnitude and direction of static friction depend on the relative sliding trend. Because friction is a force to prevent relative sliding, the direction of static friction is naturally opposite to the direction of relative sliding trend on the contact surface. Both objects are subjected to static friction, and their directions are opposite to the relative sliding trend of each object on the contact surface. The magnitude of static friction also depends on the trend of relative sliding. Without relative sliding tendency, there is no static friction, that is, the magnitude of friction is zero. As soon as there is a relative sliding trend, static friction appears. Under certain conditions, the relative sliding trend between objects is certain, and the static friction has a certain magnitude corresponding to it, which should be just enough to offset the relative sliding trend, so that the relative sliding will not really happen. Therefore, in specific problems, the magnitude of static friction is often unknown in advance, and it needs to be calculated from the dynamic motion equation according to the condition that there is no real relative sliding between objects. Once the situation changes, the trend of relative sliding between objects changes, and the magnitude of static friction automatically adjusts, so that relative sliding can never really happen. However, the automatic adjustment of static friction can not be carried out indefinitely, and its maximum limit is called maximum static friction. When the maximum static friction is not exceeded, the greater the external force, the greater the static friction. Once the maximum static friction is exceeded, the object begins to slide, and the static friction is converted into sliding friction. So what is the maximum static friction related to? Experiments show that the maximum static friction force fmax is proportional to the positive pressure n between two objects, and has nothing to do with the area of the contact surface, but with the properties of the contact surface (such as the material of the contact surface, the roughness of the contact surface, etc.). ), that is, fmax=μSN, where μS is called static friction coefficient, which depends on the material of contact surface and the surface state of contact surface. Practice has proved that fS≤fmax=μSN.

(2) Sliding friction

When the external force exceeds the maximum static friction, the object begins to slide, and the friction continues to exist, but the static friction is transformed into sliding friction. Objects slide relatively along the contact surface, and the friction on the contact surface that prevents relative sliding is called sliding friction. The direction of sliding friction is naturally opposite to that of relative sliding on the contact surface. The magnitude of sliding friction changes with the change of relative sliding speed. The relative sliding speed gradually increases from zero, while the sliding friction gradually decreases from the maximum static friction fmax = μ n. Generally speaking, sliding friction is less than static friction, which makes it more difficult to push a stationary object, but it is more labor-saving to keep moving at a constant speed after pushing, that is to say. However, when the relative sliding speed is too high, the sliding friction increases sharply. We can use the control variable method, and it is verified by experiments that when the dynamic friction coefficient is constant, the sliding friction is directly proportional to the positive pressure n on the contact surface. However, because the dynamic friction coefficient is difficult to control, the conclusion that sliding friction is proportional to the dynamic friction coefficient is only roughly verified under the condition of constant positive pressure. From this, we can get the formula: fk = μ n, where μ is called sliding friction coefficient, which depends on the material of the contact surface, the surface state of the contact surface and the relative sliding speed (as shown in the figure).

In some special cases (for example, the hardness of the material remains unchanged, the contact surface is processed, etc.). ), sliding friction

The force hardly changes with the speed of motion, and is almost equal to the maximum static friction, that is, μ = constant ≈μS When the external force is equal to the dynamic friction, the force on the object is still balanced. To make an object move, it is necessary to increase the external force.

Second, wet friction.

When an object moves relative to a liquid or gas (called a fluid), there is also friction along the contact surface to prevent relative sliding, which is called wet friction. When an object is immersed in liquid or gas, it will be subjected to wet friction in motion. At the same time, there is another effect, that is, on the contact surface, the object will be subjected to the pressure of liquid or gas, and the pressure direction is perpendicular to the contact surface, and the head-on pressure is greater than the back pressure, so the total effect of pressure on the object is to prevent the relative movement of the object. The resulting resistance is called dielectric resistance, which is generally much greater than wet friction. Dielectric resistance and wet friction are completely different, but they play the same role in the movement of objects relative to liquids or gases. Generally, the medium resistance is classified as wet friction, and their essence is not investigated. Wet friction is different from dry friction, and there is no wet friction without relative motion. Therefore, there is no static friction for the wet friction phenomenon. Because there is no static friction, even a small force can push an object to move in a liquid or gas. In the case of dry friction, the force less than the maximum static friction can not push the object at all. You can push the boat forward with a bamboo pole, but you have never seen a car push forward with a bamboo pole. That's the reason.

Once relative motion occurs, wet friction will also occur. The direction of wet friction is naturally opposite to the relative motion speed of the object. As for wet friction, it increases with the acceleration of relative motion. When the relative motion is slow, the wet friction is roughly proportional to the speed; When the relative motion is relatively fast, the wet friction force is roughly proportional to the square of the velocity.

When an object is immersed in liquid or gas, if it is pushed with a certain force, it will move gradually because there is no static friction. When an object begins to move, wet friction will occur. At first, the wet friction is less than the applied thrust, and the object continues to accelerate. With the increase of object speed, the wet friction force also increases. Finally, when an object reaches a certain speed, its corresponding wet friction is equal to the applied driving force, and the object keeps moving at this speed at a uniform speed, which is called the limit speed. If the initial speed of the object exceeds the limit speed, the wet friction force is greater than the applied driving force, and the movement slows down, and finally it reaches the limit speed and moves at a uniform speed. The limit speed is obviously related to the applied driving force. Generally, wet friction is not analyzed and studied in mechanics, and dry friction is mainly considered.

Third, the influence of friction.

When you push the table, if you don't push it, the table will move to the right, and at the same time, the table will be affected by a static friction force to the left, which will hinder its movement trend and make the table in a relatively static state. In the process of conveying goods upward by the conveyor belt, if there is no friction, the goods will slide down the inclined plane, so the objects tend to slide down the inclined plane, so the conveying brings a static friction along the inclined plane to the goods, which prevents the downward movement trend of the goods.

Without friction, we can't walk. Because I can't walk still. For example, walking on the ice, because the ice is slippery, you will be tired and sweaty if you can't walk far. If there were no friction, the road would be smoother than ice, and then people would feel better only if they fell to the ground. If there is no friction, the screw can't be tightened, and the nail nailed to the wall will automatically loosen and fall off. According to the law of universal gravitation, all objects will gather together under the action of universal gravitation. The tables and chairs at home should get together. Give a little thrust and it will spread out and slide across the ground, which is useless at all. . .

If there is no friction and medium resistance, the total amount of mechanical energy remains unchanged when the object only converts kinetic energy and potential energy. This is the role of friction. In a word, the factors affecting friction are fixed and few, but their manifestations are very diverse and complicated. Only by fully understanding and controlling these factors can we make full use of beneficial friction, avoid harmful friction and improve production and life to the maximum extent.

Fourth, the explanation of friction in high-end physics.

So far, the nature of friction is not very clear. The representative of the earliest experimental study on friction was Leonardo da Vinci in the Renaissance. He compared the friction of substances with different smoothness, and proposed that the friction between objects depends on the roughness of the surface. The rougher the surface, the greater the friction, that is, the roughness of the solid surface is the fundamental reason for the friction. This idea gradually developed into a theory-concave-convex theory. According to this theory, no matter what kind of treatment is done on the surface of an object, it is bound to leave large or small bumps. When objects with uneven surfaces touch each other, friction will inevitably occur. Someone has made this analogy: the contact between solid surfaces is like turning one mountain over and covering another. Because they mesh with each other, only the damaged convex part can slide, which is the basic principle of generating friction that hinders relative movement. This theory has been supported by many people for a long time.

Another view of friction is molecular theory. This was put forward by the British physicist De Sa Goulier. He believes that the reason of friction is that the molecular forces on the friction surface are staggered. The theory points out that the smoother the surface of an object, the closer the distance between friction surfaces, and the greater the surface molecular force, so the greater the friction force. However, this theory has not been confirmed by experiments because of the processing technology, so it is difficult for students to accept it.

After entering the 20th century, molecular theory gradually gained the support of many people. A man named Ewing first pointed out that the energy loss caused by friction is caused by the interference of molecular gravity fields on the solid surface, and has nothing to do with the degree of concavity and convexity. Hardy, another famous scholar, did a lot of experiments, which proved the correctness of molecular theory. He first polished the surfaces of two objects to be extremely smooth, and then made a friction experiment. The results show that the smoother two objects are ground, the smaller the friction between them, but when the smoothness reaches a certain level, the friction increases, and even two smooth metal surfaces can "stick" together. This just confirms the viewpoint of molecular theory: when molecules on two surfaces enter each other's intermolecular gravitational circle, there will be strong adhesion between them, which will be expressed in the form of friction. Hardy's experiment provided strong evidence for molecular theory, which was widely recognized and further developed into "bonding theory". However, the concave-convex theory has not been completely abandoned because of the progress of molecular theory and adhesion theory, and it is well-founded and reasonable with the opposing molecular theory and adhesion theory. On the basis of these two theories, some people put forward a comprehensive modern bonding theory, including concave-convex theory.

(A) concave-convex meshing theory

From15th century to18th century, scientists put forward a theory about the nature of friction. According to meshing theory, friction is caused by the rough surfaces of objects in contact with each other. When two objects contact and squeeze, many concave and convex parts on the contact surface mesh with each other. If the object slides along the contact surface, the convex parts of the two contact surfaces collide, resulting in fracture and wear, forming a movement obstacle.

(2) Attachment theory

This is a theory about the nature of friction after the concave-convex meshing theory. It was first proposed by the British scholar Desa Zoellick in 1734. He thinks that the friction of two kinds of metals with polished surfaces will increase, which can be explained by the increase of their molecular attraction when the surfaces of two objects are in complete contact.

Since the last century, with the development of industry and technology, the research on friction theory has been further deepened. By the middle of last century, a new theory of friction and adhesion was born.

According to the new friction and adhesion theory, no matter how smooth and rough the two surfaces are, there are many tiny bumps. When these two surfaces are put together, the tops of the protrusions are in contact, and there is a gap of 10-8 m or more between the contact surfaces except the protrusions. In this way, the tops of the contacted microprotrusions bear the normal pressure on the contact surface. If this pressure is very small, the top of the microprotrusion is elastically deformed; If the normal pressure is larger than a certain value (about a few thousandths of Newton on each bump) and exceeds the elastic limit of the material, the top of the micro bump will be plastically deformed and pressed into a flat top. At this time, the distance between two objects in contact with each other will be reduced to the range where molecular (atomic) attraction works, so atomic adhesion will be produced on the two tightly pressed contact surfaces. At this time, in order to make two surfaces in contact with each other slide relatively, tangential force must be applied to one of them to overcome the attraction between molecules (atoms) and cut off the contact generated in the actual contact area, thus generating friction. In modern friction theory, electrostatic action is also added. Smooth surfaces may have different charges during friction, and the electrostatic interaction between them is also a cause of friction.

To sum up, the mechanism of friction phenomenon is complex and can only be explained on the molecular scale. Due to the electromagnetic properties of molecular forces, friction is ultimately caused by electromagnetic interaction.

The above theory has denied the statement that "the smoother the surface of an object, the smaller the friction force". There is friction between very smooth surfaces. "Smooth surface" is often used in teaching, which means that there is no friction or the coefficient of friction is equal to zero, that is, there is no friction. This is a convention in teaching, but it doesn't really mean that the two sides are smooth. It is easy to push the wood block on the glass plate, but it is not easy to push the glass with the same mass as the wood block on the glass plate, which shows that the friction is increased.