Friction is ubiquitous in human life and production. Friction can be divided into sliding friction, static friction and rolling friction according to its nature. Different friction forces have different factors that affect their magnitude. Our group chooses sliding friction and static friction to study, and roughly studies the friction when an object moves in a fluid. So far, some achievements have been made.

First of all, for sliding friction, we know from textbooks that it is directly proportional to the positive pressure. Our team members used the control variable method to accurately verify the conclusion that the sliding friction is proportional to the positive pressure when the dynamic friction coefficient is constant through experiments. 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 still get the formula f = μ n.

So what determines the dynamic friction coefficient? As we know, the dynamic friction coefficient reflects the roughness of the object surface, which in turn determines the dynamic friction coefficient, which is the interaction between two objects with unsmooth contact and relative motion, so the dynamic friction coefficient is not determined by the roughness of one object surface alone, but by the roughness of the contact surface of two objects with interactive friction.

If we take a pen and a short piece of rope and wind the rope around the pen, we will find that the more times the rope is wound, the harder it is to pull, especially where there is overlap between the ropes. Are there any other factors that affect friction? We come to the conclusion that every time the rope wraps around the pen, there will be one more contact point between the rope and the pen, and there will be countless interactions between the two, that is, there will be more places where friction is generated. All the friction forces together increase the resultant force. If the ropes overlap, not only will there be interaction between the rope and the pen, but there will also be interaction between the rope and the rope, which will hinder each other's movement. At this time, the pressure between the rope and the pen includes not only the pressure between the rope and the pen, but also the pressure between the rope and the pen, so the friction increases sharply and it is difficult to pull the rope. In life, when ships dock, they always tie the piles to the shore with ropes, and also use the rope to wind several times to increase friction. But this does not include other factors that affect friction besides positive pressure and dynamic friction coefficient.

For static friction, the reason is that there is a tendency 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. Under the condition of not exceeding the maximum static friction, the greater the external force, the greater the static friction. Once the maximum static friction is exceeded, the object begins to move, and the static friction becomes sliding friction. So what is the maximum static friction related to? Experiments show that fmax=μN indicates that the maximum static friction force is directly proportional to the static friction factor and the positive pressure, and the static friction factor is slightly larger than the dynamic friction factor, because when the external force is equal to the dynamic friction force, the force on the object is still balanced, and the external force must be increased to make the object move.

As for the movement of an object in a fluid, it is mainly caused by the resistance of the fluid when it displaces the fluid, but the friction of the fluid on the side of the object can not be ignored. For the resistance when discharging fluid, we can reduce it by converting the moving object into streamline, or increase it by the opposite method. As for the friction on the side when an object moves, we know that when an object moves, it will drive the nearby fluid to move together, while the fluid a little farther away is still at rest. Thus, according to Bernoulli equation,

"= constant" means that the static fluid will exert pressure on the object, and the contact between the object and the fluid is not smooth, which will produce friction. Moreover, with the increase of speed, the pressure of moving fluid decreases, while the pressure of static fluid remains unchanged, so the pressure difference and pressure increase and the friction increases; Through similar analysis, it can be concluded that the friction increases with the increase of depth.

The factors that affect the magnitude of 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.