Why is there maximum static friction? Thank you.

First of all, whether it is dynamic friction or maximum static friction.

There are only two factors that affect its size, the positive pressure and the friction factor, here the same object so just consider the friction factor. Whereas, the static friction factor is greater than the kinetic friction factor.

Target. Because the contact surface between the object and the support is convex and uneven, when the object is in a stationary state, due to the action of gravity, the two contact surfaces will be slightly deformed, so that the two are well matched, and when the object is moving, there is only macroscopic contact between the two contact surfaces, and the microscopic deformation has not had time to occur, > the anastomosis has moved to the next position, so the static friction factor is greater than the dynamic friction factor, so the maximum static friction is not equal to the sliding friction.

There is a point to pay attention to in high school The maximum static friction is greater than or equal to the minimum dynamic friction Sometimes it is useful when solving the problem -- pour out the love Manual input I hope to help the landlord Follow-up question: Oh, thank you : This is the other information I found for you: The maximum static friction is not less than the sliding friction - the maximum static friction is greater than or equal to the sliding friction, which is determined by the definition of "maximum static friction", because the static friction that does not meet the above relationship is not the maximum static friction.

However, there is usually a situation where the maximum static friction is greater than the sliding friction, and it is irrelevant to explain this phenomenon that often occurs (but does not necessarily occur) regardless of the logical contradiction and definition mentioned above. This involves the theory of friction, which is so complex that it has not yet been conclusive. Two models can be provided to explain qualitatively, but apparently they are not sufficient.

1) Compared with two contact surfaces that slide against each other, the molecules (or atoms) on the contact surface at rest have enough time to be close to each other (within a certain range, the close proximity of the molecules means that the attraction between the molecules is greater), and so the molecular forces between each other.

The number of larger molecules will also be higher, so in order to overcome the attraction between these many molecules with larger molecular forces and move, the external force must be greater - the maximum static friction is greater than the sliding friction. 2) Imagine that the surface in contact has many bumps and bumps, and the bumps between the two sides bite each other to become a ** of friction. When they are at rest with each other, they bite each other fully and tightly; When sliding against each other, the bumps have been ground (or pressed) flat by the front side - the bumps that can be bitten together on the back side have become shallower and smaller, so the sliding friction will be less.

Supplement: If you help the landlord, I hope so.

There are two meanings to this question, I don't know which one you want to ask.

1) Why is there static friction.

Because two objects that are in contact with each other and have pressure will inevitably deform due to pressure (the technical term is called strain), although it is very small but does exist, so that the molecules on the contact surface of the two objects have overlapping parts, and there is a molecular force that attracts each other between them; Once there is a relative movement trend, it is necessary to deal with this attraction, which is static friction.

2) Why is there a maximum?

This is the best and the hardest to explain: the attraction is not infinite, there must be an upper limit.

1. The maximum static friction generally refers to the maximum static sliding friction. Kai hand.

2. The static sliding friction force is different from the general restraint reaction force, and it does not increase infinitely with the increase of the external force. When the magnitude of the external force reaches a certain value, the object is in a critical state where it is about to slide, but it does not start to slide.

3. At this time, as long as the external force is a little larger, the object will begin to slide. When the object is in a critical state of equilibrium, the static friction reaches its maximum value, which is the maximum static sliding friction, referred to as the maximum static friction.

This means that when an object changes from a critical state of rest to motion, the force used is just as great.

In the general calculation, it is considered that the maximum static friction is equal to the sliding friction bench imitation, and the column expression is: fm = n.

In fact, the sliding friction is slightly smaller than the maximum static friction, but it is similar to the phase rolling hand. The reason is that the tensile force of the object is greater than the maximum static friction force at the moment when the object is from rest to motion, and the tensile force is considered to be equal to the sliding friction force, which is equal within the allowable range of error.

Static friction is greater than kinetic friction because when the surface of an object is in contact, there are tiny irregular areas between the two surfaces called microscopic bulges and depressions. When an object is at rest, these bumps and depressions are chimeed with each other, allowing the object to remain stationary. In this case, static friction arises.

When the object begins to move, the mutual mating between the surfaces is broken, and the bulges and depressions slide relative to each other. This relative sliding reduces the friction so that the kinetic friction is less than the static friction. In motion, kinetic friction can only provide enough force to overcome the resistance of sliding friction, but it is comparatively smaller.

The reason why dynamic friction is less than static friction is that dynamic friction is affected by the sliding friction coefficient, which is affected by the static friction coefficient. Usually, the values of these two coefficients of friction are different. The coefficient of dynamic friction is generally less than the coefficient of static friction because once the object begins to move, the mutual mating between the bulge and the depression is broken, reducing friction.

In summary, the static friction force is greater than the kinetic friction force due to the surface irregularity, when the object is stationary, the microscopic bulges and depressions produce the effect of mutual mating, so that the static friction force becomes larger. When the object moves, the relative sliding between the bulge and the depression reduces the frictional force of the cracking spike and makes the dynamic friction smaller.

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Static friction and kinetic friction are two different manifestations of friction, and the main difference between them is the conditions and effects under which friction occurs. Static friction refers to the frictional force that occurs when two objects are at relative rest due to their contact with each other. Static friction usually occurs when there is no relative motion between two objects, but there is a tendency to relative motion between them.

For example, when you push a table but don't push it, the force you exert on the table creates static friction. Kinetic friction refers to the frictional force generated by the mutual contact between two objects in a state of relative motion. Kinetic friction usually occurs when relative motion has already taken place between two objects.

For example, when you push a table, the relative motion between the table and you creates kinetic friction. In addition to the different conditions under which it is generated, the effects of static friction and dynamic friction are also different. Static friction usually hinders the relative tendency of an object but does not stop the object from moving.

Kinetic friction, on the other hand, hinders the relative motion of the object and generates heat, which wears the surface of the object. In addition, the properties of static friction and dynamic friction are also different. Static friction can generally be regarded as a type of elastic force whose magnitude can be calculated by the external force exerted on the object.

Kinetic friction, on the other hand, is a damping force whose magnitude can be calculated by the coefficient of friction and the external force exerted on the object. In conclusion, static friction and kinetic friction are two different forms of friction, and the difference between them is the condition and effect under which friction occurs. Understanding their differences helps us better understand the interaction forces and motion laws between the objects in the dressing room.

1.Friction is generated between objects that are in direct contact and is premised on whether there is relative motion or a tendency to move relative between the two objects. The direction of friction is in the tangential direction of the contact surface of two objects and is opposite to the direction of the relative motion of the object or the relative tendency of motion.

Static Friction The magnitude of the static friction fs is between zero and the maximum static friction fmax.

0＜fs≤fmax

fmax＝μ′n

It is called the coefficient of static friction, which is related to factors such as the material properties, roughness, dryness and wetness of the contact surface of two objects, and is usually determined by experiments. n is the magnitude of the normal supporting force.

2.We were able to observe this phenomenon when we were experimenting.

Pull a slider with a spring scale that is stationary in a rough horizontal plane, the force gradually increases, when the force is f (that is, the maximum static friction), the slider moves up, if the slider moves at a uniform speed, then we will see that the number of the spring scale (sliding friction) is less than f, 6, in high school, my classmates and I thought: because the object has a stationary inertia at the beginning (as far as possible to maintain the original state of motion and do not change it), it is difficult to change its state of motion (from rest to motion), Therefore, the maximum static friction is larger; When the object is in motion, the object already has the inertia of motion, and the friction force will change the motion state of the object (from motion to rest), so the friction force should be smaller, so that the object can maintain motion as much as possible. So the maximum static friction is slightly greater than the sliding friction.

2. The thrust force makes the block move, and the magnitude of the force must be greater than the magnitude of the sliding friction force, otherwise the chaos cannot change the motion state of the block (stationary). The magnitude of the force is at least the magnitude of the maximum static friction. ,1,From the point of view of inertia,0,In my opinion,The two cannot be equal,That is, it is absolutely smooth,The sliding friction is 0,The static friction is not 0。

The reason should be analyzed from a microscopic point of view, when it is at rest, you want to move it molecules must stay away, but there is a gravitational force between them, hindering the distance, when moving, it is from far to near and then far, and the energy when close is canceled out when it is far away, so the maximum static friction is greater than the sliding friction. and static friction = molecular gravity + kinetic friction. 0,