How to calculate the maximum static friction of the object stack

1.

There is no calculation formula for both. The magnitude of static friction is related to the trend of relative motion between objects. The larger the trend is, the greater the static friction is. The specific value can be obtained through the force analysis of objects. The maximum static friction is slightly greater than the sliding friction, which is generally considered equal to the sliding friction. Sliding friction formula: F= μ N

Static friction:

when two objects in contact with each other have the tendency of relative sliding, but still keep relatively static, they act on each other to block the relative sliding resistance, which is called static sliding friction, or static friction for short, and is generally expressed by F. It is defined as the obstruction effect of another object on the surface of another object when one object has a relative motion trend. The magnitude of static friction can vary between 0 and Fmax (0 〈 f ≤ Fmax, where Fmax is the maximum static friction and is related to positive pressure). Generally, it should be solved by equilibrium condition or Newton's law according to the motion state of the object

sliding friction:

sliding friction is the force generated on the contact surface between two objects with relative sliding, which hinders their relative sliding. Generally speaking, it is the friction generated when one object slides on the surface of another object. It is always tangent to the contact surface and opposite to the relative motion direction of the object. The so-called relativity is still based on the reference frame of the object exerting the friction force. The value is approximately equal to the maximum static friction. When the roughness of the contact surface is the same, the sliding friction f is proportional to the positive pressure: F= μ N μ Is the dynamic friction coefficient)

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2. Pressure and friction are passive forces that do not appear when they are not necessary
this problem has nothing to do with the reference frame

what you said about the ground is very interesting. In fact, the ground is the earth. We don't consider the rotation of the earth. Then there are three objects in the universe, the earth, object a and object B. object a is sandwiched between the gravitational force of itself and the earth and the pressure of object B (which is also caused by the gravitational force). We pull object a (the earth does not follow the speed, such as using a rocket), The acceleration of a, B and the earth is the same: F / M total, the force of a on the earth is f / M total * m earth, obviously very close to 1F. The force of a on B is f / M total * m earth, which is obviously close to zero

the reason for the above explanation is that the earth is too heavy. Even though its total acceleration is close to zero, almost all the forces return to itself

but I'm a perfectionist. I have to consider another situation. In fact, when we pull objects, we always borrow from the fixed points of the earth. That is to say, while we are pulling a, we are pulling the earth in the opposite direction. These two forces are just balanced by the static friction between the earth and a and the static friction between a and the earth. So there's no acceleration at all, and there's no need to worry about rotation. At this time, the inequivalence between the B system and the ground is very obvious, and it is not who is heavy. If you put the borrowing point on B, the result is that there is friction between ab

so it has nothing to do with the reference frame.

3. The friction in static friction is called static friction. When the tangential external force increases graally but the two objects remain relatively static, the static friction increases with the increase of the tangential external force, but the increase of the static friction can only reach a certain maximum value, The maximum value of static friction is called "maximum static friction". The limit friction is expressed by Fmax. The maximum static friction is proportional to the positive pressure n between the contact surfaces of two objects, that is, F= μ N denotes the maximum static friction force by F, and n denotes the positive pressure, where the proportional constant μ It's called static friction coefficient. It's a value with no unit μ 1. The sliding friction is related to the material of the contact surface and the smoothness of its surface; It is related to the positive pressure between objects; But it has nothing to do with the size of contact area. Pay attention to the explanation of positive pressure. 2. The size of sliding friction can be calculated by formula F= μ N. Dynamic friction coefficient μ In fact, when the surface of two objects is very rough, the dynamic friction coefficient will be very large e to the staggered engagement on the contact surface; For a very smooth surface, especially a very clean surface, because molecular force plays a major role, the dynamic friction coefficient is greater. The smoother the surface is, the greater the dynamic friction coefficient is. However, in mechanics, it is often said that "the surface of an object is smooth". This is a formulation that ignores the friction force between objects. In fact, it is an idealized model, which has nothing to do with the above description The dynamic friction coefficient u is a physical quantity without unit, which can directly affect the motion state and force condition of the object. 4. The static friction increases with the increase of the external force, and is equal to the size of the external force. But the static friction cannot increase infinitely, but has a maximum value. When the external force exceeds this maximum value, the object will start to slide, This maximum static friction is called maximum static friction Fmax. 5. The direction of static friction is always tangent to the contact surface and opposite to the direction of relative motion trend of the object

4. Like sliding friction, the maximum static friction is related to the surface properties of materials and the pressure between materials. Generally speaking, the maximum static friction is greater than sliding friction. At present, there is no universal formula to calculate the maximum static friction of various materials

you can understand the maximum static friction between one or two materials through experiments. Experimental method: pull the object hard and record the tension in real time. You will find that the tension graally increases, and then suddenly slides down after reaching a certain peak value, when the object starts to move. The peak you record is the maximum static friction.

5. First of all, LZ is ambiguous. Friction is a kind of "secondary effect", that is, unless there is a tendency of relative sliding between objects, there will be no friction. As for LZ, if there is no tendency of relative sliding between the lower slider and the ground, such as being subjected to external force in the horizontal direction, no matter how many objects are put on it, There will be no friction.
when there is a trend of relative sliding, friction will occur. Generally speaking, it is considered that sliding friction is equal to the maximum static friction. The calculation formula is what you call F= μ N, μ Is the coefficient of friction, n is the positive pressure. But here f is "the maximum friction that can be proced". For example, if there is an external force= μ N. Then it can just push, if the external force & gt; μ N. Then the object will accelerate, but the friction will only last μ N. The third situation is easy to make mistakes, that is, when the external force & lt; μ When n, the friction force is equal to the external force instead of the external force μ N.
or you can understand that, μ N is like a cup. It has a fixed volume. When you pour water into the cup (applying external force) without exceeding the volume, there will be as much water as you pour. However, after exceeding the volume, no matter how much you pour, the cup can only hold these. The extra "water" will become acceleration. Adding blocks on the slider is equivalent to increasing the volume of the cup

6. ① When f ≤ μ When 1 (Mg + Mg) = 2n, a and B are relatively stationary and stationary to the ground, and F2 = f, that's right.
then when f continues to increase, the wood block and the iron block are relatively stationary and begin to slide together. At this time, (f-f2) / M = (f- μ 1 (Mg + Mg)) / (Mg + Mg)), and F2= μ In this stage, f is less than the solution value
and then f continues to increase, F2 will not change, F2 will not change= μ 2 * mg, the iron block of the board began to move relatively, and the acceleration of the board remained unchanged

7. If this is the case, object a is on the table and object B is on a. A. B receives horizontal left and right forces a and B respectively. Now calculate the static friction force: if a and B remain stationary, then the static friction force between a and B is only a, and the pull force B has no contribution to the friction force

for questioning, B and C see that the friction force = (10 + 10) * 0.15 = 3N, the maximum static friction force of C to the ground = (10 + 10 + 10) * 0.15 = 4.5n, which is greater than the friction force of B to C at this time and the tension force received by C, so C is stationary, the friction force of C to the ground = the tension force received, and the friction force of several B to C = 3N.

8. For example, if you press a wooden block on the wall by hand, no matter how much the pressure is, as long as the object is static, the static friction it receives is equal to gravity. Generally, the maximum static friction is approximately regarded as sliding friction, that is, f = UN. Obviously, when the pressure changes, the maximum static friction will change