Physics trivia questions about friction, questions about friction?

The generation of rolling friction is caused by the deformation at the contact point between the object and the plane. The object is pressed into the bearing surface by gravity, and at the same time it is also deformed by compression, so when rolling forward, the bearing surface in front of it is uplifted, which makes the point of action of the elastic force n of the support facing the object move forward from the lowest point, so the elastic force n and gravity g are not in a straight line, but form a force couple moment that hinders rolling, which is rolling friction. The magnitude of rolling friction is measured by the coupling moment, and is proportional to the positive pressure, and the proportional coefficient is called the rolling friction coefficient δ, which is numerically equivalent to the elastic force to the force arm of the center of mass of the rolling object, so it has a dimension of length; It is related to the material, hardness and other factors of the rolling object and the bearing surface, and has nothing to do with the radius.

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In other words, rolling friction is not a force, but the effect of static friction, elastic force, and other moments that hinder rotation, depending on the effect of the center of mass of the rolling object on the frictional moment.

As long as there is the ability to hinder rotation, there is a rotating kinetic energy (ek = 1 2j, j is the moment of inertia) to the direction of friction and heat generation, which naturally produces energy dissipation, so it will eventually stop, and it cannot always do uniform motion when the wheel rolls purely on the horizontal plane without the intervention of external force.

When the rotation is pure rolling, the static friction force does not do work, but it can be done with respect to the rotation and translation of the wheel, which are positive and negative, and the values are equal, and the role of static friction is to realize the energy conversion between the rotational kinetic energy and the translational kinetic energy. According to this law, the principle of judging the direction of static friction on a pure rolling wheel is obtained. The problem of determining the direction of static friction on several typical pure rolling wheels is analyzed. No.

Because f is less than the static friction, there is no relative motion between the slider and the plank; If the ground is smooth, the slider and the plank will accelerate together (the external force remains the same, the acceleration does not change).

First, let the mass of the block m, the board m, the title says that f is less than the maximum static friction, the two objects move together and there is no relative motion between the two objects, then the acceleration a=f (m+m); By the way, I'll make up for a greater than one, if f f, then the two objects will move together and the blocks on them will move with an acceleration of a=(f-f) m, and the plank will move with an acceleration of a=f m.