Physics static friction, is there friction when at rest

1 As shown in the figure: F is the direction of the force acting on A to the right, and the frictional force on the lower surface of B is seen as a whole as a whole, and the frictional force on the lower surface of B is to the left in the direction of Fb2, so it can be seen that the frictional force on the upper surface of B is to the right in the direction of FB1 (the resultant external force is zero). It can also be seen that the frictional force experienced by a is to the right in the direction of fa.

2 As shown in the figure: when the force acts on B to the right and B is stationary, B has a tendency to move to the right, then B has no tendency to move due to the friction force to the left, so A is not affected by friction.

Static friction can be analyzed in the following way: you first assume that the object is not subject to friction, analyze the relative motion of the object, and judge whether it is subject to friction (if it can remain relatively stationary without friction, it means that it is not subject to friction, otherwise it is subject to friction) and the direction of friction (the direction of friction is always opposite to the relative direction of motion).

1.Suppose the level of force experienced by a is to the right. If A is not subjected to friction by B and is only subjected to a horizontal force to the right, then A will move to the right relative to B, and now A remains at rest relative to B, indicating that A must be subjected to the friction force given to it by B to the horizontal to the left, and the friction force is balanced with the force to the right in order A to remain relatively stationary.

According to the effect of force is reciprocal, B gives the friction of A to the left horizontally, then A gives the friction of B to the right horizontally. Since the friction of B on A is horizontal to the right, it means that the friction on the ground on B is horizontal to the left (the analysis method is as above).

2.Suppose the level of force experienced by b is to the right. b is subjected to the friction of the ground horizontally to the left. There is no friction between A and B.

1. In the stationary state, when pushing A to the right without pushing, A is subjected to the static friction force of B to the left; b has a tendency to move to the right, and b is subjected to the leftward static friction of the ground.

2.In the stationary state, when B is pushed to the right without pushing, B is subjected to the leftward static friction of the ground; A has a tendency to move to the left, and B is subjected to the leftward static friction of A.

So let's assume that your force acts on the object, and if the object moves, the friction force it receives should be in which direction, and the static friction force is in which direction, take b in the figure as an example, suppose you push it from right to left, and it doesn't move, if we assume that it moves, it should move to the left, right, at this time, a brings it friction to the right, and if the lower surface hits the ground, the ground also gives it friction to the right, and then we can determine that b is subjected to the force we add, It is subjected to the rightward static friction exerted on it by A and the ground. Or you can think that it is a bit confusing to say that where the object wants to move, friction is blocking the direction of its movement.

1. Suppose the force received by A is to the right, and the analysis of A, when the force acts on A, it is stationary, and the force of A is balanced, so the bottom surface of A is subjected to the friction of B to A, and the direction is to the left, but the magnitude is equal, and the analysis of B, A has a friction force to the right of B, and B wants to remain stationary, then the ground gives B a friction force to the left, and for the overall analysis, the whole is subjected to a force, and to remain stationary, it is subject to the friction of the ground facing the whole, and the magnitude of this force is equal to the force acting on A, and the direction is opposite. The two friction forces of the contact surface of AB cancel out, so when drawing the friction force, only one left friction force is required on the contact surface of B with the ground.

2. A is only supported by gravity and B, and there is no friction, while B is supported by thrust and the friction of the ground, and the overall drawing method is the same as 1.

The difference between the two is: whether there is friction on the contact surface of AB is based on whether there is a movement trend. Fear of harming the plate.

This data is ** on the map, and the final result is subject to the latest data on the map.

Immobility is the balance of forces, A, B is a whole, the overall friction is given by the ground, and the force acting on A is the opposite, marked under B, in the AB separately, A balances, B balances, this is all analyzed.

Just grasp a few points 1Analyze the movement trend of an object 2Static friction is in the opposite direction of the object's tendency to motion.

3.The static friction of the ground against the object and the object on the object are related to the action and reaction forces.

What is the direction of the force.

Not necessarily. Not all objects at rest are subjected to frictional forces. When two objects that are in contact with each other and squeezed each other, when they are in relative motion or have a relative tendency to motion, a force is generated on the contact surface that hinders the relative motion or relative tendency of motion, and this force is called friction.

Not all objects at rest are subject to friction, for example, if an object is stationary on a horizontal tabletop, it is not subject to friction. Static friction is two objects that touch each other and squeeze, when one object has a tendency to move relative to another object, there will be a force on the contact surface of the two objects that hinders this relative tendency of motion, this force is static friction. For example, if an object is stationary on an inclined plane, it is subjected to the static friction given by the inclined plane.

Static friction refers to the absence of relative motion between two objects in contact with each other, such as a wooden block at rest on an inclined plane; Rolling friction is the rolling of two objects that are in contact with each other that generate frictional force, such as moving wheels and ground, bearings, etc.

Static friction means that there is friction, but there is no relative motion, such as when you retreat and you push a car without pushing, the static friction between the car and the ground is generated. If the car is pushed, there is rolling friction between the wheels and the ground.

Static friction is relative to kinetic friction, while sliding friction and rolling friction are relative.

1) When an object is moving on another object, a point on the object always does not leave the contact surface, and the friction generated between the object and the contact surface is sliding friction

When an object is moving on another object, a point on the object is not always on the contact surface, and the friction created between the object and the contact surface is rolling friction

2) Under the same conditions, the rolling friction is less than the sliding friction

Example: When writing with a fountain pen, the nib of the pen is always in contact with the surface of the paper and does not leave the surface of the paper, and the friction between the nib and the paper is sliding friction

When writing with a ballpoint pen, the point of contact between the ball on the nib and the surface of the paper is constantly changing, and the friction between the nib and the paper is rolling friction

Pushing heavy wooden boxes often padded logs under the wooden boxes, and rolling friction was used instead of sliding friction to reduce friction

Therefore, the key to judging sliding friction and rolling friction is to grasp whether the contact point has changed.

When two objects in contact with each other have a tendency to slide relative to each other, but remain relatively stationary, there is a resistance to the relative sliding between each other, and this resistance is called static friction. When we tried to push the table, it didn't push due to the low thrust. There is static friction between the ground and the table, once pushed.

That is, sliding friction, which China should pay special attention to, are two relatively stationary objects, and these two objects can be in motion. For example, a moving train car pushes a box on the floor without pushing it. There is also static friction between the box and the floor.

Yes, such as an object placed on an inclined plane, although it is stationary but has a tendency to move, there will be static friction.

1 Static friction.

When an object is tangent to another object along a contact surface.

Directional motion or relative motion.

There is a force between the contact surfaces of two objects that hinders their relative motion, and this force is called friction. If two objects that are in contact with each other and are relatively stationary, if they only have a relative sliding tendency under the action of external forces, but do not slide relatively, the force that prevents the relative sliding between their contact surfaces is called "static friction".

2 Conditions for the generation of static friction:

The contact surface is rough;

Two objects touching each other and squeezing each other;

There is a tendency for objects to move relative to each other.

2 Direction of static friction:

Target. Determination of the direction of the relative motion trend: Assuming that the contact surface is smooth and there is no friction, the relative motion direction of the object is looked at, and the direction of the relative motion trend is determined.

3 Magnitude of static friction:

As shown in Figure 2, the mass of the sand in the barrel is gradually increased, and the tensile force t on m gradually increases, because m is in a resting state, the static friction f is always balanced with the tensile force t, so the magnitude of the static friction gradually increases. When the mass of the sand increases to a certain level, it is found that the block suddenly moves relative to the horizontal plane, and the static friction between the block and the table no longer exists. From the above analysis, it can be seen that:

Maximum static friction: There is a maximum value of static friction, which is called maximum static friction. Its field core is equal to the minimum external force required to make the object just about to move.

The magnitude of the static friction force is not a fixed value, and the static friction force varies with the actual situation, and the magnitude is between zero and the maximum static friction fm. Its value can be determined by the state of motion of the object.

Static friction is very common. For example, the bottle in the hand and the Mao Song pen will not slip, which is the result of static friction and impulse. The ability to weave threads into cloth and sew cloth into clothes also depends on the effect of static friction between yarns. Static friction in production technology.

There are also many applications. For example, a belt conveyor relies on the static friction between the cargo and the conveyor belt to transport the cargo elsewhere.

Static friction is when the object is stationary, there is a tendency to move, and the surface that is in contact with it exerts a force that can stop the movement of the bird's core. At this point, the object can be either stationary or moving, (relative to the ground).

The static friction force varies with the change of other forces on the object, which can be found according to the balance of two forces, but cannot exceed the maximum static friction force, i.e. FM = static n (static is the static friction coefficient, n is the positive pressure of the object on the contact surface).

That is the static friction of the object on the inclined plane: the component of the object on the inclined plane and the static friction force of the object on the inclined plane in the direction of the inclined plane by gravity on the inclined plane remain at rest, and this component is mgsin

Static friction is produced by the interaction of two objects that are at rest with each other. The direction of the static friction force can be the same as the direction of the force or it can be the opposite direction of the force.

The formula for sliding friction is f=un where n is the pressure and n = mg when the ground is level

u is the sliding friction factor, which is related to the material.

Static friction is the phenomenon of pushing without moving, and the force f=f grinding. It is different when the push is, then f grinding = dynamic friction factor multiplied by mgIt should also be noted that the maximum static friction is slightly greater than the sliding friction.

The resultant force applied to the direction of the parallel contact surfaces, provided that it does not exceed the maximum static friction (f = n, i.e. the elastic force in the vertical direction of the friction factor).

Static friction is the force that arises when there is no relative motion between objects in order to keep the objects at rest or moving at a uniform speed.

Since the maximum static friction is 30N, the object is stationary at 28N.

The static friction is 28N

When the force is 35n, the object is in motion.

Therefore, when the force is reduced to 28N, the dynamic friction is 28*

When the magnitude of the force f gradually increases from zero to 28N, the magnitude of the frictional force on the wooden block is 28N

When the force is reduced from 35N to 28N, the frictional force on the block is 100*

The first question is 28n

The second question is 25n

Process. 1)28n<30n

So the block is not moving, and the force is balanced, f=f=28n

2)35n>30n

So the wooden blocks move.

for kinetic friction. f Mo =

There are many types of friction, according to the movement form of the friction pair, friction is divided into sliding friction and rolling friction, the former is the friction when two objects in contact with each other have relative sliding or relative sliding tendency, and the latter is the friction when two objects in contact with each other have relative rolling or relative rolling tendency;

According to the motion state of the friction pair, friction is divided into static friction and dynamic friction, the former is the friction when the two objects in contact with each other have a relative motion tendency and are in a critical state of rest, and the latter is the friction when two objects in contact with each other cross the critical state of rest and move relatively;

According to the lubrication state of the friction surface, friction can be divided into dry friction, boundary friction and fluid friction (see figure).

Friction can be further divided into external friction and internal friction. External friction refers to the friction between the surfaces of two objects when they move relative to each other; Internal friction refers to the friction between molecules inside an object. Dry friction and boundary friction belong to external friction, and fluid friction belongs to internal friction.

Dry friction friction when the surface of the friction pair is in direct contact and there is no friction in the presence of lubricant. Coulomb's law of friction is commonly used to express the relationship between the sliding friction force f, the normal force n and the friction coefficient f between friction surfaces: f = f n. The steel-to-steel f-value in the atmosphere is about.

Clean surfaces are accessible. According to the research of Bowden et al. in the United Kingdom, extremely clean metals (the gas on the surface is eliminated by heating, electronic bombardment, etc.) are bitten to death at the surface contact under the experimental conditions of high vacuum, and the f-value can be as high as 100. This extremely clean metal surface is immediately contaminated or oxidized as soon as it comes into contact with the atmosphere, resulting in a significant reduction in the f-number.

The methods for determining static friction include the tilt method and the traction method. Tilt method: put the object to be measured with gravity n on the inclined plane of the dual material, gradually increase the inclination angle of the inclined plane, and measure the inclination angle when the object begins to slide.

friction angle), from which the friction coefficient f=tg is obtained. Traction method: Set gravity to n

The object to be measured is placed on the plane of the dual material, pulled by force p, and the force f when the object begins to slide is the maximum static friction (at this time f = p), and the friction coefficient f = f n is obtained.

The roughness of the contact surface determines the amount of friction.

Dynamic friction can be measured on various types of testing machines (such as reciprocating friction and wear testing machines, rotating disc-pin friction and wear testing machines and four-ball friction testing machines), for which a mechanism for measuring friction or friction torque is installed on the testing machine, and the friction force is measured first, and then the friction coefficient is converted. Common measurement methods include lever method, spring method, and electrical measurement method. It is necessary to ensure that it is clean during the measurement, otherwise it will affect the measured friction.

Boundary friction and fluid friction.

The friction in the state of boundary lubrication is called boundary friction. The boundary coefficient of friction is lower than that of dry friction. The coefficient of friction in the boundary friction state depends only on the properties of the friction interface and the structure of the boundary film, and not on the viscosity of the lubricant.

Friction in the state of fluid lubrication is called fluid friction. This friction is caused by the viscosity of the fluid. The coefficient of friction is lower than that of dry friction and boundary friction (see Lubrication).