How to learn physics well? Like friction, it s hard for me to discern its direction Ask a master to

The nature of friction is to hinder the relative motion or relative motion trend between two objects in contact with each other, so when studying the direction of friction, it is necessary to take the object in contact with the research object as a reference, and judge whether there is a relative motion or relative motion trend between the research object and the reference object If there is, the frictional force experienced by the studied object must be in the opposite direction of this relative motion or relative motion trend

The direction of friction has nothing to do with the actual direction of motion of the object.

According to what I said above, it is easy to judge the direction of sliding friction, and the direction of static friction can also be judged by the following method:

1. Hypothetical method.

The condition for static friction is that there is a tendency for objects to slide relative to each other, and the direction of static friction is opposite to the relative tendency of slide between objects. Therefore, in order to determine the direction of static friction, it is necessary to determine the direction of the tendency of the relative sliding between objects.

The so-called hypothetical method is to determine the relative sliding direction of the two objects by assuming that the contact surface is smooth, and thus determining the direction of static friction.

2. Use equilibrium conditions to judge.

The relative sliding trend between some objects is not obvious, and it is not easy to judge the direction of static friction by the hypothetical method, but if the object is in equilibrium, the object equilibrium condition can be used to judge whether the object is subjected to static friction and what is its direction.

3. Judged by the state of movement.

The direction of some static friction is closely related to the state of motion of the object, so the direction of the frictional force experienced by the object can only be judged by the state of motion of the object.

4. There are two possible judgments on the direction of static friction.

There are two ways to deal with some static friction forces in two possible directions in the same problem: one is to discuss them in two directions, and the other is to solve them with absolute inequalities.

You can find some example problems to practice!

The friction force should be judged, and the grasp is relative.

It is the opposite direction of relative motion.

Specifically, the unstudied object is assumed to be stationary, and the direction of motion of the object is towards **.

The frictional force is the opposite direction.

Set to Still!!

The direction of friction is opposite to the direction of motion.

It's a very simple way to do it, and it's a very good idea, where you assume that the objects are smooth from one to the other, and then you move one object and you see the other object in motion, backwards, and the friction forward.

The direction of friction is the opposite direction of the object's movement.

If the object is stationary and has a tendency to move horizontally to the right (the object is not moving), then the direction of the static friction force on it is horizontally to the left.

Sliding friction is the opposite direction of motion of the object.

The direction of friction is opposite to the direction of motion or relative to the trend of motion.

There are assumptions and equilibrium methods to determine the direction of friction.

The hypothesis method first assumes that the contact surface of two objects is smooth, and if the two objects do not move relatively, it means that they have no relative motion tendency and no static friction. If two objects move relative to each other, it means that they originally had a tendency to move relatively. In the equilibrium method, the direction of static friction can be determined according to the two-force equilibrium conditions.

The frictional force must be hindering the relative motion (sliding friction) or the relative motion trend (static friction), so it must be opposite to the relative motion or relative motion tendency, but not necessarily the opposite direction of motion; For example, when a person walks, the sole of the shoe rubs backwards, and the force between the objects is mutual, and the ground gives the sole a forward friction force.

Judging by the state of motion, when the friction force is in the same direction as the object's motion, the object accelerates; When the frictional force is opposite to the direction of motion of the object, the object decelerates its motion.

There are two reasons for the friction between solid surfaces: the mutual attraction of atoms and molecules on the solid surface (the energy demand for the recombination of sensitive chemical bonds, the gluing force) and the resistance to each other due to the roughness of the surface between them.

Characteristic properties of friction:

The biggest distinction within friction is between static friction and other frictional forces. Some people believe that static friction should not actually be counted as friction. All other frictional forces are related to dissipation: it reduces the relative velocity of objects rubbing against each other and converts mechanical energy into heat.

The friction between solid surfaces is divided into sliding friction, rolling friction, static friction, rolling friction and rotational friction. In engineering, lubricants are used to reduce friction. If two surfaces rubbing against each other are separated by a layer of liquid, then liquid friction can occur between them, and mixed friction can also occur if the liquid is not completely isolated.

An object moving relative to a fluid is subjected to resistance. This resistance is in the opposite direction of its movement. In the case of laminar flow this resistance is proportional to its velocity, and in turbulent flow this drag force is proportional to the square of its velocity.

Sometimes an object is subjected to both drag and friction, such as a car that is in motion by both the resistance of the air and the rolling friction of its tires.

1. Direction:

Hypothetical method. Firstly, if the contact surface of two objects is smooth, if the two objects do not move relatively, it means that they have no relative motion tendency and no static friction. If two objects move relative to each other, it means that they originally had a tendency to move relatively.

And it turns out that the direction of the relative movement trend is the same as that of the relative movement when the contact surface is assumed to be smooth. The direction of the static friction is then determined according to the direction of the static friction that is opposite to the direction of the relative motion trend of the object.

Balancing method. The direction of the static friction can be determined according to the two-force equilibrium condition.

Second, size. The magnitude of sliding friction.

The calculation is performed using the formula f= fn, where fn is the positive pressure of the object, which is not necessarily equal to the gravitational force of the object, and may not even be related to gravity. Or according to the state of motion of the object, the equilibrium condition or Newton's laws can be used to solve the problem.

The magnitude of static friction.

The magnitude of the static friction can vary between 0 and f max, and should generally be solved by equilibrium conditions or Newton's laws depending on the state of motion of the object.

The nature of friction is to hinder the relative motion or relative motion trend between two objects in contact with each other, so when studying the direction of friction, it is necessary to take the object in contact with the research object as a reference, and judge whether there is a relative motion or relative motion trend between the research object and the reference object If there is, the frictional force experienced by the studied object must be in the opposite direction of this relative motion or relative motion trend

When we judge friction, it is easy to think that it is the opposite direction of motion, then it is wrong, the direction of friction is opposite to the direction of relative motion or the direction of relative motion trend. When two objects rub against each other together, it is enough to see how one of the objects moves relative to the other. In short, it is opposite to the direction of relative motion.

Relative motion is the direction of motion of an object relative to another object, for example, if a wooden block moves to the right on a stationary table, then the direction of motion of the wooden block relative to the table is to the right. Relative motion tendency is the tendency of an object to move relative to another object. For example, if a wooden block is standing still on an inclined table, then it has a tendency to roll down relative to the table.

It can be either hypothetical or two-force equilibrium.

The direction of friction is always opposite to the direction of relative motion.

The easiest way to do it: assume that the contact surface is absolutely smooth.

Magnitude of static friction:

In general, the magnitude of static friction changes with the change of external force, and its specific value must be determined by the actual situation in combination with the state of motion.

I don't know the bureau cd fish bones according to this hair.

The specific method of judging the direction of friction:

1 Take advantage of the conditions that generate friction first:

Two objects are in contact with each other;

There is an elastic force between the contact surfaces of the two objects;

The contact surface is not smooth;

There is a tendency for two objects to move relative to each other or relative to each other, and it is determined whether friction is present or not.

Frictional force. 2. Select the object of friction as the reference, judge the relative direction of motion between the two objects or the trend direction of relative motion, and then judge the direction of the corresponding sliding friction or static friction. Specifically:

If there is friction between two objects, it is necessary to judge the direction of friction force experienced by A, and judge the direction of motion or trend of motion of A (the object subjected to force) with B (the object applying force) as the reference; The direction of the frictional force experienced by A is opposite to this direction; Be careful not to use the third object as a reference, especially for problems such as belt drives, if you use a third party as a reference, it is easy to make mistakes. The same is true for determining the direction of the frictional force experienced by B.

Frictional force. 3 Based on the above analysis, we can summarize the "four-step method" for determining the direction of friction as follows:

1) Find out the object where the friction is applied.

2) Select the object to which the force is applied as the reference.

3) Judge the relative direction of motion or the direction of the relative movement trend of the stressed object.

4) Use "opposite" to determine the direction of friction.

The direction of friction is opposite to the relative direction of motion of the object (it is relative motion, not motion), so it is necessary to find the relative direction of motion of the object in the judgment process, so that the direction of friction is easy to judge.

There are 3 conditions for friction to occur: 1The objects are in contact with each other and the contact surfaces are rough and rough. 2: Objects squeeze each other, that is, there is elasticity. 3: There is a tendency for objects to move relative to each other or to move in relative ways.

The direction of friction can be the same or opposite to the direction of motion, o It cannot be considered that the direction of friction must be opposite to the direction of motion. We need to start from the concept of judging the direction of friction: friction always hinders the relative motion or relative motion tendency of an object, hinders the relative motion tendency of an object when it is static friction, and hinders the relative motion of an object when it is sliding friction.

It is believed that when we analyze friction, we need to judge the relative motion trend and relative motion.

Judgment direction: the direction of static friction is opposite to the trend of its relative motion; Kinetic friction is the opposite direction of motion.

Magnitude: static friction, there is a concept of maximum static friction, you can take a good look at the book, in general, the size of the static friction can not be obtained alone, but according to the combined external force is 0, through other known efforts to obtain static friction;

The magnitude of the dynamic friction force is the formula mun, where mu represents the coefficient of friction and n represents the elastic force between the friction surfaces.

There is also something wrong with what is said upstairs, people walk, the feet and the ground are indeed static friction, the direction of the friction between the feet and the ground is forward, which is true, that is because the feet have a tendency to slide backwards relative to the ground, precisely therefore, due to the action and reaction force, the static friction force provides the power for people to walk forward.

Analysis: The direction of friction between the front and rear wheels when a bicycle is pushed forward: Imagine that if neither the front and rear wheels of the bicycle can rotate, the wheels are forward relative to the ground, so the direction of friction should be backwards.

When a person rides on a bicycle and advances, the direction of the friction force on the front and rear wheels: the rear wheel has a tendency to rotate backwards relative to the ground under the action of the human pedal force, so the direction of the friction force on the rear wheel is forward; The front wheels are in the same situation as if the car is pushed forward, so the friction direction is backwards.

Hello! Hehe, this is simple, push the bike friction backwards, ride the bike, friction forward.

Typing isn't easy, oh!

Friction is a relatively troublesome force in mechanics because it has a lot of content and a wide range in the college entrance examination. Through visual examples, we can understand the method of judging the direction of friction and the calculation of size.

1.The object is stationary in the inclined plane, because it is stationary, the dynamic friction force is 0, and the static friction force is the component of the object's gravity along the inclined plane downward, because the object has a downward movement trend along the inclined plane, so the direction of static friction is along the inclined plane upward.

2.When two objects are stacked on an inclined plane, if they are not moving, the dynamic friction is 0, static friction: the lower surface of the object below is subjected to the static friction along the inclined surface; The upper surface of the object below experiences a static friction force downward along the inclined surface; The lower surface of the object above is subjected to static friction along the inclined surface.

Summary: In fact, force and movement are inseparable, when encountering this kind of problem, first analyze the force, then analyze the movement trend, and solve the problem calmly, the problem is not big.

Your topic should be incomplete, and if there are no other forces, it will definitely be subject to static friction. If the object is subjected to other forces, the frictional force of the object is likely to be zero.

2. It's the same problem, if two objects are stationary on the inclined plane, there will definitely be friction between the objects, but when the inclined plane is smooth, the friction between the two is 0. (Newton's second law and the holistic and segregated methods are used for this purpose).

3. Look at the research object!! Whoever studies is subjected to the force given to him by another object! The direction is judged by the "hypothetical smoothness method", that is, if two objects are smooth and the object of study will slide, the opposite direction of their relative motion is the direction of friction.