For the three problems of friction, be sure to explain the problems in parentheses

1 Static friction (because the contact point does not slide relative to the ground, if it slides, you will have to fall).

2 Basics! Definitely not! The object on the conveyor belt (upward transmission) has a downward movement trend, and the friction force is upward, otherwise the object will slide down, and the object moves upward with the conveyor belt, and the friction force is also upward, then it is in the same direction.

3 Harmful, damage bearings.

1. Static friction, because the sole and the ground do not slide relative to each other when they are in contact.

2 No, the tendency of the relative motion of the object should be taken into account. If the conveyor belt is at a constant speed, there is no friction, but if it accelerates, the object has a tendency to accelerate, and the friction force should provide acceleration, so the friction force is forward, and in the same way, if it is decelerated, the friction force is backward.

3.The axle and the wheel are fixed together, otherwise how can the axle drive the wheel. If you had to say yes, it would be static friction. It's definitely beneficial.

Maybe it's stationary, that's static friction. It may also be relative to sliding, which is sliding friction.

2 No, a moving conveyor belt is placed on a stationary object, and their friction gives the object an acceleration in the same direction as the object's velocity.

3. I don't understand your question.

1.Sliding friction, not stationary, when a person walks, the sole of the shoe is relatively sliding with the ground. 2.

Yes, the object placed on the conveyor belt has a tendency to slide relatively, but there is no movement, it belongs to static friction, the conveyor belt is forward, the friction force is backward, and vice versa. 3.Harmful, that will reduce the efficiency of the machinery, think about what the lubricant does, and you will understand (to reduce friction).

Do you know what is the maximum static friction and dynamic friction?

The kinetic friction has not changed.

If the maximum static friction is 40N, then the object does not move when subjected to a force less than or equal to 40N; When accelerating the motion, it is subject to dynamic friction, and it maintains f=un=umg, so the friction is not necessarily 20 or 30

If the maximum static friction is 30N, it also satisfies the question, understand??

In addition, the dynamic friction is slightly less than the maximum static friction.

If the box is accelerated by a force of 50n, what is the frictional force? ≥30n

The maximum static friction is about equal to the sliding friction, in the problem, when the thrust is not greater than the maximum static friction, the friction is equal to the thrust, and the static friction is exceeded, and the excess force accelerates the box.

First, the object of study is B, and it is assumed that A is stationary.

Second, the case of answer three is that the small block is not affected by an external force and remains relatively stationary with the belt.

Thirdly, the direction of the gravitational force and the resultant force of the supporting force on the small block must be in the direction of the inclined plane. The presence of friction is just a phenomenon that arises due to the action of gravity.

Object is B;

It is better to assume that A is at rest and judge the direction of motion of B relative to A, and the frictional force is the opposite direction of motion;

is downward; It should be not along the inclined plane, pay attention to the "gravity"! The gravity force is straight down, the friction force is along the inclined plane, and the resultant force is certainly not along the inclined plane.

The size of the sliding friction is only related to the size of the pressure and the roughness of the contact surface, the pulley that changes from the stationary horizontal conveyor belt to the conveyor belt rotates at a uniform speed in the counterclockwise direction, and the factors affecting the size of the friction force have not changed, so the magnitude of the friction force remains unchanged, and the speed at which the block still slides freely from point P is the same as before, the force is the reason for changing the motion state of the object, and the force received during the movement of the object remains unchanged, and the movement distance is the same as the original, so choose B

After passing through the rough stationary horizontal conveyor belt, it falls to the q point on the ground, indicating that the object is subjected to sliding friction throughout the whole process, so when the belt pulley of the conveyor belt rotates at a constant speed in the counterclockwise direction.

When the object slides up, it is still subject to sliding friction throughout the whole process, and the acceleration of the object remains unchanged twice, so, b. Still at Q o'clock.

Alternatively, the work done by the two sliding friction forces is the same, and the kinetic energy of the object changes by the same amount, so b

In the figure, the pressure does not change, and the dynamic friction factor does not change. The friction is constant. Because block kinetic energy is the same. According to the energy constancy. f (friction) s = ek (kinetic energy), the distance of the two movements is the same. So choose B

To expand on this: however, the second block will accelerate in reverse with the belt after reaching the q point. If the belt is long enough, accelerate to the same speed as the belt.

And then exercise together, there is another possibility. If the belt is short, so short that the blocks do not reach the same speed, they fly off the belt.

1. You have to distinguish the difference between static friction and dynamic friction. Only the dynamic friction will be positive, the greater the pressure, the greater the friction, i.e. f = n; The static friction is determined according to the balance of forces, holding the bottle in the hand is the static friction, and the friction is equal to the gravity of the bottle.

2. As the mass of the bottle increases, the gravity increases, and the friction is also greater. Or have you not distinguished the difference between kinetic friction and static friction, and you have not found that it will be laborious for you to hold a bottle full of water and an empty bottle for nothing?

3. This question is indeed an investigation of static friction.

Your first question, sliding friction is related to pressure and roughness, when the roughness must be, f=umg, but this formula is only applicable to sliding friction and not to static friction, which means that the bottle is balanced at this time, the combined external force is zero, and the bottle is stressed by static friction at this time, and the friction force is equal to gravity.

But if you clamp a beer bottle by hand, the bottle slides, this is the force of the bottle is sliding friction, when the force increases, the bottle no longer slides, because the friction is related to the pressure and roughness, when the roughness is constant, the pressure increases, the friction increases, so the bottle no longer slides.

The second question is the same, when the bottle is stationary in the hand, it is the static friction, not the sliding friction, do not confuse the concepts of static friction and sliding friction.

First of all, let's look at this question:

You hold the mouth of a beer bottle between your fingers and the bottle comes to rest, at which point the friction on the bottle is equal to the gravitational force of the bottle, right? Then increase the force on the mouth of the beer bottle, the bottle is still stationary, is this the friction on the bottle or equal to the gravity of the bottle, right?

1.Here comes my question:

Since friction is related to pressure and roughness.

Then here. Increased pressure.

Yes, so it is.

Friction increases.

However, it is balanced according to two forces.

The frictional force is constant.

Isn't that a paradox?

2.Continue, if the force of the finger gripping the bottle remains the same, and the bottle is still stationary when water is poured into the beer bottle, then the friction on the bottle will increase according to the balance of two forces.

However, because the friction and roughness are related to the pressure, and the pressure and roughness have not changed here, it is reasonable to say that they are constant.

Why? My own guess: Is it because the topic is static friction, and the concept in the book is about sliding friction?

Answer: Friction is related to roughness and pressure and refers to sliding friction rather than static friction. The magnitude of the static friction should be determined by the force balance, and the sliding friction can be solved by f= n.

Therefore, the above two problems can only be explained by force equilibrium, and there is sliding friction between objects when they are in relative motion or have a tendency to move relatively. So, your guess is correct. 】

Both increase friction by increasing the roughness of the surface.

When the car is driving with an acceleration of a1=10m s 2, the dynamic friction factor of the wooden box is the acceleration of the wooden box a2= g=5m s 2

The acceleration of the wooden block relative to the ground is a = 5m s 2, and the wooden block is in a deceleration motion a relative to the car'=-5m/s^2。

The wooden block will slowly move backwards, because the acceleration of the car is 10, if the wooden block wants to be equivalent to the rest of the car, the acceleration of the friction force must be 10, but the mass of the wooden block is 1kg, the friction factor is, and the acceleration is 5, so it will move like a backward motion.

The wooden block is subjected to a forward frictional force, a=gu=5 m s 2, which moves forward with respect to the ground at 5 m s 2 and backward with respect to the car. The car moves forward, and the block moves backwards relative to the car, so it is subjected to forward friction. When encountering this kind of problem, it is enough to analyze the force.

According to the basic knowledge of physics, the maximum static friction is slightly greater than the dynamic friction force, so use a force slightly greater than the static friction force to make the object have an acceleration to make it move, and then adjust the magnitude of the force to brake the friction force to make it balance, so that theoretically the originally stationary object can be made to move in a uniform straight line.

No, friction is only related to the coefficient of friction and positive pressure, unless the roughness changes, or the positive pressure changes, the friction will change. According to the horizontal plane of the topic, the positive pressure does not change, and the roughness of the contact surface does not change, and then it is judged that the motion state is sliding, OK unchanged

According to the known friction force, it is only related to the coefficient of friction and the positive pressure, and the positive pressure is unchanged, and the coefficient of friction is also unchanged, so it is still 12n···