On the question of friction and equilibrium.

The first question: if the pull does not move, it is 5n, and if the pull moves, it needs to be calculated according to the motion state. 2:

Your teacher should be talking about the object at a constant velocity. 3: Skydiving is a complex sport, in the later stage of the fall can have resistance equal to gravity, descent does not mean that there is no force, but also may be affected by the balance force.

A 25N object has a maximum friction of 25N, right?

Pulling with a force of 5N, the frictional force is 5N

Objects do not move.

Push a car weighing 1000N with a force of 100N, and if the car does not move, the force on the car in the horizontal direction is balanced, and the force of the person to the car is 100N

It is equal to the friction force of the ground against the car, which is also 100N.

When skydiving, if the air resistance is equal to gravity, the resultant force received by the person is 0, and the motion state does not change, that is, the speed does not change. When parachuting, it must be opened after a while, when the parachute is opened, the person has a certain downward falling speed, and the resultant force is 0 after opening the parachute

The speed doesn't change, so it drops.

Friction is not the result of motion, as long as an object has relative motion or a tendency to move relatively, it has friction. So there is friction when only two objects come into contact and there is no smoothness between them (in fact, absolute smoothness does not exist).

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.

When moving in a straight line at a uniform speed, the tensile force and the frictional force exist at the same time. And equally.

I guess you're in junior high school.

Later, in high school, I will learn Newton's second law and the formula of friction.

The dynamic friction factor is the same, the pressure is reduced by half, and the sliding friction is also reduced by half (proportional)."If the mass of the object is reduced by half and the uniform linear motion continues, then the tensile force is 5N"That's right.

But if you pull with a force of 10n, the object will not move in a straight line with a uniform velocity, but will move in a straight line with a uniform acceleration.

It is said that the action of force is mutual, but it is true that there is a pulling force to have friction, and there is a causal relationship here.

The first, not 5n, the friction force is related to the coefficient of friction in addition to the mass of the object.

Second, the friction force is constant, the friction of the object is related to the mass of the object and the coefficient of friction, and when the two are determined, the friction force is determined.

Third, the pressure is halved, and the friction is also halved, because the coefficient of friction remains the same, and the amount of friction is only related to the mass, and it is proportional.

Fourth, it is difficult to say whether there is friction first or there is tension and then friction, from a macroscopic physical point of view, friction is generated when there is mass and a surface in contact with each other, and the tensile force is to make the friction macroscopic expression.

The method of finding sliding friction can generally be divided into two types: direct and indirect, the direct method is to use the dynamic friction factor between the contact surfaces multiplied by the positive pressure between the contact surfaces, and the direction is opposite to the direction of relative motion; The indirect method is determined by the force and the motion characteristics in the scope of the force and motion you are talking about, if it is a constant velocity, then it can be the equilibrium condition of the force, such as: tensile force = friction to solve; If it is not balanced, it can be solved by tensile force - friction force = combined external force = mass multiplied by acceleration; Think about your above question, the mass is halved, that is, the positive pressure is halved, and the roughness of the contact surface has not changed, the dynamic friction factor remains the same, and the friction force must be halved, no matter how much force is used to pull, as long as it is moving, the friction force must be 5N.

So the pull force is 5n?

Answer: friction = friction coefficient * pressure, in this question pressure = gravity, so the mass is reduced by half, and the friction force becomes half of the original = 5n, to do a uniform linear motion, the tensile force can only = 5n;

If I keep pulling with a force of 10N, then the friction is still 10N?

Answer: See above, the friction becomes half of the original = 5

The pressure is reduced by half and the friction is the same?

A: Friction is also halved;

In short, under the uniform linear motion, does the object have the existing friction and then the pulling force (the friction force is determined to know its pulling force), or is there a pulling force and then the friction force (the friction force is determined to know the friction force)?

Answer: Friction is determined first, and it is only related to mass and coefficient of friction; Whether an object can move in a straight line at a uniform speed depends on the amount of tensile force.

Junior Physics: Factors affecting sliding friction.

How is it not in a straight line? When the block moves at a constant speed, it is in equilibrium, and the frictional force is equal to the tensile force, in opposite directions, and acts on a straight line, which seems to be the content of Newton's third law.

Grandpa Newton told us that the net force of an object moving at a uniform speed is 0, so its tension and friction are a pair of balancing forces.

Because the wooden block moves at a constant speed, the resultant force is zero, and the wooden block is subjected to gravity and support (resultant force = 0), tensile force and friction force (net force = 0), that is, tensile force and friction force are a pair of balanced forces!

Are you confusing binary balance with equilibrium?

Can you elaborate on the problem?

There was no push, so Xiao Ming was still stationary.

The frictional force increases with the thrust until it is pushed, and when the object is pushed, the frictional force remains the same, even if the thrust increases.

When there is no push, the object is at rest, it is in a state of equilibrium, and the object is balanced by force, so the thrust is equal to the friction force, and when the thrust force becomes larger, the friction force will also increase, otherwise how to balance? How do you stay still?

The magnitude of sliding friction is only related to the magnitude of the pressure and the roughness of the contact surface, and has nothing to do with other factors, so the magnitude of the thrust does not affect the sliding friction.

There is no push, so Xiao Ming is still in a static state, because it is static friction, so the friction changes with the thrust, if it is pushed, it belongs to dynamic friction, and the friction will not change with the thrust at this time.

Feel free to ask.

Xiao Hong retreated Xiao Ming, did not push, Xiao Ming was in a static state before, (when the reference object remains unchanged) did not move, it was the original static state. Some people on the Internet say that the amount of friction changes with the size of the thrust, and it should be noted that it is static friction. The magnitude of sliding friction is independent of the thrust force.

First of all, analyze the force of a... Under pressure f....b gives support n ... Downward gravity ... Upward friction ...

Re-analysis of b force... Under pressure to the right ... The wall gives the power of support... Downward gravity ... A gives downward friction ... The upward friction given by the wall...

The concept of equilibrium force: several forces acting on the same point, vector sum is 0, is a set of equilibrium forces...

Because ab is still ... So both A and B must be subject to the Equilibrium ...

If you have to be right ... There are three pairs...

a, pressure, f, support, n... Gravity and friction...

Then B is under pressure from A. and the support that the wall gives ...

A is subjected to the pressure in the horizontal direction, and B is supported by the force.

Vertical gravity, and friction from b.

b is subjected to pressure from a in the horizontal direction, a support force from the wall in the vertical direction, and gravity from the static friction between a and the wall.

But isn't it said that a block of wood is also subject to the static friction of b? I think that when analyzing A, B also has a downward trend?

1. When the train starts, the force is unbalanced, and the power is greater than the friction force. So the train has to speed up. When the power and resistance of the train are balanced, the train starts moving at a uniform speed.

It is only when the state of motion changes that the driving force does not change, and at the beginning the driving force is greater than the frictional force, and the acceleration (as learned later) can be started.

He is balanced by force, and if he is still when he suddenly appears, he will always be still; If it is in motion when it suddenly appears, it will move in a straight line at a uniform speed.

1.When it first starts, it is unbalanced, and then the speed must be balanced.

2.In terms of Newton's law, that is, without the action of external forces, the object will be stationary or move at a uniform speed, the speed of the train is changed when it is driven, and the acceleration overcomes the frictional force, mv=ft, that is, the impulse is equal to the momentum, and t is the time to start to a constant speed, when the speed reaches a certain level, the train moves at a uniform speed, at this time, it is the equilibrium force.

Friction and what force is a pair of balanced forces, this problem should be analyzed on a case-by-case basis, for example, this question is balanced with thrust, and if it slides at a uniform speed on an inclined plane, it is the component of gravity, mainly for force analysis, as long as the net force on the surface balanced with the supporting surface is 0, then the friction force is balanced with the resultant force of other forces on this surface. If this surface is not moving in a straight line at uniform speed or at rest, then there is no equilibrium force.

Rally is not.

Not when the object is not moving in a straight line at a uniform speed.

120n At this time pull = frictional force.

East friction is opposite to the direction of motion of the object.

120n gravity = support so there's only thrust.

The sliding friction force on the ground is (120N), and the direction of this force (horizontal to east), the net force of the wooden box is 0

Reason: According to Newton's third law and the principle of equilibrium, in the state of constant velocity, the resultant force is 0, and in the horizontal direction the net force is 0, then there is f=f=120n

120 East 0 In any case, the force on the object in any direction is balanced, in this problem, the force in the horizontal direction is zero, and in the horizontal direction only the thrust and friction force are said to be 120, the friction force should be opposite to the thrust, forming a state of equilibrium, so it is eastward Since the object is at a constant velocity, the net force on it is zero, and the force in any direction is zero, otherwise there will be acceleration!!

This should be combined with specific problems, such as when you push something, it becomes a drag against your push!

The frictional force of the three important forces – static friction.