The question of conservation of momentum, about the question of conservation of momentum

To be precise, momentum is not necessarily related to kinetic energy, momentum is the accumulation of force over time, and is generally in the collision problem and the collision of microscopic particles. Between the actions of both, the frictional force causes the momentum of the wooden block to decrease, and its kinetic energy decreases due to the action displacement of the wooden block due to the action of the force. Its energy is converted into frictional workmanship in the form of internal energy, and internal energy.

The frictional force acting on the plank leads to an increase in its kinetic energy. According to the definition of momentum, p=f*t acts for the same time, and the frictional force of the two is a relative force, so the increase in momentum in one part is equal to the decrease in the momentum in the other part. Hence momentum is conserved.

Whereas, energy is lost in the form of internal energy.

1) When set at the highest point on the inclined plane, the object and the inclined plane have the same velocity u.

Conservation of momentum: mv0=3mu u=v0 3

The loss of mechanical energy is converted into overcoming friction to do work to generate heat:

So f=mv0 3L-mgH L

2) Conservation by momentum, and finally the velocity is still u at rest

mgh=f（l-s）

So s=l-ghl (v0 3-gh)=v0 l (v0 -3gh).

I hope it helps you, if you have any questions, please ask o( o haha

In the past, it was very complicated to have the conservation of momentum, but now the conservation of momentum is no longer examined, and the initial velocity is calculated with a collision at most, and the focus is later, so you don't have to think too much about the conservation of momentum.

As a whole, according to the momentum theorem, the momentum of this whole is conserved without the action of external forces, right? If you still don't understand it, the content of the momentum theorem is well understood, as for energy, due to friction, kinetic energy is reduced, the reduced energy is converted into heat, and the total energy is conserved

Conserved, the friction force is the internal force, the momentum of the whole system is conserved, and the friction force does positive work on the trolley and negative work on the block. The conservation of momentum is either by internal force or without force, by external force and is zero.

In the absence of an external force or zero external force, momentum is conserved and has no relation to energy.

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Problem description: The ball is tied to the car with a non-extendable rope. When the ball is released from rest at rest in the position where the water of the suspended line collides with the water, the mechanical energy of the body composed of the ball and the trolley is conserved, but the momentum of the system is not conserved, because the resultant external force of the system is not equal to zero. But in the horizontal direction, the net external force on the system is zero.

Why? Why is the horizontal resultant external force 0? Can anyone explain the specifics?

Than the clear trace such as force analysis or something?

Analysis: The research object should be clear, whether the momentum is conserved or not is the study of the system composed of the ball and the car rather than the ball, the horizontal direction of the ball is indeed subject to tension, and the pull force also does work on the ball, the mechanical energy of the ball is not conserved, but for the system, the pull force is the internal force, and when the ball does positive work, it will definitely do negative work on the car, and does not do work on the whole, so the mechanical energy of the system is conserved.

Continuing to discuss the system, moving on a smooth horizontal plane, the horizontal direction is not subject to external forces, the vertical direction of the ball has a velocity change, there is overweight weightlessness, the vertical direction of the combined external force must not be zero, so only the horizontal direction momentum is conserved.

First of all, the horizontal plane you mentioned must also be smooth.

In fact, it takes a long time to comprehend this thing to be thorough. However, I can tell you the most basic trick, the so-called conservation of momentum of the system means that the momentum of the system is constant, and two methods can be determined: 1. The system is not subject to external force in a certain direction or the net external force is zero, and the momentum is conserved.

2 Calculate yourself to see if the total momentum of the system remains constant at any given moment. The first method is more commonly used. Smooth horizontal plane, no friction, so the system is definitely not subject to external forces in the horizontal direction and is conserved.

In the vertical direction, the inclined plane is smooth, and the wooden block must accelerate the oblique downward slide, so there must be a vertical downward acceleration, while the triangle has no vertical acceleration, so the vertical direction of the overall system has acceleration, that is, the vertical direction of the external force is not zero, so the vertical momentum of the system is increasing and is not conserved. Now I'll give you a calculation of the resultant external force in the vertical direction of the system. Assuming that the inclination angle of the inclined plane is x, the mass of the wooden block is m, and the triangular mass is m, then the pressure direction of the wooden block on the triangle is perpendicular to the contact surface, and the positive pressure is equal to mg*cos x, and the vertical component of this force should also be in *cos x, so the vertical downward pressure of the wooden block on the triangle is mg*cos x squared, and the analysis of the triangle can be seen that the vertical direction is subject to zero external force, so the support force given by the ground to him is the sum of the pressure component of the wooden block on its vertical downward pressure and the weight of the triangle itself, that is, (mg* cos x squared + mg), the gravity of the system is (mg+mg), obviously, x is not 0 degrees, then the gravity of the system is greater than the supporting force of the ground facing the triangle.

The resultant external force is the difference between these 2 forces, which is mg (1-cos x squared) and is oriented downward. The momentum of the system in the vertical direction is not conserved.

Well, I'll tell you that horizontally, the small wooden block moves to the left, giving the force to the right to the large wooden block, and the large wooden block moves to the right, so the system of the two of them together is equivalent to 'conservation' (which can be understood as cancellation). Vertically, there are only small wooden blocks facing downward, and large wooden blocks are not moving, so the whole thing is downward. So only the horizontal direction is conserved.

There's gravity at work in the vertical direction... I'm not very good at physics.

Momentum in the vertical direction is not conserved The other two problems are a little confusing I just know the surface I hope it can help you!

There is no change in momentum in the horizontal direction, but the transition between objects is called conservation of momentum in the horizontal direction, and the vertical direction is not conserved, because before and after the movement, the momentum in the vertical direction has a certain value and becomes zero, the momentum disappears, and the momentum of the system is not conserved.

Why kinetic energy cannot be conserved can actually be compared with why momentum is conserved.

Why is momentum conserved in isolated systems? Along these lines: what is the momentum theorem for a single particle?

What is the momentum theorem for mass systems? The momentum theorem of the particle system is to + the momentum theorem of each particle in the system. Because it is an isolated system, all the forces are internal forces, a pair of action and reaction forces are equal to each other at any time, and the impulse is the force multiplied by time, so the impulse that causes all the action and reaction forces is an equal and large reversal, so that the total momentum of the system does not change.

In this way, the kinetic energy theorem of a single particle is that the work of the resultant force is equal to the change of kinetic energy, and if the kinetic energy theorem of the particle system is to be derived, the kinetic energy theorem of each particle must be +. Therefore, the key to whether the kinetic energy of an isolated system can be conserved lies in whether the work of a pair of action and reaction forces can be cancelled.

So you have to clarify this problem through calculations, the key calculation is really just to calculate the sum of the work done by a pair of acting and reaction forces. Remember that the definition of work done is the force point multiplied by the displacement, assuming that two particles 1 and 2, the displacement of the force point of 1 to 2 + the displacement of the force point of 2 to 1 of 2 to 1, is the result zero?

1,.First of all, you have to make it clear that energy is conserved, and energy does not arise out of thin air or disappear out of thin air.

2.Kinetic energy is not equal to energy. Energy includes many such as kinetic energy, potential energy, thermal energy, mechanical wave energy (such as sound) and so on... A lot.

3.Kinetic energy is only related to the mass and velocity of the object.

4.For example, if two objects collide, we know that momentum is always conserved.

If the two objects are rigid, that is, there will be no deformation, at this time, before and after the collision, the kinetic energy is conserved, because the knowledge speed of the two objects changes relatively, and the kinetic energy is still fully converted into kinetic energy. If two objects are not rigid (objects in the real world are not rigid), then after the two objects collide, the kinetic energy is not conserved, because, when they collide, part of the kinetic energy is converted into other energy, such as heat energy (two balls smashed together for ten minutes, you see whether it is hot or not), kinetic energy can also be converted into mechanical wave energy (two balls collide, there must be a sound).

5.There is no need for formulas, algorithms to prove this problem, it is too complicated. If you are doing a physics problem, you must make sure that the object of study is rigid or not.

In fact, you don't need to analyze it mathematically, it may be that you don't understand the system defined by these two conservations.

Let's start with the non-conservation of kinetic energy, if there are many objects, they also have a lot of interaction forces, define them as a system, this system has no external force f, that is, the force outside the system, the force in the system is not counted, then the system is conserved, and the energy is in this system, neither increasing nor decreasing. However, once there is a strong action outside the system, the energy balance of the system is destroyed, and it is not guarded.

When talking about the conservation of momentum, note that the external force f*t is also a momentum, and the external force f is also included in the system that defines the conservation of momentum, that is to say, this system is different from the system that says above that kinetic energy is not conserved, and the two systems are different

Okay, understand.

You'd better draw a picture!

It should be that regardless of whether there is friction between m and m, choose [ b c ] Why a is not right is because gravity has done work on this system, and the system has been subjected to an external force, so the momentum is not conserved, (this is a concept).

Why d is not true, because the direction of the resultant external force is the direction of the motion of the object, and this system does not move downward. (This is also a concept).

The frictional force between m and m is not an external force but an internal force. Well, the one that follows your meridians)] So, you have to think more about the concept when you do the problem!

The topic is not complete - is the surface smooth?

The momentum of the ball is not conserved, because the elastic force of the wall has an impulse to it, which was originally MV, and after a complete elastic collision, it becomes a reverse MV, in which the change in momentum is 2MV, that is, the elastic force makes the original MV gone, and gives a backward MV.

According to the momentum theorem: ft=2mv

Hope these are helpful to you.

Momentum is a vector quantity, although the velocity of ** is the same, but it changes direction, so it is not conserved.

During Collision: Why is momentum conserved?

Let me explain it for you: I think you should analyze it in terms of the condition of conservation of momentum, and the conditions are generally divided into three cases:

In the first case, the resultant external force on the system is 0

The second case: the resultant external force is zero in a certain direction, and the momentum is conserved in this direction The third case: such as collision, the resultant external force is not zero, but because the external force is relatively small compared with the internal force at the time of collision, it can be ignored and ignored, and the system motion is conserved.

Therefore, we believe that the momentum of the system is generally conserved during the collision.

As for why momentum is conserved in these cases, there is proof in the textbook, so I will not explain it here.

Theoretically. Starting from Newton's second law, f=ma doesn't need to be explained.

Further evolution f=m* v t then v is the amount of change in velocity in time t.

In the process of collision with ft=m v, due to Newton's third law, it can be seen that the force of action and reaction of two objects is equal and the time is the same, then the change of momentum is the same, then one increases by so much, and the other decreases by so much, in short, the total momentum of the system remains unchanged, and the momentum is conserved. I don't know if this explanation is possible, the resultant external force under the conservation condition is 0 or not subject to external force, and mechanical energy and momentum are completely two categories, one is energy and the other is mechanics, so there will be a difference.

The understanding of energy is ok The condition for the balance of momentum is that there is no external force in this direction ab Two objects collide According to the force of action and reaction The two objects are subjected to opposite forces The time is the same, so the impulse ft is opposite and it is canceled out If it is not clear, send me a message.