High School Physics, Emergency Call Master

Conservation of momentum should be followed, when the car pulls the first carriage, because the gap left by the hook of each carriage is d, so only the head of the car and the first carriage are forced, and the front and the first carriage are regarded as a whole and are not subject to external force, so the conservation of momentum should be followed. (If there are deficiencies, you can check the relevant information).

For this kind of problem, don't pay attention to momentum, but look at the change in velocity, because momentum is mv, m must not change, v change is known, and the change of momentum will be known. When the cable between the locomotive and the first carriage is straightened, the speed of the locomotive is to change, strictly speaking, it is to be reduced, the kinetic energy lost, note that it is kinetic energy, not momentum, and the kinetic energy lost is converted into the heat energy generated by friction when all carriages start. At this time, the locomotive continues to accelerate, because the heat energy is generated by friction, the amount of heat energy is only related to friction, and the friction is only related to the weight of the object, so when the locomotive continues to accelerate, when the kinetic energy is so large that there is a surplus in addition to the part that is converted into heat energy, the whole train will start.

So while pulling the first carriage, the velocity is reduced due to the loss of kinetic energy, so the momentum decreases and is not conserved.

You are a mistake in the concept of pulling not the first car but the whole car Just because the force seems to be the first car in motion, but in fact it is not, the whole car has begun to move It is the friction that causes the car behind to not move This topic is also the principle of force conduction, of course, it is not suitable for you to use it now, about reversing and starting this is the interaction of force to avoid violent friction and collision of objects at rest in the process of sudden acceleration, which is done for the safety of the train. As for the conservation of momentum, it can also be seen as the conservation of momentum in the case of sudden acceleration, which is a conceptual understanding, but since it is high school physics, it is not so complicated and in-depth, you can understand it more simply.

Since there are external forces, such as friction, the momentum is not conserved.

The function of reversing is 1. The front of the car moves first, and then pulls the body to move; 2. Turn static friction into dynamic friction, reduce friction, and better drag ......

Hope it helps.

When pulling, it can be considered that the time is relatively short. In other words, once there is a force between the locomotive head and the first train car, it can be regarded as a co-velocity in a very short time (the conservation of momentum can be applied, and the friction and tension f are ignored), and the purpose of friction and tension is to calculate the speed at this time, so it should be this model

Damn, this upstairs and downstairs, people are going to solve the problem, right, and strictly say that it should be reduced, if you don't count one for me, really, why don't you consider the change in the quality of the theory of relativity!!

If you want to calculate this problem, you can only calculate it according to conservation

Let's talk about the last question first, momentum is not conserved, because the condition for the conservation of momentum is that it is not subject to external forces, and the external force is zero or the external force is negligible, which is obviously not suitable here.

For the benefits of reversing, there are two aspects that can be considered, one is the hook, backwards and then start, it is easy to let the front of the car start first, to avoid the difficulty of starting all together, and the other consideration is to act on the resistance to do power.

Float by, reminiscing about your high school days.

I want to do this problem for you, but I don't have a diagram, and I don't know the structure and location of the hook.

(1) Do you know the displacement 80, the initial velocity 8, the acceleration, and the time?

Do you know the time, the initial velocity, and the final velocity?

2) When the speed of the vehicle is less than the speed of the debris flow, the speed of the debris flow and the speed of the vehicle has been decreasing. until the car accelerates to the same speed as the mudslide.

Therefore, as long as the debris flow is ensured before the vehicle reaches the desired speed in (1), the displacement does not exceed that of the vehicle, there will be no danger.

The critical condition is just equal to).