碰撞小记(Collision notes)

碰撞小记

2022.5.15

2022.5.15

未经特殊说明, 碰撞的两个物体重量相等

Part I

我始终没有发现动能究竟有什么大用,直到高二的时候学弹性碰撞, 我才发现我对碰撞始终是有误解的, 我一直认为两个物体碰撞, 速度大的物体对速度小的施力,速度大的减速,速度小的加速, 直到共速。共速后相对静止,没有作用力, 所以始终保持共速。

Part II

但事实并非如此,两个铁球相撞,很神奇,速度直接就交换了!

那确实有些神秘啊, 究竟是怎么一回事呢?物理书上这样说, 因为碰撞之后动量要守恒, 动能也要守恒,因为没有转化为内能。然后将这两个方程联立, 就解出了速度。很神秘啊。

作为一个力学体系里没有动能的人,我肯定不能如同这种说法。首先就算我承认动能守恒好吧,但是就和动量一样, 动能守恒是一种结果,而非原因。我们知道最后是动量守恒的, 但是我们不能据此推断速度, 逻辑顺序不能反。

那么物体就来了, 为什么不是共速, 而是交换呢?这两者都满足动量守恒, 但为什么是后者呢?为什么共速之后还有力的作用呢?难道是我力学体系错了吗, 动能这个玩意其实是有大用的?

由于各种事情, 我直到今天才能够好好的思考这个问题。但是必须明确的一点是, 我的体系绝对正确!绝 对 正 确!

Part III

这算是一个误解吗?但是我当时和他们讨论的时候他们都是这么
认为的呢。或许是老师讲错了, 或许是教材瞎搞, 反正就是一个普遍的误解 :弹性碰撞是发生在刚体上的, 即弹性碰撞的过程中没有形变。后来我发现所谓弹性碰撞, 只要碰撞物体发生的形变能够完全恢复就可以了。事实上这也大概是官方的定义把。

Part IV

进入正题。其实结论很简单吧,毕竟问题在于问题本身。

所谓弹性碰撞, 有这么几个过程同时发生。

其一, 两物体间产生作用力,速度趋于平衡,直至共速。
其二, 产生的形变逐渐恢复,产生的力与之前发生的作用力相同。

故共速。

先写到这, 下次想明白点再好好写写。

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Collision notes

2022.5.15

2022.5.15

Without special instructions, the two objects colliding have the same weight

Part I

I never found out what great use kinetic energy can do. Until I learned elastic collision in my sophomore year of senior high school, I found that I had always misunderstood collision. I always thought that two objects collided. The object with high speed exerted force on the object with low speed, decelerated the object with high speed, accelerated the object with low speed, and then reached the same speed. After the common speed, it is relatively stationary and has no force, so it always maintains the common speed.

Part II

But this is not the case. It’s amazing that two iron balls collide, and the speed is directly exchanged!

It’s really mysterious. What’s going on? According to the physics book, momentum and kinetic energy should be conserved after collision, because they are not converted into internal energy. Then the two equations are combined to solve the velocity. It’s mysterious.

As a person without kinetic energy in the mechanical system, I certainly can’t say it like this. First of all, even if I admit the conservation of kinetic energy, well, just like momentum, the conservation of kinetic energy is a result, not a cause. We know that momentum is conserved in the end, but we can’t infer speed from it, and the logical order can’t be reversed.

So the object is coming. Why not co velocity, but exchange? Both satisfy the conservation of momentum, but why the latter? Why does the common speed still play a powerful role? Is my mechanical system wrong? Kinetic energy is actually of great use?

Due to various things, I can’t think about this problem until today. But it must be clear that my system is absolutely correct! Absolutely right!

Part III

Is this a misunderstanding? But that’s what they did when I discussed it with them
Think so. Perhaps the teacher is wrong, or the teaching materials are fooling around. Anyway, it is a common misunderstanding: elastic collision occurs on a rigid body, that is, there is no deformation in the process of elastic collision. Later, I found that the so-called elastic collision, as long as the deformation of the colliding object can be completely restored. In fact, this is probably the official definition.

Part IV

Get to the point. In fact, the conclusion is very simple. After all, the problem lies in the problem itself.

The so-called elastic collision occurs simultaneously.

First, there is a force between the two objects, and the speed tends to balance until the common speed.
Second, the resulting deformation gradually recovers, and the resulting force is the same as the previous force.

Therefore, common speed.

Write this first and write it well next time.