这是我们第一次看到能量是如何涉及到守恒过程中。

This is the first time we see how energy is involved in the conservation process.

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若没有外力，作用于系统整体，在碰撞过程中，动量就是守恒的。

And if there is no net external force on the system as a whole during the collisions, then momentum is conserved.

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理论过程的模型是从第一原则得到的，这一原则利用物质和能量守恒以及基本法则来决定模型。

Models of theoretical processes are derived from first principles, using material and energy balances and fundamental laws to determine the model.

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