Bilingual edition: English is preserved and Chinese follows each unit. Terminology uses the confirmed v20260916 glossary; automated semantic review remains traceable.
中英双语版：英文原文完整保留，中文紧随对应单元；术语采用 v20260916 确认表，自动语义审校结果可追溯。

Limitations on Newton's 2nd Law

牛顿第二定律的局限性

One of the best known relationships in physics is Newton's 2nd Law

物理学中最为人所知的关系之一是牛顿第二定律

but, though extremely useful, it is not a fundamental principle like the conservation laws. F must be the net external force, and even then a more fundamental relationship is

但尽管非常有用，它并不是像守恒定律那样的基本原理。F 必须是净外力，即使如此，更基本的关系仍然是

The net force should be defined as the rate of change of momentum; this becomes

净力应定义为动量的变化率；这成为

only if the mass is constant. Since the mass changes as the speed approaches the speed of light, F=ma is seen to be strictly a non-relativistic relationship which applies to the acceleration of constant mass objects. Despite these limitations, it is extremely useful for the prediction of motion under these constraints.

只有在质量保持不变的情况下才成立。由于当速度接近光速时质量会发生变化，因此F=ma被看作是严格非相对论的关系，适用于质量恒定物体的加速度。尽管有这些局限性，它在预测在这些约束下物体的运动时仍然非常有用。
 Index

Newton's laws concepts
牛顿运动定律的概念
 
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Limitations on Newton's 2nd Law

牛顿第二定律的局限性

One of the best known relationships in physics is Newton's 2nd Law

物理学中最为人所知的关系之一是牛顿第二定律

but, though extremely useful, it is not a fundamental principle like the conservation laws. F must be the net external force, and even then a more fundamental relationship is

但尽管非常有用，它并不是像守恒定律那样的基本原理。F 必须是净外力，即使如此，更基本的关系仍然是

The net force should be defined as the rate of change of momentum; this becomes

only if the mass is constant. Since the mass changes as the speed approaches the speed of light, F=ma is seen to be strictly a non-relativistic relationship which applies to the acceleration of constant mass objects. Despite these limitations, it is extremely useful for the prediction of motion under these constraints.

净力应被定义为动量的变化率；这只有在质量恒定时才成立。由于当速度接近光速时质量会发生变化，因此F=ma被看作是严格非相对论的关系，适用于质量恒定物体的加速度。尽管有这些局限性，它在预测在这些约束下物体的运动时仍然非常有用。
 Index

Newton's laws concepts
牛顿运动定律的概念
 
HyperPhysics***** Mechanics
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R Nave
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Variable Mass Applications

变质量应用

The generalization of Newton's 2nd Law to apply to variable mass systems takes the form

牛顿第二定律对可变质量系统的推广形式为

The term involving the derivative of the mass is responsible for the thrust in rocket propulsion and must be included in any problem where the mass changes.

涉及质量导数的项负责火箭推进中的推力，必须在任何质量发生变化的问题中包含该项。
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