📚 Conservation of Momentum in Explosions and Collisions | 爆炸与碰撞中的动量守恒
Momentum is one of the most powerful tools in physics. In closed systems, momentum is conserved during collisions and explosions, even when kinetic energy is not. This article explains the principle, the different types of collisions, and how to solve CIE A-Level problems with confidence.
动量是物理学中最强大的工具之一。在封闭系统中，即使动能不守恒，动量在碰撞和爆炸过程中也总是守恒的。本文将解释这一定律、碰撞的不同类型，并指导你自信地解决 CIE A-Level 相关题目。
1. The Law of Conservation of Momentum | 动量守恒定律
Momentum is defined as the product of mass and velocity: p = m v. Since velocity is a vector, momentum is also a vector with direction.
动量定义为质量与速度的乘积：p = m v。由于速度是矢量，动量也是具有方向的矢量。
For a system with no external force, the total momentum before an event equals the total momentum after the event:
对于不受外力的系统，事件发生前后的总动量相等：
m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
Here u represents initial velocity and v represents final velocity. The law follows from Newton's third law: during a collision, the forces between two objects are equal and opposite, so the impulses are equal and opposite, and the total momentum remains constant.
这里 u 表示初速度，v 表示末速度。该定律由牛顿第三定律推导：碰撞中两个物体之间的力等大反向，因此冲量等大反向，总动量保持不变。
2. Types of Collisions | 碰撞的类型
In CIE A-Level physics, collisions are classified by whether kinetic energy is conserved.
在 CIE A-Level 物理中，碰撞按动能是否守恒进行分类。
Elastic collision: both momentum and kinetic energy are conserved.
弹性碰撞：动量和动能均守恒。
Inelastic collision: momentum is conserved, but kinetic energy is not; some energy is converted to heat, sound, or deformation.
非弹性碰撞：动量守恒，但动能不守恒；部分动能转化为热量、声音或形变能。
Perfectly inelastic collision: the two objects stick together after collision; this is the case of maximum kinetic energy loss.
完全非弹性碰撞：碰撞后两物体粘合在一起；这是动能损失最大的情况。
For any isolated system, momentum conservation applies to all three types. The key is to know how much kinetic energy is lost.
对于任何孤立系统，动量守恒适用于所有三种类型。关键在于确定动能损失了多少。
3. Elastic Collisions in One Dimension | 一维弹性碰撞
For a one-dimensional elastic collision between two masses m₁ and m₂, the relative speed of approach equals the relative speed of separation:
对于质量分别为 m₁ 和 m₂ 的一维弹性碰撞，接近的相对速度等于分离的相对速度：
u₁ − u₂ = −(v₁ − v₂)
This equation is often easier to use than the full solution. Combined with momentum conservation, it allows you to find the final velocities.
这个方程通常比完整的解更容易使用。结合动量守恒，就可以求出末速度。
in a one-dimensional elastic collision, the two objects simply exchange velocities. Newton's cradle is a beautiful demonstration of this result.
一个特殊情况：在一维弹性碰撞中，若 m₁ = m₂，则两个物体只是交换速度。牛顿摆就是这一结果的精彩演示。
4. Perfectly Inelastic Collisions | 完全非弹性碰撞
In a perfectly inelastic collision, the two objects move together with a common final velocity v.
在完全非弹性碰撞中，两个物体以相同末速度 v 一起运动。
