📚 Equilibrium of a Stationary Particle | 静止质点的受力平衡条件
In A-level Mathematics, the mechanics unit asks us to model real objects as point masses. When a particle is completely at rest, we can apply a simple but powerful condition involving the forces acting on it. This article explains the equilibrium conditions for a stationary particle, how to draw free-body diagrams, and how to solve problems involving forces in two dimensions.
在 A-level 数学的力学部分中，我们常把真实物体建模为质点。当一个质点完全静止时，可以对它所受的力应用一个简单而有力的条件。本文将解释静止质点的平衡条件、如何绘制受力分析图，以及如何求解二维力平衡问题。
1. What is a Particle? | 什么是质点？
A particle is a body so small that its size and shape can be ignored. It has mass but no dimensions. Consequently, forces on a particle act at a single point, so there is no rotation and no moments.
质点是一种小到可以忽略其大小和形状的物体。它有质量但无尺寸。因此，作用在质点上的力都作用于同一点，不会产生转动，也不需要讨论力矩。
When a particle remains at rest, we say it is in static equilibrium. This is a special case of equilibrium in which its velocity is always zero.
当质点保持静止时，我们称它处于静态平衡。这是平衡的特殊情况，即它的速度始终为零。
2. Forces as Vectors | 力是矢量
A force is a vector quantity because it has both magnitude and direction. When several forces act on a particle, their combined effect is found by vector addition.
力是既有大小又有方向的矢量。多个力作用于质点时，其合效果通过矢量加法求得。
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