📚 Forces & Shape | 力与形变
In physics, a force is a push or pull that can change the motion, direction, or shape of an object. When forces act on objects, they can cause stretching, compressing, or bending — all forms of deformation. Understanding the relationship between forces and shape is essential for the Edexcel IGCSE Physics syllabus, particularly when studying Hooke’s Law and elastic behaviour.
在物理学中，力是对物体的推或拉，它可以改变物体的运动状态、方向或形状。当力作用在物体上时，可以使物体发生拉伸、压缩或弯曲——这些都是形变的形式。理解力与形状之间的关系是 Edexcel IGCSE 物理大纲的重要内容，尤其是在学习胡克定律和弹性行为时。
1. What is a Force? | 什么是力？
A force is a vector quantity, meaning it has both magnitude and direction. Forces are measured in newtons (N). When multiple forces act on an object, they can be combined to produce a resultant force. If the resultant force is zero, the object is said to be in equilibrium.
力是矢量，意味着它既有大小又有方向。力的单位是牛顿（N）。当多个力作用在一个物体上时，它们可以合成为一个合力。如果合力为零，则称该物体处于平衡状态。
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Magnitude: how large the force is, measured in newtons (N).
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大小：力的大小，以牛顿（N）为单位测量。
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Direction: where the force is applied, represented by arrows in diagrams.
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方向：力的作用方向，在图中用箭头表示。
A force can be represented by an arrow: the length of the arrow represents the magnitude, and the direction of the arrow represents the direction of the force. For example, to represent a force of 10 N acting to the right, an arrow 5 cm long could be used with a scale of 1 cm = 2 N.
力可以用箭头表示：箭头的长度代表力的大小，箭头的方向代表力的方向。例如，要表示一个大小为10 N、方向向右的力，可以用5 cm长的箭头，比例尺为1 cm = 2 N。
2. Contact and Non-Contact Forces | 接触力与非接触力
Forces can be classified into two main categories: contact forces and non-contact forces. Contact forces require physical contact between two objects, while non-contact forces act at a distance.
力可以分为两大类：接触力和非接触力。接触力需要两个物体之间有物理接触，而非接触力可以在一定距离外作用。
| Type 类型 | Examples 示例 |
| Contact force 接触力 | Friction, tension, air resistance, normal contact force 摩擦力、张力、空气阻力、法向接触力 |
| Non-contact force 非接触力 | Gravity, magnetic force, electrostatic force 重力、磁力、静电力 |
When studying forces and shape, tension forces (pulling forces) and compression forces (pushing forces) are of particular interest. A tension force stretches an object, while a compression force squeezes it.
在研究力与形状时，张力（拉伸力）和压力（压缩力）尤其重要。张力使物体伸长，而压力使物体被挤压。
3. Effects of Forces on Shape | 力对形状的影响
When a force is applied to an object, it can change the object’s shape. This is called deformation. There are three main types of deformation: stretching, compressing, and bending.
当力作用在物体上时，可以改变物体的形状，这称为形变。形变主要有三种类型：拉伸、压缩和弯曲。
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Stretching: the object becomes longer, e.g., pulling a spring or an elastic band.
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拉伸：物体变长，例如拉弹簧或橡皮筋。
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Compressing: the object becomes shorter, e.g., squeezing a sponge or compressing a spring.
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压缩：物体变短，例如挤压海绵或压缩弹簧。
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Bending: the object curves, e.g., bending a ruler or a metal bar.
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弯曲：物体弯曲，例如弯曲直尺或金属条。
An important concept here is extension, defined as the increase in length of an object when a force is applied:
这里一个重要的概念是伸长量，定义为施加力后物体长度的增加量：
extension = new length − original length
伸长量 = 新长度 − 原长度
For a spring, if the original length is 10 cm and after applying a force the new length is 14 cm, the extension is 4 cm.
对于弹簧，如果原长为10 cm，施加力后新长度为14 cm，则伸长量为4 cm。
4. Hooke’s Law | 胡克定律
Hooke’s Law states that the extension of an object (such as a spring) is directly proportional to the applied force, provided that the limit of proportionality is not exceeded.
胡克定律指出：在不超过比例极限的条件下，物体（如弹簧）的伸长量与所受外力成正比。
F = kx
Where F is the applied force in newtons (N), k is the spring constant in newtons per metre (N/m), and x is the extension in metres (m).
其中 F 是施加的力，单位为牛顿（N）；k 是劲度系数（弹簧常数），单位为牛顿每米（N/m）；x 是伸长量，单位为米（m）。
For example, if a spring has a spring constant of 50 N/m and is extended by 0.2 m, the force applied is:
例如，如果一根弹簧的劲度系数为50 N/m，伸长量为0.2 m，则施加的力为：
F = 50 × 0.2 = 10
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