📚 Force Diagrams: Drawing and Interpreting Forces | 力的示意图：绘制与解读
Force diagrams are one of the most useful tools in KS3 physics. They allow you to show the size and direction of every force acting on an object in a clear, visual way. If you master force diagrams early, topics such as balanced forces, acceleration, friction and terminal velocity become much easier to understand.
力的示意图是 KS3 物理中最有用的工具之一。它们让你能够以清晰、直观的方式表示作用在物体上的每个力的大小和方向。如果你尽早掌握力的示意图，平衡力、加速度、摩擦力和终极速度等主题会变得更容易理解。
1. What Is a Force? | 什么是力？
A force is a push or a pull that acts on an object. Forces can change the shape, speed or direction of an object. For example, kicking a ball applies a force that changes the ball's motion.
力是作用在物体上的推力或拉力。力可以改变物体的形状、速度或运动方向。例如，踢足球时会施加一个改变足球运动状态的力。
Force is measured in newtons (N). Forces are vector quantities, which means they have both size (magnitude) and direction. A force of 10 N to the right is different from a force of 10 N to the left.
力的单位是牛顿（N）。力是矢量，这意味着它既有大小（量值）又有方向。一个向右的 10 N 力与一个向左的 10 N 力是不同的。
2. Why Use Force Diagrams? | 为什么要使用力的示意图？
A force diagram helps you show all the forces acting on a single object at the same time. Without a diagram, it is easy to forget a force such as friction or normal reaction.
力的示意图帮助你同时显示作用在单个物体上的所有力。如果没有示意图，很容易漏掉摩擦力或支持力（法向反作用力）等力。
Force diagrams also help you work out the resultant force. The resultant force decides whether an object stays still, moves at constant speed, speeds up or slows down.
力的示意图还能帮助你求出合力。合力决定物体是保持静止、匀速运动、加速还是减速。
3. Drawing Force Arrows | 绘制力箭头
In a force diagram, each force is drawn as an arrow. The arrow must point in the direction that the force acts.
在力的示意图中，每个力都画成一个箭头。箭头必须指向力作用的方向。
a shorter arrow. Always label each arrow with the name and size of the force, for example "friction = 5 N".
箭头的长度也很重要。较大的力画成较长的箭头，较小的力画成较短的箭头。务必用力的名称和大小标记每个箭头，例如“摩擦力 = 5 N”。
4. Choosing a Scale | 选择标度
r example, you might choose 1 cm = 10 N. Then a 30 N force would be drawn as a 3 cm arrow.
当你按比例绘制力的示意图时，需要决定每厘米代表多少牛顿。例如，你可以选择 1 cm = 10 N。那么一个 30 N 的力就画成 3 cm 长的箭头。
The scale must be large enough to fit the page but small enough to show differences clearly. Always use the same scale for all force arrows on one diagram.
标度必须大到足以在页面上画出，但又小到能清楚显示差异。在同一个示意图中，所有力箭头都必须使用相同的标度。
5. Common Forces in KS3 Diagrams | KS3 示意图中的常见力
You will meet several forces again and again in KS3 science. The table below shows the most common ones and their typical directions.
在 KS3 科学中，你会反复遇到几种力。下表显示了最常见的力及其典型方向。
| Force | 中文名称 | Typical direction | Example |
|---|---|---|---|
| Weight (gravitational force) | 重力（重量） | Downwards towards Earth | A falling ball |
| Normal reaction | 支持力（法向反作用力） | Perpendicular to the surface | A book on a table |
| Friction | 摩擦力 | Opposite to motion or sliding | A box sliding on the floor |
| Air resistance (drag) | 空气阻力 | Opposite to motion through air | A parachute falling |
| Tension | 拉力（张力） | Along a rope or cable | A bucket hanging on a rope |
| Thrust | 推力 | In the direction of motion or intended motion | A rocket moving upwards |
Remember that weight is not the same as mass. Weight is a force measured in newtons, while mass is measured in kilograms.
请记住，重量不等于质量。重量是一种力，单位是牛顿；而质量的单位是千克。
6. Balanced Forces | 平衡力
When two forces on an object are equal in size but act in opposite directions, we say the forces are balanced. The resultant force is zero.
当作用在物体上的两个力大小相等、方向相反时，我们称这些力是平衡的。合力为零。
If the forces on an object are balanced, the object does not speed up or slow down. It either remains stationary or continues moving at constant velocity.
如果物体所受的力是平衡的，物体就不会加速或减速。它要么保持静止，要么继续以恒定速度运动。
A common example is a book resting on a table. Its weight pulls down, and the normal reaction from the table pushes up. These two forces are equal and opposite.
一个常见的例子是放在桌子上的书。它的重力向下拉，而桌面的支持力向上推。这两个力大小相等、方向相反。
7. Unbalanced Forces and Resultant Force | 非平衡力与合力
When one force is larger than the opposite force, the forces are unbalanced. There is a resultant force in the direction of the larger force.
当一个力大于与之相反的力时，这些力是不平衡的。在较大力的方向上会产生一个合力。
Resultant force = larger force − smaller force
如果两个方向相反的力大小不同，合力等于较大力减去较小力。例如，向右 40 N 的力和向左 15 N 的力产生的合力为向右 25 N。
An unbalanced force causes an object to accelerate, decelerate or change direction. This is Newton's second law idea at KS3 level.
非平衡力会使物体加速、减速或改变方向。这就是 KS3 阶段所涉及的牛顿第二定律思想。
8. Free-Body Diagrams | 隔离体图
A free-body diagram is a simplified force diagram that shows only the object and the forces acting on it. The object is often drawn as a small box or a dot.
隔离体图是一种简化的力示意图，它只显示物体和作用在该物体上的力。物体通常画成一个小方框或一个圆点。
All force arrows start from the centre of the object. The surroundings, such as the table or the floor, are not drawn. This makes the diagram clearer and easier to read.
所有力箭头都从物体的中心开始。桌子或地面等周围环境不画出来。这使示意图更清晰、更容易阅读。
9. Step-by-Step Drawing Guide | 分步骤绘制指南
Follow these steps to draw a correct force diagram:
按照以下步骤绘制正确的力示意图：
Identify the object you are studying and draw it as a box or dot.
确定你要研究的物体，并将它画成一个方框或圆点。
List all the forces acting on that object from the surroundings.
列出周围环境作用在该物体上的所有力。
Choose a suitable scale if you need to show accurate sizes.
如果需要准确表示力的大小，选择合适的标度。
Draw one arrow for each force, starting from the centre of the object.
为每个力画一个箭头，从物体的中心开始。
Label every arrow with the force name and its size in newtons.
标记每个箭头，写上力的名称及其牛顿数值。
Check whether the forces are balanced or unbalanced.
检查力是平衡的还是非平衡的。
10. Worked Examples | 典型实例
Example 1: A box resting on a table. The box has a weight of 20 N downwards. The table pushes up with a normal reaction of 20 N. The two arrows are equal in length and opposite in direction, so the forces are balanced.
实例 1：桌上的箱子。箱子受到向下的重力 20 N，桌面对它施加向上的支持力 20 N。两个箭头长度相等、方向相反，因此力是平衡的。
Example 2: A car accelerating along a road. The thrust from the engine is 3000 N forwards. Friction and air resistance together give 1500 N backwards. The arrows are not equal: the forward arrow is longer. There is a resultant force of 1500 N forwards, so the car accelerates.
实例 2：沿道路加速的汽车。发动机提供的推力向前，大小为 3000 N。摩擦力和空气阻力合计向后，大小为 1500 N。两个箭头不相等，向前的箭头更长。因此有向前 1500 N 的合力，汽车加速。
11. Common Mistakes and Exam Tips | 常见错误与考试技巧
One common mistake is drawing an arrow for a force that the object exerts on something else. For example, in a diagram of a book on a table, do not draw the force of the book pushing down on the table unless you are studying the table.
一个常见错误是画出了物体施加在其他物体上的力。例如，在桌上书本的示意图中，除非你研究的是桌子，否则不要画书本向下压桌子的力。
is forgetting to label the arrows. In CIE exams, marks are often given for labels such as "weight = 20 N" or "friction = 5 N". Always use a ruler and a pencil so your arrows are straight and clear.
另一个错误是忘记给箭头标注。在 CIE 考试中，标注如“重量 = 20 N”或“摩擦力 = 5 N”通常可以得分。始终使用尺子和铅笔画图，使箭头笔直且清晰。
Finally, check that longer arrows represent larger forces. If two forces are equal, their arrows must be exactly the same length.
最后，检查较长的箭头是否表示较大的力。如果两个力相等，它们的箭头长度必须完全相同。
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