📚 KS3 CIE Force Diagrams Mark Scheme | KS3 CIE 受力图评分标准
Force diagrams are a key part of the Cambridge Lower Secondary Science curriculum. They test how well you can show forces acting on a single object using labelled arrows. This article explains the mark scheme points that CIE examiners look for in KS3 force diagram questions.
受力图是剑桥初中科学课程的重点内容。它考查你能否用带标签的箭头表示作用在一个物体上的力。本文讲解 CIE 考官在 KS3 受力图题中关注的评分标准要点。
1. What a Force Diagram Shows | 受力图表示什么
A force diagram shows all the forces acting on one object. It is sometimes called a free-body diagram. The object is usually drawn as a simple box, a ball, or a car, and each force arrow starts on that object.
受力图显示作用在一个物体上的所有力。它有时被称为隔离体图。物体通常画成一个简单的方框、球或车，每个力的箭头都从该物体上出发。
Mark schemes award marks only for forces that act on the object, not forces the object applies to other objects. Adding extra arrows can lose marks because they do not belong on the diagram.
评分标准只认可作用在物体上的力，而不是物体施加给其他物体的力。添加多余的箭头会失分，因为它们不属于该受力图。
2. Arrows and Direction | 箭头与方向
Direction is the most common mark in force diagram questions. Every arrow must point in the direction the force acts. Weight always acts vertically downward towards the centre of the Earth.
方向是受力图题目中最常见的得分点。每个箭头必须指向力的作用方向。重力总是竖直向下指向地心。
Normal contact force acts perpendicularly to the surface. Friction or air resistance opposes the direction of motion. Tension acts along a rope or string, pulling away from the object.
法向接触力垂直于接触面。摩擦力或空气阻力与运动方向相反。张力沿绳子或线方向，远离物体。
CIE mark schemes usually require the arrowhead to be clearly shown. A line without an arrowhead may not gain the direction mark.
CIE 评分标准通常要求清晰画出箭头尖端。没有箭头的线可能无法得到方向分。
3. Magnitude and Relative Length | 大小与相对长度
Relative arrow length should represent the size of the force. If a push is 10 N and friction is 5 N, the push arrow should be twice as long as the friction arrow.
箭头的相对长度应表示力的大小。如果推力为 10 N、摩擦力为 5 N，则推力箭头应为摩擦力箭头的两倍长。
When forces are equal, arrows must be the same length. This shows balanced forces and is a very common mark scheme requirement.
当力相等时，箭头必须等长。这表示平衡力，也是评分标准中非常常见的要求。
If numerical values are given in the question, use them to decide arrow lengths. If only directions are given, show a clear difference where one force is larger.
如果题目给出了数值，用数值来决定箭头长度。如果只给出方向，则在一方较大时清楚画出长度差异。
4. Labelling Forces Correctly | 正确标注力
Use exact force names: weight, normal contact force, friction, air resistance, upthrust, tension, thrust, magnetic force, and electrostatic force. Do not write vague labels like “force”.
使用准确的力名称：重力、法向接触力、摩擦力、空气阻力、浮力、张力、推力、磁力和静电力。不要写模糊的“力”。
Do not confuse weight with mass. Weight is measured in newtons (N); mass is measured in kilograms (kg). Weight is the force due to gravity.
不要混淆重力与质量。重力的单位是牛顿 (N)；质量的单位是千克 (kg)。重力是万有引力产生的力。
In mark schemes, an arrow labelled “weight” or “W” is accepted, but “gravity” can also be accepted if it means the gravitational force on the object. Avoid “mass” as a force label.
在评分标准中，标为“weight”或“W”的箭头可得分，如果“gravity”指物体受到的重力，也可接受。避免用“mass”作为力的标签。
5. Contact and Non-contact Forces | 接触力与非接触力
Non-contact forces include weight, magnetic force and electrostatic force. They can act through empty space without the objects touching.
非接触力包括重力、磁力和静电力。它们可以在物体不接触的情况下通过真空空间作用。
Contact forces include friction, air resistance, tension, upthrust and normal contact force. They only act when objects touch each other.
接触力包括摩擦力、空气阻力、张力、浮力和法向接触力。它们仅在物体相互接触时起作用。
If a mark scheme asks for a non-contact force, “weight” is the safest answer because it is always present on Earth. “Magnetic” and “electrostatic” also score if the situation involves them.
如果评分标准要求写出一种非接触力，“重力”是最稳妥的答案，因为它在地球上始终存在。如果情境涉及磁力或静电力，它们也能得分。
6. Balanced and Unbalanced Forces | 平衡力与非平衡力
Balanced forces mean the resultant force is zero. The object is either stationary or moving at a constant speed in a straight line.
平衡力意味着合力为零。物体要么静止，要么沿直线匀速运动。
Unbalanced forces produce a resultant force that is not zero. The object will accelerate, decelerate, or change direction.
非平衡力会产生不等于零的合力。物体将加速、减速或改变方向。
Resultant force = 0 for balanced forces
In a force diagram, balanced forces are shown by equal arrows in opposite directions. Unbalanced forces are shown by one longer arrow in one direction.
在受力图中，平衡力由方向相反的等长箭头表示。非平衡力由一个方向上的较长箭头表示。
7. Resultant Force Calculations | 合力计算
For forces along the same straight line, subtract the smaller force from the larger force. The direction of the resultant force is the direction of the larger force.
对于沿同一直线的力，用较大的力减去较小的力。合力的方向是较大力的方向。
Example: 12 N to the right and 7 N to the left. The resultant force is 12 N − 7 N = 5 N to the right.
示例：向右 12 N，向左 7 N。合力为 12 N − 7 N = 向右 5 N。
Resultant force = larger force − smaller force
For perpendicular forces at KS3, questions usually ask you to draw the forces rather than calculate a diagonal resultant. Always use the values given to compare arrow lengths.
对于 KS3 的垂直力问题，通常只要求画图而不是计算斜向合力。始终根据给出的数值比较箭头长度。
8. Common Mark Scheme Points | 常见评分标准得分点
Typical CIE mark scheme points include the following: the arrow starts on
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