📚 IGCSE Physics: Kinetics Key Points | IGCSE物理：动力学考点精讲
Kinetics, also known as kinematics in the IGCSE Physics syllabus, is the study of motion without considering the forces that cause it. This topic lays the foundation for understanding how objects move, from a sprinter accelerating down a track to a ball dropped from a height. Mastering the key concepts of distance, displacement, speed, velocity, acceleration, and the graphical representation of motion is essential for success in your examination. This revision guide breaks down every important point into clear explanations, worked examples, and exam tips to help you achieve top marks.
动力学，在IGCSE物理课程中也常称为运动学，是研究物体运动而不考虑引起运动的作用力的学科。这一主题为理解物体如何运动奠定了基础，从短跑运动员在跑道上的加速到从高处落下的球。掌握距离、位移、速率、速度、加速度以及运动的图解表示等关键概念，对于在考试中取得成功至关重要。本复习指南将每一个重要知识点分解为清晰易懂的解释、例题和考试技巧，助你取得优异成绩。
1. Scalar and Vector Quantities | 标量与矢量
In physics, quantities are divided into scalars and vectors. A scalar quantity has magnitude (size) only, while a vector quantity has both magnitude and direction. Distance is a scalar — it tells you how much ground an object has covered, but not the direction. Displacement is a vector — it is the straight-line distance in a given direction from the starting point to the finishing point. Speed is scalar (e.g., 20 m/s), whereas velocity is vector (e.g., 20 m/s north). Recognising this difference is crucial for solving motion problems correctly.
在物理学中，量分为标量和矢量。标量只有大小，矢量既有大小又有方向。距离是标量——它告诉你物体经过了多少路程，但不涉及方向。位移是矢量——它是从起点到终点的直线距离，并带有给定的方向。速率是标量（如 20 m/s），而速度是矢量（如 20 m/s 向北）。正确区分这一点对于解答运动问题至关重要。
- Scalar examples: distance, speed, mass, time, energy. | 标量例子：距离、速率、质量、时间、能量。
- Vector examples: displacement, velocity, acceleration, force, momentum. | 矢量例子：位移、速度、加速度、力、动量。
- Adding vectors requires consideration of direction; scalars are simply added numerically. | 矢量相加需要考虑方向；标量直接数字相加。
2. Distance, Displacement, Speed and Velocity | 距离、位移、速率和速度
Distance is the total length of the path travelled by an object. Displacement is the shortest distance between the initial and final positions, together with the direction. The average speed is calculated as total distance divided by total time taken: speed = distance / time. The average velocity is total displacement divided by time: velocity = displacement / time. An object moving in a circle at constant speed has a constantly changing velocity because its direction is continuously changing.
距离是物体所经过路径的总长度。位移是起点与终点之间的最短距离，外加方向。平均速率等于总距离除以总时间：速率 = 距离 / 时间。平均速度等于总位移除以时间：速度 = 位移 / 时间。一个以恒定速率作圆周运动的物体，其速度是不断变化的，因为它的方向在持续改变。
For constant speed in a straight line, instantaneous speed equals average speed. In IGCSE problems, you may be asked to find the distance from a speed-time graph or calculate average speed for a journey with multiple legs. Always make sure you use the correct total time and total distance.
对于直线上的匀速运动，瞬时速率等于平均速率。在IGCSE题目中，你可能需要从速度-时间图求出距离，或计算包含多段行程的平均速率。始终确保使用正确的总时间和总距离。
3. Acceleration | 加速度
Acceleration is defined as the rate of change of velocity. It is a vector quantity, measured in metres per second squared (m/s²). The formula is: a = (v – u) / t, where u is initial velocity, v is final velocity, and t is time. Positive acceleration means speeding up in the positive direction; negative acceleration (deceleration) means slowing down. An object moving in a circle at constant speed is still accelerating because its direction changes — this is called centripetal acceleration.
加速度被定义为速度的变化率。它是一个矢量，单位为米每二次方秒 (m/s²)。公式为：a = (v – u) / t，其中 u 为初速度，v 为末速度，t 为时间。正加速度意味着朝正方向加速；负加速度（减速度）意味着减速。一个以恒定速率作圆周运动的物体仍然在加速，因为其方向在变化——这叫做向心加速度。
In velocity-time graphs, acceleration is the slope (gradient) of the line. Constant acceleration produces a straight sloping line; changing acceleration gives a curve. The area under the velocity-time graph represents the displacement.
在速度-时间图中，加速度是直线的斜率（梯度）。恒定加速度产生一条倾斜的直线；变化的加速度给出曲线。速度-时间图下方的面积代表位移。
4. Equations of Motion for Uniform Acceleration | 匀加速直线运动的运动方程
When an object moves in a straight line with constant acceleration, you can use the four kinematic equations, often called ‘suvat’ equations. The symbols are: s = displacement, u = initial velocity, v = final velocity, a = acceleration, t = time. The equations are:
当物体以恒定的加速度沿直线运动时，你可以使用四个运动学方程，通常称为“suvat”方程。符号含义为：s = 位移，u = 初速度，v = 末速度，a = 加速度，t = 时间。方程如下：
v = u + a t
s = (u + v) t / 2
s = u t + ½ a t²
v² = u² + 2 a s
Always list the known quantities and the one you need to find before choosing the appropriate equation. Remember: deceleration means negative a. These equations only work for constant acceleration and straight-line motion. IGCSE exam questions often involve a car braking, a train leaving a station, or a falling object (near Earth’s surface, a = g = 9.8 m/s² downwards).
在选择合适的方程之前，始终列出已知量和待求量。记住：减速度意味着负的 a。这些方程仅适用于恒定加速度和直线运动。IGCSE 考题常涉及汽车刹车、火车离站或落体（接近地表时，a = g = 9.8 m/s² 向下）。
5. Free Fall and Acceleration Due to Gravity | 自由落体与重力加速度
Free fall is the motion of an object where the only force acting on it is gravity. Near the Earth’s surface, all objects fall with the same constant acceleration, g = 9.8 m/s² (to 2 significant figures, often taken as 10 m/s² in some exams). This acceleration is always directed vertically downwards. Motion of a falling object can be analysed using the suvat equations, with a = g = 9.8 m/s². For an object dropped from rest, u = 0; for an object thrown upwards, u is positive, a = -g.
自由落体是物体仅受重力作用下的运动。靠近地球表面，所有物体都以相同的恒定加速度下落，g = 9.8 m/s²（保留两位有效数字，部分考试中常取 10 m/s²）。这个加速度总是竖直向下。可以使用 suvat 方程分析落体的运动，其中 a = g = 9.8 m/s²。对于从静止下落的物体，u = 0；对于上抛物体，u 为正，a = -g。
A common misconception is that heavier objects fall faster; in the absence of air resistance, all objects fall at the same rate. The famous demonstration of a feather and a hammer on the Moon confirmed this. In everyday situations, air resistance affects light objects more significantly, so a piece of paper falls slower than a stone.
一个常见的误解是较重的物体下落更快；在没有空气阻力的情况下，所有物体以相同的加速度下落。月球上羽毛和锤子的著名实验证实了这一点。在日常生活中，空气阻力对轻物体的影响更显著，因此一张纸比一块石头下落得慢。
6. Motion Graphs: Displacement-Time Graphs | 运动图像：位移-时间图
A displacement-time (d-t) graph shows how displacement changes with time. The gradient (slope) of the line gives the velocity. A horizontal line indicates zero velocity (object is stationary). A straight sloping line means constant velocity. A curve indicates changing velocity, i.e. acceleration. The steeper the gradient, the greater the velocity. If the graph goes back down (negative gradient), the object is moving in the opposite direction.
位移-时间 (d-t) 图显示位移如何随时间变化。直线的梯度（斜率）给出速度。水平线表示速度为零（物体静止）。倾斜直线表示匀速运动。曲线表示速度在变化，即存在加速度。梯度越陡，速度越大。如果图像下降（负梯度），物体向反方向运动。
Exam tip: never confuse displacement-time with distance-time. Distance-time graphs can only go up or stay flat; displacement-time graphs can go up, down, or be flat, showing direction changes. Use the area under the curve? No, area has no meaning in d-t graphs.
考试技巧：切勿将位移-时间图与距离-时间图混淆。距离-时间图只能上升或保持水平；位移-时间图可以上升、下降或保持水平，显示方向的改变。利用曲线下的面积？不，面积在 d-t 图中没有意义。
7. Motion Graphs: Velocity-Time Graphs | 运动图像：速度-时间图
A velocity-time (v-t) graph is one of the most powerful tools for solving motion problems. The gradient of a v-t graph gives the acceleration. A horizontal line means constant velocity (zero acceleration). A straight sloping line means constant acceleration. The area under the graph between two times gives the total displacement during that interval. To find distance travelled, take the magnitude of areas (i.e., treat all areas as positive, regardless of negative velocity).
速度-时间 (v-t) 图是解决运动问题最强大的工具之一。v-t 图的梯度给出加速度。水平线意味着恒定速度（零加速度）。倾斜直线意味着恒定加速度。两个时间点之间图像下的面积给出该时间间隔内的总位移。要求得行驶距离，则取各面积的绝对值（即把所有面积都视为正值，忽略负速度）。
If the graph line crosses the time axis, the object is changing direction. A constant positive acceleration appears as a straight line sloping upwards; negative acceleration (deceleration) slopes downwards. The area of a trapezium or triangle can be calculated using simple geometry. In exams, be ready to read values from the graph and apply suvat equations if needed.
如果图像线与时间轴相交，物体正在改变方向。恒定的正加速度表现为一条向上倾斜的直线；负加速度（减速度）向下倾斜。梯形或三角形的面积可用简单的几何公式计算。在考试中，准备好从图上读取数值，并在需要时应用 suvat 方程。
8. Terminal Velocity | 终端速度
When an object falls through a fluid (liquid or gas), it experiences air resistance or drag, which increases with speed. Initially, the only force is weight (mg), so the object accelerates at g. As speed increases, drag increases, reducing the net force and acceleration. Eventually, drag force equals weight, net force becomes zero, and acceleration stops. The object then falls at a constant maximum speed called terminal velocity. A skydiver’s terminal velocity in a spread-eagle position is about 55 m/s (120 mph).
当物体在流体（液体或气体）中下落时，会受到空气阻力或拖曳力，其大小随速度增加而增加。开始时，只有重力 (mg)，因此物体以 g 加速。随着速度增加，阻力增大，使净力与加速度减小。最终，阻力等于重力，净力为零，加速度停止。物体随后以恒定的最大速度下落，该速度称为终端速度。跳伞运动员在四肢伸展姿势下的终端速度约为 55 m/s（120 英里/小时）。
A velocity-time graph for a falling object with air resistance shows a curve that starts steep (high acceleration) and gradually levels off to a horizontal line (terminal velocity). Without air resistance, the graph would be a straight line with constant slope g. IGCSE often asks to sketch or interpret such graphs and explain the forces involved.
对于有空气阻力的下落的物体，其速度-时间图显示一条曲线，开始时很陡（高加速度），并逐渐趋于水平线（终端速度）。如果没有空气阻力，图像将是一条恒定斜率为 g 的直线。IGCSE 常要求绘制或解读此类图像，并解释所涉及的力。
9. Interpreting Multiple Leg Journeys | 多段行程的解读
Real-life motion often involves moving, stopping, and changing direction. You can model such journeys using a sequence of straight-line segments on a velocity-time graph. The total displacement is the sum of signed areas (positive areas minus negative areas), while the total distance is the sum of the absolute values of all areas. Average speed = total distance / total time; average velocity = total displacement / total time. Always check if the graph gives positive and negative velocities.
现实生活中的运动常常涉及移动、停止和改变方向。你可以利用速度-时间图上的一系列直线段来模拟这类行程。总位移是带符号面积之和（正面积减去负面积），而总距离是所有面积绝对值之和。平均速率 = 总距离 / 总时间；平均速度 = 总位移 / 总时间。始终检查图像是否给出正速度和负速度。
A typical exam question: the first part of the journey is acceleration from rest to 20 m/s in 10 s, then constant speed for 30 s, then deceleration to rest in 20 s. You would calculate the area under the graph using shapes (triangle + rectangle + triangle) to find distance travelled. Then find average speed for the whole trip.
典型的考题：行程的第一部分是从静止开始，在 10 秒内加速到 20 m/s，然后匀速运动 30 秒，再在 20 秒内减速至静止。你会利用形状（三角形 + 矩形 + 三角形）计算图像下的面积以得出行驶距离。然后求出整个行程的平均速率。
10. Relative Motion | 相对运动
Relative velocity is the velocity of one object as observed from another moving object. If two objects move along the same straight line, the relative velocity is found by vector addition or subtraction. For example, if car A is moving at 20 m/s east and car B at 10 m/s east, the velocity of A relative to B is 20 – 10 = 10 m/s east. If they move towards each other, relative speed is the sum of speeds. This concept is helpful in problems involving overtaking, catching up, or head-on collisions.
相对速度是指从另一个运动物体观察到的某物体的速度。如果两个物体沿同一直线运动，相对速度通过矢量加法或减法求得。例如，汽车 A 以 20 m/s 向东行驶，汽车 B 以 10 m/s 向东行驶，则 A 相对于 B 的速度为 20 – 10 = 10 m/s 向东。如果它们相向而行，相对速率是两个速率的和。这一概念有助于解决涉及超车、追赶或正面碰撞的问题。
IGCSE questions may ask about the time for one car to overtake another or the time for two trains to pass each other. Use relative velocity together with distance to find the time: t = distance / relative velocity. Remember that direction matters; choose a sign convention consistently.
IGCSE 考题可能会问及一辆汽车超越另一辆车所需的时间，或两列火车相会所需的时间。利用相对速度和距离来求时间：t = 距离 / 相对速度。记住方向非常重要；始终一致地选择符号约定。
11. Practical Skills in Kinetics | 动力学部分的实验技能
The IGCSE Physics exam often includes questions on practical experiments to measure speed and acceleration. A classic experiment uses a trolley rolling down a ramp, with light gates or ticker tape to record motion. For speed, you measure the time taken for the trolley to travel between two points of known distance. For acceleration, the trolley starts from rest and a light gate measures the time to pass a known length, or ticker tape dots are analysed for spacing. The distance between dots on ticker tape increases with speed.
IGCSE 物理考试常包含测量速率和加速度的实验操作题。一个经典实验是利用手推车从斜坡上滑下，使用光电门或打点计时器来记录运动。测量速率时，你需要测量手推车经过两个已知距离点之间的时间。对于加速度，手推车从静止开始运动，光电门测量其通过某段已知长度的时间，或者通过分析打点纸带上点之间的间距。纸带上点距的增加意味着速率在增加。
Students must be able to describe how to set up the apparatus, what measurements to take, how to reduce errors (e.g., by repeating timings and taking an average, avoiding parallax error), and how to calculate results. Ensure you are familiar with both analogue (ticker tape) and digital methods (data loggers, light gates).
学生必须能够描述如何搭建实验装置、需要测量哪些数据、如何减少误差（例如，通过重复计时求平均值、避免视差），以及如何计算结果。确保你熟悉模拟（打点计时器）和数字方法（数据记录器、光电门）。
12. Exam Tips and Common Pitfalls | 考试技巧与常见错误
To excel in IGCSE kinetics, always convert units to SI (metres, seconds, m/s, m/s²). If a question gives time in minutes or distance in cm, convert them before calculation. Distinguish clearly between distance and displacement, speed and velocity. In suvat problems, define the positive direction at the start and stick to it. For graphs, label axes correctly and note whether the graph is displacement-time or velocity-time. Check if the question asks for distance or displacement, average speed or average velocity.
要在 IGCSE 动力学部分脱颖而出，始终将单位转换为国际单位制（米、秒、m/s、m/s²）。如果题目给的时间以分钟为单位或距离以厘米为单位，要在计算前进行换算。明确区分距离与位移、速率与速度。在 suvat 问题中，一开始就规定正方向，并始终保持一致。对于图像，正确标注坐标轴，并注意图像是位移-时间图还是速度-时间图。检查题目要求的是距离还是位移，平均速率还是平均速度。
A frequent mistake is forgetting that acceleration can be negative. Always substitute deceleration as a negative value. Another error is using the formula v = s/t for accelerated motion — this only works for constant velocity. Finally, show your working step by step; even if the final answer is wrong, you may earn marks for correct method and equation selection.
一个常见错误是忘记加速度可以为负值。始终将减速度代作负值。另一个错误是对加速运动使用公式 v = s/t —— 这仅适用于匀速运动。最后，要逐步展示解题过程；即使最终答案错误，你也可能因为正确的方法和方程选择而获得分数。
Published by TutorHao | IGCSE Physics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导