📚 Speed, Velocity and Acceleration | 速度、速率与加速度
When we talk about motion, words like speed, velocity and acceleration are used every day. In IGCSE Science, these terms have precise definitions and form the foundation for understanding everything from moving cars to falling objects. This article explains the key ideas of speed, velocity and acceleration, including graphs, equations and real-life applications, all aligned with the Edexcel IGCSE Science specification.
当我们谈论运动时，“速率”“速度”和“加速度”这些词每天都会被用到。在IGCSE科学课程中，这些术语有严格的定义，它们构成了理解从行驶的汽车到自由落体等一切运动的基础。本文解释速率、速度与加速度的核心概念，包括运动图像、计算公式和实际应用，全部紧扣Edexcel IGCSE科学大纲。
1. Scalar and Vector Quantities | 标量与矢量
Physical quantities are divided into scalars and vectors. A scalar quantity has only magnitude (size), such as distance, speed, mass and time. A vector quantity has both magnitude and direction, such as displacement, velocity, acceleration and force.
物理量分为标量和矢量。标量只有大小，例如路程、速率、质量和时间。矢量既有大小又有方向，例如位移、速度、加速度和力。
Understanding this difference is essential because distance and displacement, or speed and velocity, are not the same. Distance is how much ground an object has covered, while displacement tells us the straight-line distance from start to finish in a specific direction. Similarly, speed just measures how fast something moves, but velocity measures how fast and in which direction.
理解这一区别至关重要，因为路程和位移、速率和速度并不相同。路程是物体所经过的路径总长度，而位移则告诉我们从起点到终点的直线距离并指明具体方向。同样，速率只衡量运动快慢，而速度既衡量快慢也指明方向。
2. Defining Speed | 定义速率
Speed is a scalar quantity that describes the rate at which an object moves. It is defined as the distance travelled per unit time. The common unit is metres per second (m/s), but we also use kilometres per hour (km/h).
速率是一个标量，描述物体运动的快慢。它定义为单位时间内所通过的路程。常用单位是米每秒（m/s），但我们也使用千米每小时（km/h）。
The formula for constant speed is:
speed = distance ÷ time
This equation can be rearranged to find distance or time if the other two quantities are known.
这个公式可以变形，如果已知其他两个量，就能求出路程或时间。
3. Average Speed Calculation | 平均速率的计算
ften calculate average speed. Average speed = total distance travelled ÷ total time taken.
在现实生活中，物体很少做匀速运动。我们常常计算平均速率。平均速率 = 总路程 ÷ 总时间。
For example, if a car travels 120 km in 2 hours, its average speed is 60 km/h, even if it stopped briefly or changed speed. In the exam, you must be able to rearrange the equation and use the correct units.
例如，如果一辆汽车在2小时内行驶120 km，它的平均速率是60 km/h，即使中途短暂停车或改变速度。在考试中，你必须能够变换公式并使用正确的单位。
4. Distance-Time Graphs | 距离–时间图像
A distance-time graph shows how far an object has travelled over a period of time. Time is plotted on the horizontal axis and distance on the vertical axis.
距离–时间图像显示物体在一段时间内运动的路程。时间标在横轴上，路程标在纵轴上。
The gradient of a distance-time graph represents speed. A straight line means constant speed; a steeper line means a higher speed. A horizontal line indicates the object is stationary.
距离–时间图像的斜率表示速率。一条直线代表匀速；线越陡，速率越大。水平线表示物体静止。
5. Speed and Velocity | 速率与速度
While speed only considers the magnitude, velocity is a vector that also states the direction. For example, a person walking 5 m/s due north has a velocity of 5 m/s north. If the person turns around and walks south at 5 m/s, the speed remains 5 m/s but the velocity changes to 5 m/s south.
速率只考虑大小，而速度是一个矢量，同时说明方向。例如，一个人以5 m/s向正北走，其速度为5 m/s北。如果此人转身以5 m/s向南走，速率仍为5 m/s，但速度变为5 m/s南。
In circular motion, even if the speed is constant, the velocity is constantly changing because the direction changes. This leads to the concept of acceleration towards the centre.
在圆周运动中，即使速率恒定，速度也不断变化，因为方向在变化。这引出了向心加速度的概念。
6. Acceleration | 加速度
Acceleration is a vector quantity defined as the rate of change of velocity. An object accelerates if its speed increases, decreases (deceleration) or changes direction. In everyday language, acceleration means speeding up, but in physics, any change in velocity is acceleration.
加速度是一个矢量，定义为速度的变化率。物体的速度增加、减小（减速度）或方向改变，都会产生加速度。在日常用语中，加速度指变快，但在物理学中，任何速度的变化都是加速度。
Deceleration, or negative acceleration, occurs when the final velocity is less than the initial velocity. The unit of acceleration is metres per second squared (m/s²).
当末速度小于初速度时，发生减速（或称负加速度）。加速度的单位是米每二次方秒（m/s²）。
7. The Acceleration Equation | 加速度方程
The acceleration of an object moving in a straight line can be calculated with:
a = (v – u) ÷ t
e taken. This equation is given on the Edexcel formula sheet, but you must know how to use it and rearrange it.
式中 a = 加速度，v = 末速度，u = 初速度，t = 时间。这个方程出现在Edexcel公式表上，但你必须知道如何运用和变形。
accelerates from 4 m/s to 10 m/s in 3 seconds, the acceleration is (10 − 4) ÷ 3 = 2 m/s².
例如，如果一名自行车手在3秒内从4 m/s加速到10 m/s，则加速度为 (10 − 4) ÷ 3 = 2 m/s²。
8. Velocity-Time Graphs | 速度–时间图像
Velocity-time graphs provide rich information. The gradient represents acceleration, while the area under the graph represents the distance travelled (or displacement). A straight sloping line shows constant acceleration; a horizontal line shows constant velocity.
速度–时间图像提供了丰富的信息。斜率代表加速度，图像下的面积代表所经过的路程（或位移）。倾斜的直线表示匀加速度；水平线表示匀速。
If the line slopes downwards, the object is decelerating. A curved line means the acceleration is changing. In IGCSE questions, you may need to calculate distance from a velocity-time graph by finding the area of simple shapes such as rectangles and triangles.
如果线段向下倾斜，物体在减速。曲线表示加速度在变化。在IGCSE考题中，你可能需要通过求矩形和三角形等简单图形的面积，从速度–时间图像中计算路程。
9. Free Fall and Gravity | 自由落体与重力
Near the Earth’s surface, all objects fall with the same acceleration due to gravity, provided air resistance is negligible. This acceleration, denoted by g, is about 9.8 m/s², often rounded to 10 m/s² for IGCSE calculations.
在地球表面附近，如果忽略空气阻力，所有物体都以相同的重力加速度下落。这个加速度用 g 表示，约为 9.8 m/s²，在IGCSE计算中常取近似值 10 m/s²。
erates uniformly. The equations of motion can be used to predict its velocity and distance fallen after a certain time. In the absence of air resistance, a feather and a hammer will hit the ground at the same time.
当物体从静止释放时，初速度 u = 0，它做匀加速运动。运动学方程可以用来预测某一时刻后的速度和下落距离。在没有空气阻力时，羽毛和锤子将同时着地。
10. Safety and Stopping Distances | 安全与停车距离
Speed and acceleration are directly related to road safety. The total stopping distance of a vehicle is the sum of the thinking distance (reaction time) and the braking distance. Speed is a crucial factor because braking distance increases with the square of the speed.
速率和加速度与道路安全直接相关。车辆的总停车距离是反应距离（反应时间内行驶的距离）与制动距离之和。速率是关键因素，因为制动距离随速率的平方而增加。
This means that if a car doubles its speed, the braking distance can become four times longer. IGCSE questions often link velocity and acceleration to real-world contexts like stopping distances, so you should be able to interpret factors such as road conditions, driver fatigue and vehicle mass.
这意味着如果车速加倍，制动距离可能会变为原来的四倍。IGCSE试题常常将速度和加速度与现实情境（如停车距离）联系起来，因此你需要能够解释路况、驾驶员疲劳和车辆质量等因素的影响。
11. Practical Investigations | 实验探究
In the laboratory, you can measure speed by using a stopwatch and a measured distance, or with light gates connected to a data logger. For acceleration, a trolley rolling down a ramp is a classic experiment. You can record the time taken to travel between two points and calculate velocity changes.
在实验室中，你可以用秒表和已知距离来测量速率，或者使用与数据采集器相连的光电门。研究加速度时，小车沿斜面滚落是经典实验。记录小车通过两点间的时间，并计算速度变化。
Plotting distance-time or velocity-time graphs from experimental data helps strengthen the link between theory and practice. Remember to repeat readings and take averages to reduce random errors.
根据实验数据绘制距离–时间图或速度–时间图，有助于加强理论与实践的衔接。务必重复测量并取平均值以减少随机误差。
12. Common Misconceptions and Exam Tips | 常见误区与考试提示
Many students confuse speed and velocity: remember velocity includes direction. Another pitfall is assuming zero acceleration means the object is stationary; in fact, zero acceleration means constant velocity, which could be a steady speed in a straight line.
许多学生混淆速率和速度：请记住速度包含方向。另一个易错点是认为加速度为零意味着物体静止；事实上，加速度为零意味着速度恒定，可以是沿直线匀速运动。
When using equations, always list the known quantities and check they are in the correct units—seconds for time, metres per second for velocity, and metres for distance. Finally, drawing and interpreting graphs is a key skill; practice finding gradients and areas.
使用公式时，务必列出已知量并检查单位是否正确——时间用秒、速度用米每秒、路程用米。最后，绘制和解读图像是一项关键技能；多练习求斜率和面积。
Published by TutorHao | Science Revision Series | aleveler.com
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