📚 How Is Speed Measured | 速度是如何测量的？
Speed tells us how quickly an object moves. In KS3 science, we measure speed by finding the distance travelled and the time taken. This article explains the key methods, units and graphs used to measure speed accurately.
速度告诉我们物体运动的快慢。在 KS3 科学中，我们通过测量物体移动的距离和所用的时间来测量速度。本文将解释测量速度的主要方法、单位和图表。
1. What Speed Means | 速度的含义
Speed is a measure of how far an object travels in a certain amount of time. A faster object covers more distance in the same time, or the same distance in less time.
速度是衡量物体在一定时间内移动多远的量。较快的物体在相同时间内会移动更远的距离，或者在更短的时间内移动相同的距离。
Speed is a scalar quantity, which means it only has size (magnitude) and no direction. If direction is included, we call it velocity.
速率是一个标量，这意味着它只有大小而没有方向。如果加上方向，我们就称之为速度（velocity）。
In everyday language, people often mix up speed and velocity. In physics, however, this difference matters when describing motion in a straight line or around a curve.
在日常语言中，人们经常混淆速率和速度。然而在物理学中，当描述直线或曲线运动时，这种区别很重要。
2. The Speed Equation | 速度公式
The formula for average speed is:
平均速度的公式是：
average speed = total distance ÷ total time
In symbols, this is written as:
用符号表示为：
v = d ÷ t
Here, v stands for speed, d stands for distance, and t stands for time. You can rearrange the formula to find distance or time.
其中 v 代表速度，d 代表距离，t 代表时间。你可以变换公式来求距离或时间。
distance = speed × time
time = distance ÷ speed
Always write down the formula first, substitute the numbers with units, and then calculate the answer.
始终先写下公式，代入带单位的数值，然后计算答案。
3. Units of Speed | 速度的单位
In science, the standard unit of speed is metres per second, written as m/s or m s⁻¹. For everyday travel, kilometres per hour (km/h) is common.
在科学中，速度的标准单位是米每秒，写作 m/s 或 m s⁻¹。在日常交通中，常用千米每小时（km/h）。
Other units include miles per hour (mph) and centimetres per second (cm/s). Always check the units before calculating, because mixing m and km will give a wrong answer.
其他单位包括英里每小时（mph）和厘米每秒（cm/s）。在计算之前务必检查单位，因为将米和千米混用会得出错误答案。
| Object | Typical speed |
|---|---|
| Walking | 1.4 m/s |
| Running | 5 m/s |
| Car on a road | 25 m/s |
| Sound in air | 343 m/s |
| Light in a vacuum | 300,000,000 m/s |
Using the correct unit is just as important as getting the number right.
使用正确的单位与算对数字同样重要。
4. Measuring Distance and Time | 测量距离和时间
To measure speed directly, you need a distance and a time. In the lab, a metre rule or measuring tape measures distance, and a stopwatch measures time.
要直接测量速度，你需要距离和时间。在实验室中，米尺或卷尺测量距离，秒表测量时间。
For longer distances, you can use trundle wheels or GPS devices. For very short times, electronic timers are more accurate than hand-held stopwatches.
对于较长的距离，可以使用测距轮或 GPS 设备。对于非常短的时间，电子计时器比手持秒表更准确。
When measuring a running person, mark a start line and a finish line. Measure the distance between them, then use a stopwatch to record the time from start to finish.
测量跑步者时，标记起点线和终点线。测量两者之间的距离，然后用秒表记录从起点到终点的时间。
Human reaction time is a source of error in stopwatch measurements. Repeating the measurement helps to improve reliability.
人的反应时间是秒表测量中的一个误差来源。重复测量有助于提高可靠性。
5. Average Speed vs Instantaneous Speed | 平均速度与瞬时速度
Average speed is the total distance divided by the total time for a whole journey. Instantaneous speed is the speed at one particular moment.
平均速度是总距离除以总时间。瞬时速度是某一特定时刻的速度。
A car may have an average speed of 60 km/h on a journey, but its instantaneous speed changes when it speeds up, slows down or stops.
一辆汽车在旅途中的平均速度可能是 60 km/h，但它的瞬时速度在加速、减速或停车时会发生变化。
A speedometer shows instantaneous speed, while a calculation using total journey distance and total journey time gives average speed.
速度表显示瞬时速度，而用总路程和总时间计算得到的是平均速度。
In the lab, if an object moves at a steady speed, its average speed and instantaneous speed are the same.
在实验室中，如果物体匀速运动，那么它的平均速度和瞬时速度相同。
6. Distance-Time Graphs | 距离-时间图
A distance-time graph shows how distance changes with time. The gradient (slope) of the line equals the speed.
距离-时间图显示距离随时间的变化。图线的斜率等于速度。
A straight, sloping line means constant speed. A horizontal line means the object is stationary. A steeper line means a higher speed.
一条笔直倾斜的线表示匀速运动。水平线表示物体静止。线越陡表示速度越大。
gradient = change in distance ÷ change in time = speed
To find the gradient, pick two points on the line, subtract their distances and subtract their times, then divide the distance change by the time change.
要计算斜率，选择线上的两个点，用它们的距离差除以时间差。
A curved line on a distance-time graph means the speed is changing. If the curve gets steeper, the object is accelerating.
距离-时间图上的曲线表示速度在变化。如果曲线越来越陡，说明物体在加速。
7. Light Gates and Motion Sensors | 光门与运动传感器
A light gate sends a beam of light across a track. When an object breaks the beam, the timer starts; when the object passes, the timer stops.
光门在轨道上发射一束光。当物体遮住光束时，计时器开始计时；当物体通过后，计时器停止。
If you know the length of the card on the object, you can calculate speed using:
如果你知道物体上卡片的长度，就可以用以下公式计算速度：
speed = card length ÷ interruption time
Light gates are more accurate than stopwatches because they remove human reaction time.
光门比秒表更准确，因为它消除了人的反应时间。
Motion sensors use ultrasound to track position continuously. A computer plots distance and time, then calculates speed from the data.
运动传感器使用超声波连续跟踪位置。计算机记录距离和时间数据，然后根据这些数据计算速度。
8. Ticker Tape Timers | 打点计时器
A ticker tape timer makes dots on a paper tape at regular time intervals, usually 50 dots per second. The distance between dots shows the motion.
打点计时器在纸带上以固定的时间间隔打点，通常每秒 50 个点。点与点之间的距离显示了运动情况。
Equal spacing means constant speed. Increasing spacing means acceleration, and decreasing spacing means deceleration.
间距相等表示匀速运动。间距增大表示加速，间距减小表示减速。
To calculate speed from a ticker tape, measure the distance between a group of dots, then divide by the time taken for that interval. For 50 dots per second, each gap is 0.02 s.
要从打点纸带计算速度，测量一组点之间的距离，然后除以该间隔所用的时间。对于每秒 50 个点，每个间隔为 0.02 秒。
speed = distance between dots ÷ time interval
Ticker tape timers are useful for studying acceleration and deceleration in the lab.
打点计时器在实验室中常用于研究加速和减速运动。
9. Speedometers and Radar Guns | 速度表与雷达测速枪
A speedometer in a car measures instantaneous speed by using the rotation of the wheels or gears. It displays speed in km/h or mph.
汽车中的速度表通过车轮或齿轮的转动来测量瞬时速度，并以 km/h 或 mph 显示速度。
Police use radar guns to measure the speed of moving vehicles. The radar gun sends out radio waves, which reflect off the vehicle and change frequency depending on its speed.
警察使用雷达测速枪来测量行驶车辆的速度。雷达枪发射无线电波，电波经车辆反射后，其频率根据车辆速度而变化。
This change in frequency is called the Doppler effect. By measuring the frequency shift, the radar gun calculates the vehicle's speed.
这种频率变化称为多普勒效应。通过测量频率偏移，雷达枪计算出车辆的速度。
Radar guns are portable and can measure speed from a distance, which makes them useful for roadside speed checks.
雷达枪便于携带，能在远处测量速度，因此适用于路边测速检查。
10. Unit Conversions and Accuracy | 单位换算与准确度
To convert from km/h to m/s, divide by 3.6. To convert from m/s to km/h, multiply by 3.6.
从 km/h 转换为 m/s，除以 3.6。从 m/s 转换为 km/h，乘以 3.6。
72 km/h = 72 ÷ 3.6 = 20 m/s
10 m/s = 10 × 3.6 = 36 km/h
Measurement accuracy depends on human reaction time and instrument precision. Repeating measurements and taking an average reduces random errors.
测量准确度取决于人的反应时间和仪器的精确度。重复测量并取平均值可以减少随机误差。
Systematic errors, such as a stopwatch that runs slow or a ruler with a damaged zero end, affect every measurement in the same way.
系统误差，例如秒表走得慢或尺子的零端损坏，会以相同的方式影响每一次测量。
Always align your eye perpendicular to the scale when reading a ruler to avoid parallax error, and use the same start and finish points for all trials.
读取尺子时，视线应与刻度垂直，以避免视差误差，并且在所有试验中使用相同的起点和终点。
Published by TutorHao | Physics Revision Series | aleveler.com
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