📚 Calculating Speed | 速度的计算
Speed tells us how fast an object is moving. In KS3 science, you need to be able to calculate speed, convert between units, and read distance-time graphs. This revision guide explains the key ideas clearly with worked examples and common mistakes to avoid.
速度告诉我们一个物体运动得有多快。在 KS3 科学中，你需要学会计算速度、换算单位并阅读距离-时间图。本复习指南通过清晰的要点、例题和常见错误，帮助你掌握核心概念。
1. What is speed? | 什么是速度？
Speed is a measure of how much distance an object travels in a certain amount of time. If a cyclist covers 20 metres every second, the speed is 20 metres per second.
速度是衡量物体在一定时间内通过多少距离的物理量。如果一名骑自行车的人每秒前进 20 米，那么速度就是 20 米每秒。
Speed is a scalar quantity, which means it has size (magnitude) but no direction. For example, saying 'the car is moving at 10 m/s' tells you the speed, but not the direction of travel.
速度是标量，这意味着它只有大小而没有方向。例如，说'汽车以 10 米/秒行驶'只告诉你速度的大小，但没有说明运动方向。
2. The speed equation | 速度公式
The relationship between distance, time and speed can be written as:
距离、时间与速度之间的关系可以写成：
speed = distance ÷ time
You can rearrange this equation to find distance or time:
你可以把这个公式变形来求距离或时间：
distance = speed × time
time = distance ÷ speed
Make sure the units match before you calculate. If distance is in metres and time is in seconds, speed will be in metres per second (m/s).
计算前要确保单位一致。如果距离以米为单位、时间以秒为单位，那么速度的单位就是米每秒 (m/s)。
3. Units of speed | 速度的单位
Speed can be measured in several units. The most common units in science are metres per second (m/s) and kilometres per hour (km/h).
速度可以用多种单位表示。科学中最常用的单位是米每秒 (m/s) 和千米每小时 (km/h)。
- metres per second (m/s) — 米每秒
- kilometres per hour (km/h) — 千米每小时
- miles per hour (mph) — 英里每小时
In physics, m/s is the standard SI unit of speed. However, when you travel by car, km/h or mph are often more convenient because the distances and times are larger.
在物理学中，米每秒是速度的标准国际单位。但在乘车出行时，千米每小时或英里每小时通常更方便，因为距离和时间都更大。
4. Using the formula triangle | 使用公式三角形
A formula triangle helps you remember how to rearrange the speed equation. Draw a triangle and put D at the top, with S and T in the bottom two corners.
公式三角形可以帮助你记住速度公式的变形。画一个三角形，将 D 放在顶部，S 和 T 放在底部两个角。
ime. To find distance, cover D and you get S × T. To find time, cover T and you get D ÷ S.
要求速度时，遮住 S。三角形显示 D 除以 T，所以速度 = 距离 ÷ 时间。要求距离时，遮住 D，得到 S × T。要求时间时，遮住 T，得到 D ÷ S。
5. Converting units | 单位换算
You often need to convert between m/s and km/h. The key conversion is:
你经常需要在米每秒和千米每小时之间进行换算。关键换算关系是：
1 m/s = 3.6 km/h
urs. One metre is 0.001 km and one second is 1/3600 of an hour, so 0.001 ÷ (1/3600) = 3.6.
这是因为要把米换算成千米、秒换算成小时。1 米 = 0.001 千米，1 秒 = 1/3600 小时，因此 0.001 ÷ (1/3600) = 3.6。
| Conversion | 换算 | 中文 |
|---|---|
| 1 m/s = 3.6 km/h | 1 米每秒 = 3.6 千米每小时 |
| 1 km/h ≈ 0.28 m/s | 1 千米每小时约等于 0.28 米每秒 |
6. Average speed and instantaneous speed | 平均速度与瞬时速度
Average speed is the total distance travelled divided by the total time taken. A car may speed up and slow down during a journey, but average speed gives one useful value for the whole trip.
平均速度是总路程除以总时间。汽车在行驶过程中可能会加速和减速，但平均速度给出了整个行程的一个有用数值。
Instantaneous speed is the speed at a particular moment. A speedometer in a car shows instantaneous speed, not average speed.
瞬时速度是某一特定时刻的速度。汽车上的速度表显示的是瞬时速度，而不是平均速度。
7. Distance-time graphs | 距离-时间图
A distance-time graph shows how distance changes with time. Time is plotted on the horizontal axis and distance on the vertical axis.
距离-时间图表示距离如何随时间变化。通常时间标在横轴上，距离标在纵轴上。
The gradient (slope) of a distance-time graph represents speed. A steeper line means a higher speed. A flat horizontal line means the object is stationary.
距离-时间图的斜率代表速度。线越陡，速度越大。一条水平的直线表示物体静止。
8. Worked example: car journey | 例题：汽车行程
A car travels 150 km in 2 hours. Calculate its average speed.
一辆汽车在 2 小时内行驶了 150 千米。计算它的平均速度。
Using speed = distance ÷ time:
使用公式 speed = distance ÷ time：
speed = 150 km ÷ 2 h = 75 km/h
The car's average
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