📚 Distance-Time Graphs: Interpreting Motion | 距离-时间图：解读运动
A distance-time graph is one of the most useful tools in KS3 Physics. It shows how the distance travelled by an object changes as time passes, and it allows you to describe motion, calculate speed, and compare different parts of a journey.
距离-时间图是 KS3 物理中最有用的工具之一。它展示物体走过的距离如何随时间变化，让你能够描述运动、计算速度，并比较一段旅程的不同部分。
1. What Is a Distance-Time Graph? | 什么是距离-时间图？
A distance-time graph is a visual record of motion. The horizontal x-axis represents time, while the vertical y-axis represents distance from a chosen starting point. Each point on the graph shows the distance the object has travelled at a particular time.
距离-时间图是运动的可视化记录。水平 x 轴表示时间，竖直 y 轴表示从选定起点开始的距离。图上的每个点都显示物体在某一特定时刻所走过的距离。
When the points are joined, the line tells the story of a journey. For example, a line rising to the right shows that distance is increasing as time increases, which means the object is moving away from the start.
将这些点连接起来后，线条就讲述了旅程的过程。例如，一条向右上方上升的线表示距离随时间增加，说明物体正在远离起点。
2. Axes and Units | 坐标轴与单位
Time is usually plotted on the x-axis in seconds (s), minutes (min) or hours (h). Distance is plotted on the y-axis in metres (m) or kilometres (km). You must check the label and scale on each axis before reading or plotting values.
时间通常以秒（s）、分钟（min）或小时（h）为单位画在 x 轴上。距离以米（m）或千米（km）为单位画在 y 轴上。在读取或绘制数值之前，必须检查每个坐标轴的标签和刻度。
If one large square represents 10 m on the y-axis and 2 s on the x-axis, then a point halfway along a square does not automatically represent 5 m and 1 s. Always calculate the value of one small square first.
如果 y 轴上一大格代表 10 m，x 轴上一大格代表 2 s，那么格中间的一点并不自动代表 5 m 和 1 s。务必先算出一小格代表的数值。
- X-axis: time in seconds (s), minutes (min) or hours (h) | X 轴：时间，单位为秒（s）、分钟（min）或小时（h）
- Y-axis: distance in metres (m) or kilometres (km) | Y 轴：距离，单位为米（m）或千米（km）
3. Reading Values from the Graph | 从图中读取数值
To find the distance at a certain time, locate the time on the x-axis, draw a vertical construction line up to the plotted line, then draw a horizontal line across to the y-axis. The value where this line meets the y-axis is the distance travelled.
要找出某一时刻的距离，先在 x 轴上找到该时间，画一条竖直的辅助线向上到达图中曲线，再画一条水平线到 y 轴。这条线与 y 轴相交处的数值就是所走的距离。
To find the time at which a certain distance was reached, reverse the process: start on the y-axis, move horizontally to the line, then drop vertically to the x-axis.
要找出到达某一距离所用的时间，反过来操作：从 y 轴开始，水平移动到图线，再竖直向下到 x 轴。
For example, if the graph passes through the point (4 s, 20 m), then after 4 seconds the object has travelled 20 metres.
例如，如果图线经过点（4 s, 20 m），那么 4 秒后物体已经走了 20 米。
4. Gradient Equals Speed | 斜率等于速度
The gradient of a distance-time graph represents speed. Gradient is calculated as the vertical change divided by the horizontal change between two points on a straight line.
距离-时间图的斜率表示速度。斜率是直线上两点之间竖直变化量除以水平变化量。
speed = distance ÷ time
On a distance-time graph, vertical change is distance, and horizontal change is time. Therefore the gradient equals speed. A steeper line has a larger gradient, so it represents a higher speed.
在距离-时间图上，竖直变化量是距离，水平变化量是时间。因此斜率等于速度。线越陡，斜率越大，代表的速度也越大。
Units of speed depend on the units used on the axes. If distance is in metres and time is in seconds, speed is in metres per second (m/s). If distance is in kilometres and time is in hours, speed is in kilometres per hour (km/h).
速度的单位取决于坐标轴使用的单位。如果距离以米为单位、时间以秒为单位，速度就是米每秒（m/s）。如果距离以千米为单位、时间以小时为单位，速度就是千米每小时（km/h）。
5. Horizontal Line Means Stationary | 水平线表示静止
A flat horizontal line on a distance-time graph means that distance is not changing while time passes. This tells you the object is stationary: it is not moving at all.
距离-时间图上的水平直线表示随着时间推移距离没有变化。这说明物体是静止的：它完全没有移动。
The gradient of a horizontal line is zero, so the speed is zero. In a real journey, a horizontal section could represent a pause at a bus stop, a break during a walk, or waiting at traffic lights.
水平线的斜率为零，因此速度为零。在真实旅程中，水平段可以表示在公交车站停留、步行中休息或等红绿灯。
Do not confuse a horizontal line on a distance-time graph with a horizontal line on a speed-time graph. On a distance-time graph, horizontal always means stationary.
不要把距离-时间图上的水平线与速度-时间图上的水平线混淆。在距离-时间图上，水平线始终表示静止。
6. Constant Speed: Straight Sloped Line | 匀速：倾斜直线
A straight line that slopes upwards at a constant angle represents constant speed. The object covers equal distances in equal time intervals throughout this section.
以恒定角度向上倾斜的直线表示匀速运动。物体在这段路程中每经过相等的时间间隔就走过相等的距离。
Because the line is straight, its gradient is the same everywhere. This means the speed does not change. The steeper the straight line, the greater the constant speed.
因为线是直的，所以它的斜率处处相同。这意味着速度没有变化。直线越陡，匀速运动的速度越大。
ravels 100 m every 10 s, the graph is a straight line, and the speed is 100 ÷ 10 = 10 m/s.
例如，如果一名骑自行车的人每 10 秒走 100 米，图线就是一条直线，速度为 100 ÷ 10 = 10 m/s。
7. Acceleration and Deceleration: Curved Lines | 加速与减速：曲线
When a distance-time graph curves upwards and becomes steeper, the gradient is increasing. This means the object is speeding up, or accelerating.
当距离-时间图向上弯曲并且越来越陡时，斜率在增大。这意味着物体在加速，也就是速度越来越快。
When the curve becomes less steep as time goes on, the gradient is decreasing. This means the object is slowing down, or decelerating.
当曲线随着时间的推移变得越来越平缓时，斜率在减小。这意味着物体在减速，也就是速度越来越慢。
In KS3, you do not usually need to calculate acceleration from a distance-time graph. You only need to describe whether the object is speeding up, slowing down, or moving at a steady speed.
在 KS3 阶段，你通常不需要从距离-时间图中计算加速度。你只需要描述物体是在加速、减速还是匀速运动。
8. Calculating Speed from a Graph | 从图中计算速度
To calculate speed from a straight section of a distance-time graph, choose two clear points on that section. Subtract the smaller distance from the larger distance to find the change in distance. Subtract the smaller time from the larger time to find the change in time. Then divide change in distance by change in time.
要从距离-时间图的直线段计算速度，请在该段上选择两个清晰点。用较大距离减去较小距离，得到距离变化量；用较大时间减去较小时时间，得到时间变化量；然后用距离变化量除以时间变化量。
speed = (30 m - 10 m) ÷ (6 s - 2 s) = 20 m ÷ 4 s = 5 m/s
This method works for any straight section of the graph. For curved sections, the speed is changing, so a single speed value cannot describe the entire curve.
这个方法适用于图上的任何直线段。对于曲线段，速度在不断变化，所以单个速度值不能描述整段曲线。
Always include units in your final answer. If the distance change is in metres and the time change is in seconds, the speed will be in m/s.
最终答案一定要写上单位。如果距离变化量以米为单位，时间变化量以秒为单位，速度就是 m/s。
9. Comparing Speeds on the Same Graph | 在同一图中比较速度
When two or
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