Edexcel A-Level Physics: Motion – Displacement, Speed, Velocity and Acceleration | 爱德思 A-Level 物理：运动学——位移、速率、速度与加速度

📚 Edexcel A-Level Physics: Motion – Displacement, Speed, Velocity and Acceleration | 爱德思 A-Level 物理：运动学——位移、速率、速度与加速度

In Edexcel A-Level Physics, Topic 2.1.1 introduces the language of motion. You need to distinguish clearly between scalar and vector quantities, and use displacement, speed, velocity and acceleration in calculations and graphs. This topic underpins mechanics, projectiles, forces and later work on momentum and circular motion.

在爱德思 A-Level 物理中，第 2.1.1 节介绍运动学的基本语言。你需要清楚区分标量与矢量，并能运用位移、速率、速度和加速度进行计算与图像分析。这一主题是力学、抛体运动、力以及后续动量与圆周运动的基础。

1. Scalar and Vector Quantities | 标量与矢量

A scalar quantity has magnitude only, such as distance, speed, mass and energy. A vector quantity has both magnitude and direction, such as displacement, velocity, acceleration and force. Direction is essential when adding vectors: two perpendicular velocities of 3 m s⁻¹ north and 4 m s⁻¹ east give a resultant velocity of 5 m s⁻¹ at 53.1° to the east.

标量只有大小，如路程、速率、质量和能量；矢量既有大小又有方向，如位移、速度、加速度和力。方向在矢量合成中至关重要：例如 3 m s⁻¹ 向北与 4 m s⁻¹ 向东的速度合成后，合速度为 5 m s⁻¹，方向为东偏北 53.1°。

2. Displacement vs Distance | 位移与路程

Distance is the total length of the path travelled and is always positive. Displacement is the straight-line change in position from start to finish; it can be positive, negative or zero. For one complete lap of a 400 m running track, distance = 400 m but displacement = 0 because the starting and finishing points are the same.

路程是物体运动路径的总长度，始终为正；位移是从起点到终点的直线位置变化，可以为正、负或零。绕 400 m 跑道一整圈，路程为 400 m，但位移为 0，因为起点与终点重合。

3. Speed and Velocity | 速率与速度

Speed is the rate of change of distance, calculated as total distance divided by time. Velocity is the rate of change of displacement, so it must use total displacement in a given direction. Average velocity can therefore be zero even when average speed is not, such as in a round trip.

速率是路程对时间的变化率，用总路程除以时间计算。速度是位移对时间的变化率，因此必须使用给定方向上的总位移。平均速度可能为零而平均速率不为零，例如往返运动。

v = Δs ÷ Δt

In this equation, Δs is displacement, not distance. Instantaneous velocity is the gradient of a displacement-time graph at a specific point. A positive gradient means motion in the positive direction; a negative gradient means motion in the negative direction.

在这个方程中，Δs 是位移，不是路程。瞬时速度是位移-时间图像在某一点的切线斜率。正斜率表示向正方向运动；负斜率表示向负方向运动。

4. Acceleration | 加速度

Acceleration is the rate of change of velocity. Its SI unit is m s⁻². Acceleration is a vector, so it has both magnitude and direction. When an object slows down, the acceleration is often called deceleration or retardation, provided the chosen positive direction is the direction of motion.

加速度是速度对时间的变化率，其国际单位为 m s⁻²。加速度是矢量，既有大小又有方向。当物体减速时，如果选运动方向为正，加速度通常称为减速度或负加速度。

a = Δv ÷ Δt

An object can accelerate even while moving backwards. If its velocity changes from -5 m s⁻¹ to -10 m s⁻¹, the acceleration is -2.5 m s⁻² if the positive direction is unchanged, showing that the object is speeding up in the negative direction.

物体在倒退时也可以加速。如果速度从 -5 m s⁻¹ 变为 -10 m s⁻¹，在正方向不变时加速度为 -2.5 m s⁻²，说明物体正在负方向上加速。

5. Equations of Motion | 运动学方程

For uniform acceleration, Edexcel requires you to use the four SUVAT equations. Define u = initial velocity, v = final velocity, a = acceleration, t = time, and s = displacement. These equations only work when acceleration is constant.

对于匀变速直线运动，爱德思考纲要求使用四个 SUVAT 方程。定义 u = 初速度，v =

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