A-Level Mathematics | A-Level 数学：一维变加速度运动分析

📚 A-Level Mathematics | A-Level 数学：一维变加速度运动分析

In A-Level Mechanics, motion is often studied under constant acceleration using the standard SUVAT equations. However, real-world problems frequently involve acceleration that changes with time, velocity, or displacement. This article provides a systematic approach to analysing one-dimensional motion with variable acceleration, using calculus methods that are essential for exam success.

在 A-Level 力学中，我们通常使用标准 SUVAT 方程研究匀加速运动。然而，现实问题中的加速度常常随时间、速度或位移而变化。本文提供一种系统化的方法，运用微积分工具分析一维变加速度运动，这是考试取得高分的关键技能。

1. From Constant to Variable Acceleration | 从匀加速到变加速

The SUVAT equations, such as v = u + at and s = ut + ½at², are only valid when acceleration is constant. When acceleration varies, these formulas break down. In such cases, we must return to the fundamental definitions: acceleration is the rate of change of velocity, and velocity is the rate of change of displacement.

SUVAT 方程（如 v = u + at 和 s = ut + ½at²）仅在加速度恒定时成立。当加速度变化时，这些公式不再适用。此时我们必须回到基本定义：加速度是速度的变化率，速度是位移的变化率。

The fundamental relationships are expressed as derivatives:

a = dv/dt, v = ds/dt, a = v·(dv/ds)

The third relationship, a = v·(dv/ds), is particularly useful when acceleration is given as a function of displacement. It connects acceleration directly to the spatial rate of change of velocity.

第三个关系式 a = v·(dv/ds) 在加速度表示为位移的函数时特别有用，它将加速度与速度关于位移的变化率直接联系起来。

2. Types of Variable Acceleration Problems | 变加速度问题的类型

Variable acceleration problems in A-Level exams generally fall into three categories, depending on how acceleration is expressed:

A-Level 考试中的变加速度问题通常分为三类，取决于加速度的表达方式：

a as a function of t (time): a = f(t) — integrate to find v(t) and s(t).
a as a function of x (displacement): a = f(x) — use a = v·(dv/ds) and integrate with respect to s.
a as a function of v (velocity): a = f(v) — separate variables and integrate.

a 为 t（时间）的函数：a = f(t) — 积分求 v(t) 和 s(t)。
a 为 x（位移）的函数：a = f(x) — 使用 a = v·(dv/ds) 对位移积分。
a 为 v（速度）的函数：a = f(v) — 分离变量后积分。

Recognising the type quickly is essential, because the integration strategy differs fundamentally for each case. In particular, when acceleration is a function of velocity, we often need to set up differential equations such as dv/dt = f(v) and solve by separation of variables.

快速识别类型至关重要，因为每种情况的积分策略完全不同。特别地，当加速度是速度的函数时，我们通常需要建立形如 dv/dt = f(v) 的微分方程，并通过分离变量法求解。

3. Finding Velocity from Acceleration | 由加速度求速度

When acceleration is given as a function of time, a = f(t), the velocity is obtained by integrating acceleration with respect to time:

当加速度是时间的函数 a = f(t) 时，速度通过对加速度关于时间积分得到：

v(t) = ∫ f(t) dt + C

The constant of integration C is determined by the initial condition, typically v(0) = u or v(t₀) = v₀. Without an initial condition, the velocity function is not uniquely determined, so always look for the initial velocity in the problem statement.

积分常数 C 由初始条件确定，通常是 v(0) = u 或 v(t₀) = v₀。没有初始条件，速度函数就不是唯一确定的，因此务必注意题目中给出的初速度。

Worked Example: A particle moves in a straight line with acceleration a = 6t – 4 m/s². Given that v = 2 m/s when t = 0, find v(t).

例题：质点沿直线运动，加速度 a = 6t – 4 m/s²。已知 t = 0 时 v = 2 m/s，求 v(t)。

v(t) = ∫(6t – 4)dt = 3t² – 4t + C

Using v(0) = 2 gives C = 2, hence v(t) = 3t² – 4t + 2.

由 v(0) = 2 得 C = 2，因此 v(t) = 3t² – 4t + 2。

4. Finding Displacement from Velocity | 由速度求位移

Once the velocity function v(t) is known, the displacement is obtained by a second integration:

一旦求得速度函数 v(t)，位移通过对速度再次积分得到：

s(t) = ∫ v(t) dt + D

The second constant D is determined by the initial displacement, often s(0) = 0 if the origin is chosen at the starting point. Note that this gives displacement (change in position), not necessarily total distance travelled.

第二个积分常数 D 由初始位移确定，若原点选在起点通常 s(0) = 0。注意，这里得到的是位移（位置变化量），不一定是总路程。

Continuing the Example: With v(t) = 3t² – 4t + 2, if the particle starts at the origin, then:

接上例：若 v(t) = 3t² – 4t + 2，且质点从原点出发，则：

s(t) = ∫(3t² – 4t + 2)dt = t³ – 2t² + 2t

Since s(0) = 0, no additional constant is needed. This cubic displacement function completely describes the motion of the particle over time.

因为 s(0) = 0，无需额外加常数。这个三次位移函数完整描述了质点随时间的运动状态。

5. Definite Integrals for Specific Intervals | 定积分求特定区间

When the question asks for displacement or distance travelled over a specific time interval, definite integration is often more direct:

当题目要求特定时间区间内的位移或路程时，定积分往往更直接：

s(t₂) – s(t₁) = ∫₍t₁₎^₍t₂₎ v(t) dt

This gives the net displacement between t₁ and t₂. If the question asks for total distance, you must check whether the particle changes direction within the interval, which happens when v(t) = 0. If it does, split the integral at those points and sum the absolute values.

该式给出从 t₁ 到 t₂ 的净位移。如果题目要求的是总路程，必须检查质点是否在区间内改变方向（v(t) = 0 时发生折返）。若发生了折返，需要在折返点分割积分区间，并取各部分绝对值之和。

This is a classic exam trap: many students integrate and take the answer as distance, losing marks when the particle reverses direction. Always sketch or analyse the sign of v(t) first.

这是一个经典的考试陷阱：许多学生直接积分并将结果当作路程，在质点折返时丢分。务必先画出草图或分析 v(t) 的符号。

6. Acceleration as a Function of Displacement | 加速度为位移的函数

When a = f(x), the relationship a = v·(dv/ds) becomes extremely valuable. Rearranging this gives:

当 a = f(x) 时，关系式 a = v·(dv/ds) 极具价值。整理后得：

v dv = f(x) dx

Integrating both sides yields a relationship between velocity and displacement, independent of time:

两边积分得到速度与位移之间的关系，该关系与时间无关：

½v² = ∫ f(x) dx + C

Worked Example: A particle moves with acceleration a = -4x m/s². Find v as a function of x if the particle starts from rest at x = 2 m.

例题：质点以加速度 a = -4x m/s² 运动。若质点从 x = 2 m 处由静止出发，求 v 关于 x 的函数。

½v² = ∫(-4x)dx = -2x² + C

Using v = 0 at x = 2: 0 = -8 + C, so C = 8. Hence ½v² = 8 – 2x², giving v² = 16 – 4x².

利用 x = 2 时 v = 0：0 = -8 + C，故 C = 8。因此 ½v² = 8 – 2x²，即 v² = 16 – 4x²。

This equation describes simple harmonic motion-like behaviour where velocity becomes zero at x = ±2, confirming the particle oscillates between these boundaries.

该方程描述了类似简谐运动的特性：速度在 x = ±2 处为零，证实质点在两个边界之间往复运动。

7. Acceleration as a Function of Velocity | 加速度为速度的函数

When a = f(v), we treat the problem as a differential equation. The two primary approaches are:

当 a = f(v) 时，我们将问题视为微分方程。两种主要方法是：

Method 1 — Find time: If a = dv/dt = f(v), separate variables:

方法 1 — 求时间：若 a = dv/dt = f(v)，分离变量：

dt = dv / f(v) ⟹ t = ∫ dv / f(v) + C

Method 2 — Find displacement: If a = v·(dv/ds) = f(v), then:

方法 2 — 求位移：若 a = v·(dv/ds) = f(v)，则：

ds = v dv / f(v) ⟹ s = ∫ v dv / f(v) + C

This is often called the “resistance proportional to velocity” scenario common in mechanics problems involving drag. The resulting integrals often involve natural logarithms, so be comfortable with logarithmic integration.

这常被称为“阻力与速度成正比”的情形，常见于涉及阻力的力学问题。所得积分通常包含自然对数，因此必须熟练掌握对数积分。

8. Finding Maximum and Minimum Values | 求最大值和最小值

Exam questions routinely ask for maximum or minimum velocity, displacement, or acceleration. The strategy is to find stationary points using differentiation:

考试题常要求最大或最小速度、位移或加速度。解题策略是使用微分求驻点：

Maximum displacement: set v = ds/dt = 0, check the sign change of v to confirm a maximum.
Maximum velocity: set a = dv/dt = 0, verify with the second derivative that it is a maximum.
Maximum acceleration: set da/dt = 0 if a = f(t).

最大位移：令 v = ds/dt = 0，通过 v 的符号变化确认是否为最大值。
最大速度：令 a = dv/dt = 0，用二阶导数验证是否为最大值。
最大加速度：若 a = f(t)，令 da/dt = 0。

For a = v·(dv/ds), maximum speed occurs when a = 0, provided the acceleration changes sign at that point. This is particularly relevant in SHM problems where maximum speed occurs at the equilibrium position where acceleration vanishes.

对于 a = v·(dv/ds)，最大速度发生在 a = 0 处，前提是加速度在该点改变符号。这在简谐运动问题中尤为相关——最大速度出现在加速度为零的平衡位置。

9. Graphical Interpretation | 图形分析

Understanding the graphical relationships is vital for interpreting problems and verifying answers:

理解图形关系对于解读问题和验证答案至关重要：

Graph	Gradient / Slope	Area Under Curve
s-t (displacement-time)	velocity v	not usually relevant
v-t (velocity-time)	acceleration a	displacement s
a-t (acceleration-time)	jerk (not in A-Level)	change in velocity Δv

For variable acceleration, these graphs are curves, not straight lines. The slope at any point on a v-t graph gives the instantaneous acceleration, and the area under the v-t curve between two times gives the displacement.

对于变加速度，这些图形是曲线而非直线。v-t 图上任意一点的斜率给出瞬时加速度，而 v-t 曲线在时间区间下的面积给出位移。

10. Typical Exam Pitfalls | 常见考试陷阱

Below are the most common mistakes students make in variable acceleration questions, along with strategies to avoid them:

以下是学生在变加速度问题中最常见的错误以及规避策略：

Using SUVAT for variable acceleration: Always check that acceleration is constant before applying v = u + at.
Forgetting integration constants: Always apply initial conditions to find C and D.
Confusing distance with displacement: Distance is always positive; displacement can be negative when the particle reverses direction.
Misidentifying the correct differential equation: Use a = dv/dt when a depends on t, and a = v·(dv/ds) when a depends on s.
Wrong limits in definite integrals: Check whether the limits are time values or displacement values.

对变加速度使用 SUVAT 方程：应用 v = u + at 前务必确认加速度恒定。
忘记积分常数：务必利用初始条件求出 C 和 D。
混淆路程与位移：路程恒为正；位移在质点折返时可能为负。
选错微分方程：a 依赖 t 时用 a = dv/dt；a 依赖 s 时用 a = v·(dv/ds)。
定积分上下限错误：检查上下限是时间值还是位移值。

11. Summary of Problem-Solving Strategy | 解题策略总结

To approach any variable acceleration problem methodically, follow this five-step process:

为了有条不紊地解决任何变加速度问题，请遵循以下五步流程：

Identify the form of acceleration: Is a given as a function of t, s, or v?
Choose the correct relationship: a = dv/dt for time-based; a = v·(dv/ds) for displacement-based; separate variables for v-based.
Integrate: Apply the chosen method, keeping the integration constant.
Determine constants: Substitute initial conditions to evaluate all integration constants.
Answer the exact question asked: Distance vs displacement, maximum vs minimum, specific time vs general expression.

识别加速度的形式：a 是 t、s 还是 v 的函数？
选择正确的关系式：基于时间用 a = dv/dt；基于位移用 a = v·(dv/ds)；基于速度用分离变量法。
积分：使用所选方法，保留积分常数。
确定常数：代入初始条件求所有积分常数。
准确回答题目所求：路程还是位移、最大还是最小、特定时刻还是一般表达式。

Mastering these techniques will give you a solid foundation for tackling not only kinematics problems but also circular motion and simple harmonic motion, both of which rely on the calculus of variable acceleration.

掌握这些技巧不仅能为解决运动学问题打下坚实基础，也为后续学习圆周运动和简谐运动提供了保障，因为两者都依赖于变加速度的微积分分析。

12. Final Exam Tips | 考前锦囊

In the final moments before your exam, remember these key points:

临考之前，请记住以下关键要点：

Read the question carefully to identify whether acceleration is constant or variable before choosing your method.
Write down the differential equation or integral setup explicitly — examiners award method marks even if your integration is imperfect.
Always return to the original question after solving to verify you have answered what was asked (units, direction, specific time).
Practise past papers to become familiar with the commonly tested scenarios.

仔细审题，在选择方法前先确认加速度是恒定还是变化的。
明确写出微分方程或积分表达式——即使积分不完美，考官也会给方法分。
解完题后务必回到原题，验证是否回答了所问内容（单位、方向、特定时刻）。
多做历年真题，熟悉常考场景。

With a solid grasp of the calculus of motion, you can confidently handle any variable acceleration problem that appears on your A-Level exam.

扎实掌握运动的微积分方法后，你就能自信从容地应对 A-Level 考试中出现的任何变加速度问题。

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A-Level Mathematics | A-Level 数学：一维变加速度运动分析