Alevel物理 力学 SUVAT 动量 能量 圆周 SHM
Why Mechanics Matters in A-Level AQA Physics
Mechanics is the backbone of A-Level Physics ： it accounts for roughly 30% of the AQA specification (Sections 3.4 和 3.6). From SUVAT equations to simple harmonic motion, these topics form the foundation for Paper 1 and Paper 2 exam questions. 力学是A-Level物理的核心模块，占AQA考纲约30%，涵盖运动学方程、牛顿定律、动量和能量守恒，直至圆周运动和简谐运动。
Whether you are targeting a B or an A*, mastering mechanics requires more than memorising formulas ： it demands systematic problem-solving methods and deep conceptual understanding. 要拿到高分，仅背诵公式远远不够，你需要系统化的解题方法和深刻的概念理解。This guide covers the AQA specification for Mechanics, walking through worked examples and common exam question patterns. 本指南覆盖AQA力学考纲，配有详细例题和常见考试题型分析。
1. SUVAT Equations: The Core Toolkit 运动学五大方程
The five SUVAT equations are the starting point for nearly every mechanics problem in AQA Paper 1. You must know when to use each one. 五个SUVAT方程是几乎所有力学题目的起点，你必须清楚每个方程的使用场景。
Equation 1
v = u + at
No displacement s
Equation 2
s = ut + ½at²
No final velocity v
Equation 3
s = ½(u+v)t
No acceleration a
Equation 4
v² = u² + 2as
No time t
Equation 5
s = vt - ½at²
No initial velocity u
How to choose (选择技巧): Write down the four variables you know (s, u, v, a, t) and the one you need to find. Pick the equation that connects them. AQA exam questions often give three variables and ask for the fourth ： the "fill-in-the-blank" approach. 先列出已知的三个量和待求的一个量，选择包含这四个量的方程即可。
Worked Example 例题: A ball is launched vertically upward at 15 m/s from the ground. Find the maximum height reached (g = 9.81 m/s²). 一个球以15 m/s的初速度竖直向上抛出，求最大高度。
Solution 解答: At maximum height, v = 0. Known: u = 15, v = 0, a = -9.81. Need: s. Use v² = u² + 2as: 0 = 15² + 2(-9.81)s = s = 225 / 19.62 = 11.47 m. 在最高点v=0，选用v² = u² + 2as，代入得最大高度11.47米。
2. Free-Body Diagrams and Newton's Laws 受力分析与牛顿定律
Newton's Three Laws are the conceptual heart of mechanics. Every force problem starts with a clear free-body diagram. 牛顿三定律是力学的概念核心，每道受力题都从清晰的受力分析图开始。
Newton's First Law (牛顿第一定律): An object remains at rest or in uniform motion unless acted on by a resultant force. In equilibrium problems, ΣF = 0. 物体在合力为零时保持静止或匀速直线运动。平衡问题中合力为零。
Newton's Second Law (牛顿第二定律): F = ma. The resultant force equals mass times acceleration. This is the most-used equation in mechanics. 合力等于质量乘以加速度，这是力学中使用频率最高的方程。
Newton's Third Law (牛顿第三定律): For every action there is an equal and opposite reaction. These forces act on DIFFERENT objects ： a common exam trap. 作用力与反作用力大小相等方向相反，但作用在不同物体上，这是常见考试陷阱。
Free-body diagram method (受力分析四步法):
Step 1: Isolate the object ： draw it as a dot or box. 隔离物体，画成点或方框。
Step 2: Draw all forces acting ON it ： weight (mg, always downward), normal reaction (perpendicular to surface), tension (along string), friction (opposing motion). 画出所有作用在物体上的力。
Step 3: Choose coordinate axes ： typically parallel and perpendicular to the slope for inclined plane problems. 选坐标系，斜面问题通常沿斜面和垂直于斜面方向。
Step 4: Resolve forces along axes and apply F = ma in each direction. 沿坐标轴分解力，分别在两方向应用F=ma。
3. Momentum and Impulse 动量与冲量
Momentum (p = mv) is conserved in all collisions and explosions when no external resultant force acts. This principle is tested heavily in AQA Paper 1. 动量守恒是无外力作用时所有碰撞和爆炸过程的核心原理，是AQA Paper 1高频考点。
Momentum 动量
p = mv
kg·m/s, vector quantity
Impulse 冲量
Δp = F·Δt
Area under F-t graph
Conservation 守恒
m₁u₁+m₂u₂ = m₁v₁+m₂v₂
Elastic: KE conserved
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