📚 Year 9 Edexcel Physics: Formulas and Theorems Quick Reference Guide | Year 9 Edexcel 物理：公式定理速查手册
This quick reference guide covers all the key formulas and principles you need for Year 9 Edexcel Physics. Each section presents an equation with clear variable definitions, units and a worked example to help you revise efficiently.
本速查手册涵盖了 Year 9 Edexcel 物理的全部核心公式与原理。每个小节都给出方程、清晰的变量定义、单位以及一个具体例子，帮助你高效复习。
1. Speed Formula | 速度公式
Speed describes how fast an object moves. It is a scalar quantity, meaning it only has magnitude and no direction.
速度描述物体运动的快慢。它是标量，只有大小，没有方向。
v = d / t
v = speed (m/s), d = distance travelled (m), t = time taken (s).
v = 速度（米/秒），d = 移动距离（米），t = 所用时间（秒）。
Example: A cyclist covers 300 m in 20 s. Speed = 300 ÷ 20 = 15 m/s.
例子：一名骑行者 20 s 内移动 300 m，速度 = 300 ÷ 20 = 15 m/s。
2. Acceleration | 加速度
Acceleration measures how quickly velocity changes. A positive value means the object is speeding up; a negative value (deceleration) means it is slowing down.
加速度衡量速度变化的快慢。正值表示物体在加速，负值（减速度）表示物体在减速。
a = (v − u) / t
= acceleration (m/s²), v = final velocity (m/s), u = initial velocity (m/s), t = time (s).
a = 加速度（米/秒²），v = 末速度（米/秒），u = 初速度（米/秒），t = 时间（秒）。
Example: A car accelerates from 5 m/s to 25 m/s in 4 s. a = (25 − 5) / 4 = 5 m/s².
例子：一辆轿车在 4 s 内从 5 m/s 加速到 25 m/s，加速度 a = (25 − 5) / 4 = 5 m/s²。
3. Newton's Second Law | 牛顿第二定律
The resultant force acting on an object is equal to the object's mass multiplied by its acceleration. This law links force, mass, and motion.
作用在物体上的合力等于物体的质量乘以其加速度。该定律将力、质量和运动联系在一起。
F = m × a
F = resultant force (N), m = mass (kg), a = acceleration (m/s²).
F = 合力（牛顿），m = 质量（千克），a = 加速度（米/秒²）。
Example: A 2 kg toy car accelerates at 3 m/s². The force needed is F = 2 × 3 = 6 N.
例子：一辆 2 kg 的玩具车以 3 m/s² 的加速度运动，所需力为 F = 2 × 3 = 6 N。
4. Weight and Gravity | 重量与重力
Weight is the gravitational force acting on an object. It depends on the object's mass and the gravitational field strength of the planet. On Earth, g is approximately 10 N/kg (or 9.8 N/kg for more precise work).
重量是作用在物体上的引力，取决于物体的质量和所在星球的重力场强度。在地球上，g 约为 10 N/kg（精确计算时常取 9.8 N/kg）。
W = m × g
W = weight (N), m = mass (kg), g = gravitational field strength (N/kg).
W = 重量（牛顿），m = 质量（千克），g = 重力场强度（牛/千克）。
Example: A 5 kg object on Earth has weight W = 5 × 10 = 50 N.
例子：一个 5 kg 的物体在地球上的重量 W = 5 × 10 = 50 N。
5. Work Done |
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