📚 Year 9 CCEA Physics: Quick Reference Guide to Formulas and Principles | Year 9 CCEA 物理：公式定理速查手册
This quick reference guide brings together the essential formulas, principles and relationships you need to know for Year 9 CCEA Physics. Each section presents the key equations with clear definitions and typical units, helping you revise efficiently and apply your knowledge to numerical problems and experiments.
本速查手册汇集了 Year 9 CCEA 物理课程中必须掌握的核心公式、定理和数量关系。每一节都给出了关键公式及其清楚的定义和常用单位，帮助你高效复习，并能将知识运用到计算题和实验题中。
1. Speed and Acceleration | 速度与加速度
Average speed is a scalar quantity that tells you how fast an object is moving overall. It is found by dividing the total distance travelled by the total time taken.
v = s / t
平均速度是一个标量，表示物体整体运动的快慢。它等于总路程除以总时间。
Acceleration measures how quickly velocity changes. It is a vector, so both its size and direction matter. Acceleration is calculated from the change in velocity divided by the time over which the change occurs.
a = (v - u) / t
加速度衡量速度变化的快慢，是矢量，因此大小和方向都很重要。加速度等于速度的变化量除以发生这段变化所用的时间。
2. Forces and Newton's Second Law | 力与牛顿第二定律
Newton's second law describes the link between the resultant force acting on an object, its mass and its acceleration. A larger force produces a larger acceleration, while a larger mass leads to a smaller acceleration for the same force.
F = m a
牛顿第二定律描述了物体所受合力、质量与加速度之间的关系。力越大加速度越大，相同的力作用在质量更大的物体上则产生的加速度越小。
The unit of force is the newton (N). One newton is the force needed to give a mass of 1 kg an acceleration of 1 m/s².
1 N = 1 kg m s⁻²
力的单位是牛顿（N）。1 牛顿就是使 1 kg 的物体产生 1 m/s² 加速度所需要的力。
3. Mass, Weight and Gravity | 质量、重量与重力
Mass is the amount of matter in an object and is measured in kilograms. Weight is the gravitational force acting on that mass and depends on the gravitational field strength, g.
W = m g
质量是物体所含物质的多少，单位是千克。重量是作用在该质量上的重力，其大小取决于引力场强度 g。
On Earth, g is approximately 9.8 N/kg, but it is often rounded to 10 N/kg in classroom calculations. Weight is a vector and always acts towards the centre of the Earth.
g ≈ 9.8 N/kg (or 10 N/kg)
在地球表面，g 约为 9.8 N/kg，课堂计算常取 10 N/kg。重量是矢量，方向总是竖直向下指向地心。
4. Energy Stores and Transfers | 能量储存与转移
Kinetic energy is the energy an object has because of its motion. It depends on both the mass of the object and the square of its speed.
KE = ½ m v²
动能是物体由于运动而具有的能量。其大小取决于物体的质量和速度的平方。
Gravitational potential energy is the energy stored in an object raised above the ground. It increases with mass, height and the gravitational field strength.
GPE = m g h
重力势能是物体被举高时所储存的能量。质量越大、高度越高、引力场强度越大，重力势能就越大。
Efficiency describes how well energy is transferred usefully. It can be expressed as a percentage by comparing useful output energy to total input energy.
Efficiency = (useful output energy / total input energy) × 100%
效率描述能量转换的有用程度。通过将有用输出能量与总输入能量进行比较，可以用百分数表示效率。
5. Work Done and Power | 做功与功率
Work is done when a force moves an object. The amount of work depends on the size of the force and the distance moved in the direction of the force.
W = F d
力使物体移动时便做了功。功的大小取决于力的大小和物体沿力的方向移动的距离。
Power is the rate at which work is done or energy is transferred. An appliance with a higher power transfers more energy every second.
P = W / t or P = E / t
功率是做功或转移能量的快慢。功率越大的用电器，每秒钟转移的能量就越多。
6. Electrical Circuits: Current, Voltage, Resistance | 电路：电流、电压、电阻
Ohm's law links the potential difference across a conductor, the current flowing through it and its resistance. For an ohmic conductor at constant temperature, the ratio V/I remains constant.
V = I R
欧姆定律给出了导体两端电压、通过它的电流和它的电阻之间的关系。对于温度恒定的欧姆导体，V/I 的比值保持不变。
When resistors are connected in series, the total resistance is simply the sum of the individual resistances. The same current passes through each component.
Rₜₒₜₐₗ = R₁ + R₂ + ...
电阻串联时，总电阻等于各个电阻之和。串联电路中各处电流相等。
For resistors in parallel, the total resistance is less than the smallest individual resistance. The reciprocal formula is used.
1/Rₜₒₜₐₗ = 1/R₁ + 1/R₂ + ...
电阻并联时，总电阻小于其中最小的一个电阻，须使用倒数公式计算。
7. Electrical Power and Energy | 电功率与电能
Electrical power can be calculated using the current through a component and the potential difference across it. This formula works for any circuit element.
P = I V
用电器的电功率可用通过它的电流和它两端的电压来计算。该公式适用于任何电路元件。
When you know the current and resistance, power can also be found using this alternative form. It shows that power depends on the square of the current.
P = I² R
如果已知电流和电阻，电功率也可用此式求出。可以看出功率与电流的平方成正比。
A third useful form of the power equation involves voltage and resistance alone, which is handy when current is not given directly.
P = V² / R
第三种有用的功率公式只包含电压和电阻，在未直接给出电流时非常方便。
8. Density of Materials | 材料的密度
Density links the mass of a substance to the volume it occupies. Materials with tightly packed particles tend to have higher densities.
ρ = m / V
密度将物质的质量与其占据的体积联系起来。微粒排列紧密的材料密度往往较大。
In many practical investigations you measure mass with a balance and volume by displacement or by multiplying dimensions for regular solids, then calculate density.
ρ in kg/m³, m in kg, V in m³
在许多实验探究中，你会用天平测质量，用排水法或测量规则物体的尺寸来求体积，再计算出密度。密度单位常用 kg/m³。
9. Pressure in Solids and Liquids | 固体与液体压强
Pressure is the force acting per unit area. A high pressure can be achieved by applying a large force over a small area.
p = F / A
压强是单位面积上所受的压力。通过在小面积上施加较大的力可获得高压强。
In a liquid, pressure increases with depth and also depends on the density of the liquid and the gravitational field strength.
p = ρ g h
液体内部的压强随深度增加而增大，且与液体的密度和引力场强度有关。
10. Moments and Levers | 力矩与杠杆
The moment of a force measures its turning effect. It depends on the size of the force and the perpendicular distance from the pivot to the line of action of the force.
M = F d
力矩量度的是力的转动效应。其大小取决于力的大小和支点到力作用线的垂直距离。
When a lever or any object is balanced, the sum of the clockwise moments about the pivot equals the sum of the anticlockwise moments. This is the principle of moments.
Σ clockwise moments = Σ anticlockwise moments
当杠杆或任何物体平衡时，支点两侧的顺时针力矩之和等于逆时针力矩之和。这就是力矩平衡原理。
11. Wave Properties | 波的特性
The wave equation relates the speed of a wave to its frequency and wavelength. It is true for all types of wave, including sound, water and light waves.
v = f λ
波速公式将波速、频率和波长联系起来。它对一切波都成立，包括声波、水波和光波。
Frequency is measured in hertz (Hz). One hertz means one complete wave cycle per second. Wavelength is the distance between two adjacent points that are in phase, such as two crests.
1 Hz = 1 s⁻¹
频率的单位是赫兹（Hz）。1 Hz 表示每秒完成一个完整的波周期。波长是相邻两个同相点之间的距离，比如两个相邻波峰间的距离。
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