📚 Year 9 CCEA Physics: Quick Reference Formula & Theorem Handbook | Year 9 CCEA 物理：公式定理速查手册
This handbook brings together all the essential formulas and principles you need in Year 9 CCEA Physics. Memorising these equations and understanding when to use them will boost your confidence in calculations and help you spot patterns across topics. Keep it handy as a quick revision aid.
本手册汇集了 Year 9 CCEA 物理课程中所有重要的公式和定理。记住这些方程并理解其应用场景，不仅可以提高你的计算信心，还能帮助你在各个主题之间发现规律。请将它作为快速复习的好帮手。
1. Speed | 速度
Speed tells you how quickly an object moves and is calculated by dividing the distance travelled by the time taken. The word formula is:
速度表示物体运动的快慢，用通过的距离除以所用时间来计算。文字公式为：
speed = distance ÷ time
In symbols, we often write v = d / t. The SI unit of speed is metre per second (m/s), but you will also see kilometre per hour (km/h).
用符号表示，常写作 v = d / t。速度的国际单位是米每秒（m/s），但你也会遇到千米每小时（km/h）。
Always check that the distance and time are in matching units before calculating. For example, if a cyclist covers 300 m in 20 s, her speed is 15 m/s.
计算前务必检查距离和时间的单位是否匹配。例如，一名自行车手在 20 秒内行驶了 300 米，她的速度为 15 m/s。
2. Acceleration | 加速度
Acceleration is the rate at which speed changes. When an object speeds up, slows down or changes direction, it accelerates.
加速度描述速度变化的快慢。当物体加速、减速或改变方向时，它都在做加速运动。
acceleration = (final speed – initial speed) ÷ time
In symbols: a = (v – u) / t, where v is the final speed, u is the initial speed and t is the time. The unit is metre per second squared (m/s²).
符号公式为：a = (v – u) / t，其中 v 为末速度，u 为初速度，t 为时间。单位是米每二次方秒（m/s²）。
A negative acceleration means the object is decelerating. For instance, a train slowing from 30 m/s to 10 m/s in 5 s has an acceleration of –4 m/s².
负加速度表示物体在减速。例如，一列火车在 5 秒内速度从 30 m/s 降到 10 m/s，其加速度为 –4 m/s²。
3. Weight and Gravitational Force | 重量与引力
Weight is the force of gravity acting on an object and must not be confused with mass. Mass is the amount of matter and is measured in kilograms; weight is a force and is measured in newtons (N).
重量是作用在物体上的重力，切勿与质量混淆。质量是物质的多少，以千克为单位；重量是一种力，以牛顿（N）为单位。
weight = mass × gravitational field strength
In symbols: W = m × g arth, g ≈ 10 N/kg (or more precisely 9.8 N/kg). A 5 kg object therefore weighs about 50 N.
符号公式：W = m × g。在地球表面，g 约等于 10 N/kg（更精确为 9.8 N/kg）。因此一个 5 kg 的物体重量约为 50 N。
Remember that mass does not change when you go to the Moon, but the weight does because g is smaller there.
记住，当你去到月球时，质量不变，但重量会改变，因为那里的 g 更小。
4. Density | 密度
Density compares the mass of a material to the volume it occupies. It helps explain why some objects float while others sink.
密度用于比较物质的质量与其所占体积，可以解释为什么有些物体会漂浮而另一些会下沉。
density = mass ÷ volume
Symbol formula: ρ = m / V. The SI unit is kilogram per cubic metre (kg/m³), but in the lab you may use gram per cubic centimetre (g/cm³).
符号公式：ρ = m / V。国际单位是千克每立方米（kg/m³），但在实验室中你可能使用克每立方厘米（g/cm³）。
und from length × width × height. For an irregular object, you can use a displacement can.
对于规则固体，体积可用 长 × 宽 × 高 求得。对于不规则物体，你可以使用排水法测量。
5. Pressure | 压强
Pressure measures how concentrated a force is over an area. A large force spread out over a large area produces a small pressure, while the same force on a tiny area gives a huge pressure.
压强表示力在面积上的集中程度。同样的力分布在大的面积上产生的压强小，而集中在很小的面积上则产生巨大的压强。
pressure = force ÷ area
In symbols: p = F / A. The unit of pressure is the pascal (Pa), where 1 Pa = 1 N/m².
符号公式：p = F / A。压强的单位是帕斯卡（Pa），1 Pa = 1 N/m²。
This formula explains why sharp knives cut easily (small area) and why snow shoes prevent you from sinking (large area).
这个公式可以解释为什么锋利的刀容易切东西（面积小），以及为什么雪鞋能防止下陷（面积大）。
6. Work Done | 做功
In physics, work is done when a force moves an object in the direction of the force. If there is no movement, no work is done, even if a large force is applied.
在物理学中，当力使物体沿着力的方向移动时，我们就说做了功。如果没有移动，即使施加了很大的力，也没有做功。
work done = force × distance moved in the direction of the force
In symbols: W = F × d. Work is a form of energy and is measured in joules (J). Lifting a 10 N weight vertically by 2 m does 20 J of work.
符号公式：W = F × d。功是能量的一种形式，单位是焦耳（J）。将一个 10 N 的重物竖直提升 2 m，做功 20 J。
7. Kinetic Energy and Gravitational Potential Energy | 动能与重力势能
Energy comes in many stores. Two important mechanical energy stores are kinetic energy (energy of motion) and gravitational potential energy (energy due to height in a gravity field).
能量有许多储存方式。两个重要的力学能量储存是动能（运动的能量）和重力势能（因在重力场中的高度而具有的能量）。
kinetic energy = ½ × mass × speed²
gravitational potential energy = mass × gravitational field strength × height
In symbols: Ek = ½ m v² and Ep = m g h. Both are measured in joules. Notice that work done to raise an object equals the gain in gravitational potential energy (ignoring friction).
符号公式：Ek = ½ m v²，Ep = m g h。两者的单位都是焦耳。注意，将一个物体举起所做的功等于它增加的重力势能（忽略摩擦）。
8. Power | 功率
Power is the rate at which work is done or energy is transferred. A powerful machine transfers a lot of energy in a short time.
功率是做功或能量转换的快慢。强大的机器能在短时间内转换大量能量。
power = work done ÷ time taken
Also: power = energy transferred ÷ time. In symbols: P = W / t or P = E / t. The unit is the watt (W), where 1 W = 1 J/s.
也可表示为：功率 = 转移的能量 ÷ 时间。符号公式：P = W / t 或 P = E / t。单位是瓦特（W），1 W = 1 J/s。
For example, a motor that does 600 J of work in 30 s has a power of 20 W.
例如，一台电动机在 30 秒内做了 600 J 的功，其功率为 20 W。
9. Electric Current and Charge | 电流与电荷量
Electric current is a flow of electric charge. The size of the current tells you how much charge passes a point in a circuit each second.
电流是电荷的流动。电流的大小表示每秒钟通过电路某一点的电荷量。
charge = current × time
In symbols: Q = I × t. Charge is measured in coulombs (C), current in amperes (A) and time in seconds (s). A current of 2 A flowing for 10 s carries 20 C of charge.
符号公式：Q = I × t。电荷的单位是库仑（C），电流的单位是安培（A），时间的单位是秒（s）。2 A 的电流流动 10 秒携带的电荷为 20 C。
10. Voltage, Resistance and Ohm's Law | 电压、电阻与欧姆定律
Voltage (potential difference) pushes current around a circuit, while resistance opposes the flow of current. In a metal wire at constant temperature, the current is directly proportional to the voltage.
电压（电势差）驱动电流在电路中流动，而电阻阻碍电流。在温度不变的金属导线中，电流与电压成正比。
voltage = current × resistance
Symbol equation: V = I × R. Voltage is measured in volts (V), current in amperes (A) and resistance in ohms (Ω). This relationship is called Ohm's law.
符号方程：V = I × R。电压以伏特（V）为单位，电流以安培（A）为单位，电阻以欧姆（Ω）为单位。这一关系称为欧姆定律。
Use the triangle method if you need to rearrange: R = V / I, I = V / R.
如果需要变形，可使用三角形法：R = V / I，I = V / R。
11. Wave Speed | 波速
Waves transfer energy from one place to another without transferring matter. The speed of a wave depends on its frequency and wavelength.
波将能量从一个地方传递到另一个地方，而不传递物质。波速取决于波的频率和波长。
wave speed = frequency × wavelength
In symbols: v = f × λ. Frequency is measured in hertz (Hz), wavelength in metres (m), and wave speed in metres per second (m/s).
符号公式：v = f × λ。频率的单位是赫兹（Hz），波长的单位是米（m），波速的单位是米每秒（m/s）。
This equation works for all types of wave, including sound waves, water waves and light waves.
该方程适用于所有类型的波，包括声波、水波和光波。
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