📚 Year 9 CAIE Physics: Core Knowledge Summary | Year 9 CAIE 物理：核心知识点梳理
This article provides a structured overview of the most important topics in Year 9 CAIE Physics. Each section introduces a key concept, explains it clearly, and is followed by a matching explanation in Chinese. Use this guide to strengthen your foundation before moving to IGCSE level.
本文梳理了 Year 9 CAIE 物理课程中最重要的知识点。每个板块先给出英文讲解，再提供对应的中文解释。通过这份中英双语复习指南，你可以在进入 IGCSE 阶段前打下扎实的基础。
1. Measurements and Units | 测量与单位
All physical quantities are expressed using the International System of Units (SI). The base units you must remember are the metre (m) for length, kilogram (kg) for mass, second (s) for time, and kelvin (K) for temperature.
所有物理量都采用国际单位制 (SI)。必须记住的基本单位有：长度用米 (m)，质量用千克 (kg)，时间用秒 (s)，温度用开尔文 (K)。
Derived units such as metres per second (m/s) for velocity and newtons (N) for force are built from these base units. Always include correct units with numerical answers.
导出单位由基本单位组合而成，如速度用米每秒 (m/s)，力用牛顿 (N)。答题时数字后面一定要带上正确的单位。
tch may read to 0.01 s. Repeated readings help reduce random errors, but systematic errors require instrument recalibration.
测量时，仪器的精度会限制测量结果。直尺通常可读到最接近的毫米 (±1 mm)，数字秒表可能读到 0.01 s。多做几次测量可以减小随机误差，但系统误差需要通过校准仪器来消除。
2. Describing Motion | 运动的描述
Speed is the distance travelled per unit time. The formula is:
速度是单位时间内通过的路程。其公式为：
v = d ÷ t
is a vector and includes direction. Acceleration is the rate of change of velocity: a = (v – u) / t, where u is initial velocity, v is final velocity, and t is time.
速率是标量，只有大小。而速度是矢量，既有大小又有方向。加速度是速度变化的快慢：a = (v – u) / t，其中 u 是初速度，v 是末速度，t 是时间。
Distance–time graphs show how far an object has moved. A straight line indicates constant speed; a horizontal line means the object is stationary. Speed–time graphs give acceleration from the slope and distance from the area under the graph.
距离–时间图像显示物体运动的路程。直线表示匀速运动，水平线表示物体静止。速度–时间图像中，斜率代表加速度，图像下的面积代表通过的距离。
3. Forces and Their Effects | 力及其作用效果
A force is a push or a pull. Forces can change the speed, direction, or shape of an object. Common forces include weight (gravity), friction, tension, and normal reaction. The unit of force is the newton (N).
力是推或拉的作用。力可以改变物体的速度、方向或形状。常见的力有重力、摩擦力、张力和支持力。力的单位是牛顿 (N)。
Forces are vectors and can be added using arrows. The resultant force is the single force that has the same effect as all the individual forces combined. When forces on an object are balanced, the object is either stationary or moving at constant velocity.
力是矢量，可以用箭头相加。合力是效果等同于所有分力共同作用的那一个力。当物体受力平衡时，物体要么静止，要么做匀速直线运动。
Newton’s First Law states that an object continues in its state of rest or uniform motion unless acted upon by a resultant force. This explains why seat belts are needed in cars.
牛顿第一定律指出，除非受到合外力的作用，物体会保持静止或匀速直线运动状态。这说明了为什么乘车时需要系安全带。
4. Newton’s Second Law, Mass and Weight | 牛顿第二定律、质量与重量
Newton’s Second Law links force, mass, and acceleration:
牛顿第二定律将力、质量和加速度联系在一起：
F = m × a
A larger force produces a larger acceleration, but a larger mass results in a smaller acceleration for the same force.
力越大，加速度越大；但在同一个力作用下，质量越大，加速度越小。
red in kilograms; weight is measured in newtons. Do not confuse mass and weight: mass is constant, weight varies with location.
重量是作用在物体上的重力。计算公式为 W = m × g，其中 g 是引力场强度（地球表面约为 9.8 N/kg）。质量用千克表示，重量用牛顿表示。不要混淆质量和重量：质量不变，重量会随位置不同而变化。
5. Density and Pressure | 密度与压强
Density tells us how much mass is packed into a given volume. The formula is:
密度表示单位体积内所含质量的多少。公式为：
ρ = m ÷ V
The unit of density is kg/m³, though g/cm³ is also common. Objects with density less than water float; those with greater density sink.
密度的单位是 kg/m³，也常用 g/cm³。密度小于水的物体会漂浮，密度大于水的物体会下沉。
Pressure is the force acting per unit area:
压强是单位面积上受到的压力：
P = F ÷ A
s high pressure because the area is small, while snowshoes reduce pressure by increasing area.
压强的单位是帕斯卡 (Pa)，1 Pa = 1 N/m²。尖钉子因受力面积小而压强很大，而雪鞋通过增大面积来减小压强。
th and density, and is given by P = ρg h. This is why dam walls are thicker at the bottom.
液体内部的压强随深度和液体密度的增加而增大，计算公式为 P = ρg h。这就是大坝底部修得更厚的原因。
6. Work, Energy, and Power | 功、能与功率
Work is done when a force moves an object in the direction of the force. The formula is:
当一个力使物体沿力的方向移动时，力就对物体做了功。公式为：
W = F × d
Work is measured in joules (J). 1 J is the work done when a force of 1 N moves an object 1 m. Energy is the capacity to do work and also has the unit joule.
功的单位是焦耳 (J)。1 J 就是 1 N 的力使物体移动 1 m 所做的功。能量是做功的能力，单位也是焦耳。
, height, and g: GPE = m g h. Energy can be stored or transferred but is always conserved.
动能 (KE) 取决于质量和速度：KE = ½ m v²。重力势能 (GPE) 取决于质量、高度和 g：GPE = m g h。能量可以被储存或转移，但总量总是守恒的。
k or transferring energy: P = W / t. The unit of power is the watt (W), where 1 W = 1 J/s.
功率是做功或能量转移的快慢：P = W / t。功率的单位是瓦特 (W)，1 W = 1 J/s。
7. Energy Transfers and Conservation | 能量的转化与守恒
Energy can exist in many forms: chemical, kinetic, thermal, light, sound, electrical, and nuclear. The principle of conservation of energy states that energy cannot be created or destroyed, only converted from one form to another.
能量有多种形式：化学能、动能、热能、光能、声能、电能和核能。能量守恒定律指出，能量既不会凭空产生也不会凭空消失，只会从一种形式转化为另一种形式。
Sankey diagrams show energy transfers with arrows whose width represents the amount of energy. In any real process, some energy is always transferred to the thermal store of the surroundings, which is often called wasted energy.
桑基图用箭头的宽度表示能量的大小，展示能量转化过程。在任何实际过程中，总有一部分能量会转化为周围环境的内能，这部分能量常被称为“浪费掉的能量”。
expressed as a percentage: Efficiency = (useful energy output / total energy input) × 100%.
效率是有用输出能量占总输入能量的比例，常用百分数表示：效率 = (有用输出能量 / 总输入能量) × 100%。
8. Thermal Energy Transfer | 热传递
Heat can be transferred in three ways: conduction, convection, and radiation. Conduction is the transfer of thermal energy through a solid without the movement of the material itself. Metals are good conductors because they have free electrons.
热传递有三种方式：传导、对流和辐射。传导是热量通过固体传递而物质本身不发生移动。金属因含有自由电子而成为热的良导体。
Convection occurs in liquids and gases. Warmer, less dense fluid rises, and cooler, denser fluid sinks, creating a convection current. This is how radiators heat a room.
对流发生在液体和气体中。较热、密度较小的流体会上升，较冷、密度较大的流体会下沉，形成对流循环。暖气片就是利用这种方式加热房间。
Infrared radiation does not require a medium; it can travel through a vacuum. Dull, black surfaces are good absorbers and emitters of radiation, while shiny, silver surfaces are poor absorbers but good reflectors.
红外辐射不需要介质，能在真空中传播。暗黑色表面是好的吸收体和发射体，而光亮银白色表面吸收辐射的能力很差，但反射率高。
9. Wave Basics | 波的基础
Waves transfer energy without transferring matter. Transverse waves (e.g. light, water waves) have oscillations perpendicular to the direction of energy travel. Longitudinal waves (e.g. sound) have oscillations parallel to the direction of energy travel.
波传播能量而不传播物质。横波（如光波、水波）的振动方向与能量传播方向垂直。纵波（如声波）的振动方向与能量传播方向平行。
Key wave properties include wavelength (λ), frequency (f), amplitude, and wave speed (v). The wave equation links them:
波的基本特征包括波长 (λ)、频率 (f)、振幅和波速 (v)。三者之间的关系由波速方程给出：
v = f × λ
Frequency is measured in hertz (Hz) and represents the number of waves per second. Reflection occurs when waves bounce off a surface. Refraction is the change in direction when waves enter a medium of different speed.
频率的单位是赫兹 (Hz)，表示每秒通过的波数。反射是波碰到障碍物返回原介质的现象。折射是波进入传播速度不同的介质时方向发生改变的现象。
10. Sound | 声音
Sound is a longitudinal wave produced by vibrating objects. It travels fastest in solids, slower in liquids, and slowest in gases because particles are closer together in solids. Sound cannot travel through a vacuum.
声音是由物体振动产生的纵波。声音在固体中传播最快，液体中较慢，气体中最慢，这是因为固体中粒子排列更紧密。声音不能在真空中传播。
The pitch of a sound depends on its frequency: a higher frequency gives a higher pitch. The loudness depends on the amplitude of the wave. Echoes are sound reflections that arrive at least 0.1 s after the original sound.
声音的音调取决于频率：频率越高，音调越高。响度取决于波的振幅。回声是声音反射后比原声延迟至少 0.1 s 后听到的声音。
The human hearing range is approximately 20 Hz to 20 000 Hz. Ultrasound, with frequencies above 20 000 Hz, has many uses, including medical imaging and industrial cleaning.
人耳的听觉范围大约在 20 Hz 至 20 000 Hz 之间。频率高于 20 000 Hz 的超声波有许多应用，如医学成像和工业清洗。
11. Light: Reflection and Refraction | 光的反射与折射
Light travels in straight lines in a uniform medium. The law of reflection states that the angle of incidence (i) equals the angle of reflection (r), both measured from the normal (a line perpendicular to the surface).
光在均匀介质中沿直线传播。反射定律指出，入射角 (i) 等于反射角 (r)，这两个角都以法线（垂直于表面的线）为准。
Plane mirrors produce virtual, upright, and laterally inverted images. The image is the same distance behind the mirror as the object is in front.
平面镜生成的像是虚像，正立且左右颠倒。像到镜面的距离等于物到镜面的距离。
from air to glass), it bends towards the normal. The relationship between angles is given by Snell's law: n₁ sin i = n₂ sin r.
光从一种透明介质进入另一种透明介质时会发生折射，传播速度和方向都会改变。光进入光密介质（如从空气到玻璃）时，光线向法线偏折。角度的关系由斯涅耳定律给出：n₁ sin i = n₂ sin r。
White light can be split into a spectrum of colours by a prism through dispersion. Total internal reflection occurs when the angle of incidence inside a denser medium exceeds the critical angle; it is the principle behind optical fibres.
白光通过棱镜会发生色散，分解成彩色光谱。当光在光密介质中入射角大于临界角时，会发生全内反射，这是光纤通信的基本原理。
12. Basic Electricity | 电学基础
Electric current is the flow of charge. In metals, it is a flow of electrons. Current (I) is measured in amperes (A) using an ammeter connected in series. Voltage (V), or potential difference, is the energy transferred per unit charge and is measured in volts (V) with a voltmeter connected in parallel.
电流是电荷的定向移动。在金属中，电流是电子的流动。电流 (I) 的单位是安培 (A)，用串联在电路中的电流表测量。电压 (V)（即电势差）是每单位电荷转移的能量，单位是伏特 (V)，用并联的电压表测量。
Resistance (R) opposes the flow of current and is measured in ohms (Ω). Ohm's law states:
电阻 (R) 阻碍电流的流动，单位是欧姆 (Ω)。欧姆定律表示为：
V = I × R
The resistance of a wire depends on its length, cross-sectional area, and the material. In a series circuit, the current is the same everywhere, but the total voltage is shared. In a parallel circuit, the voltage across each branch is the same, but the total current splits.
导线的电阻取决于其长度、横截面积和材料。串联电路中各处电流相等，但总电压被各用电器分摊。并联电路中各支路两端电压相等，但总电流被各支路分流。
s fuses and circuit breakers prevent overheating by breaking the circuit when the current is too high.
电功率的计算公式是 P = I V。电路中的能量转移用 E = I V t 计算。保险丝和断路器这类安全装置能在电流过大时断开电路，防止过热。
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