📚 AS Physics Core Revision: Mechanics, Waves and Electricity | AS 物理核心复习：力学、波与电学
This AS Physics revision guide brings together the core topics that most candidates meet early in the course. It focuses on mechanics, materials, electricity, waves and quantum phenomena, with clear definitions and the key equations you need to apply in the exam.
本 AS 物理复习指南汇总了大多数考生在课程前期接触的核心专题。它重点关注力学、材料、电学、波和量子现象，给出清晰的定义以及你在考试中需要应用的关键方程。
1. Units and Measurements | 单位与测量
AS Physics questions frequently test your ability to use SI base units and derived units correctly. The seven SI base units are the metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol) and candela (cd). Every other unit, such as the newton (N) or joule (J), can be expressed as a combination of these base units.
AS 物理试题经常考查你是否能正确使用 SI 基本单位和导出单位。七个 SI 基本单位是米 (m)、千克 (kg)、秒 (s)、安培 (A)、开尔文 (K)、摩尔 (mol) 和坎德拉 (cd)。其他所有单位，例如牛顿 (N) 或焦耳 (J)，都可以表示成这些基本单位的组合。
You should also be confident with standard prefixes such as nano (10⁻⁹), micro (10⁻⁶), milli (10⁻³), centi (10⁻²), kilo (10³), mega (10⁶) and giga (10⁹). Converting between prefixes is a basic skill, but many candidates lose marks by rushing through unit conversions before substituting values into formulas.
你还应该熟练使用标准前缀，例如纳 (10⁻⁹)、微 (10⁻⁶)、毫 (10⁻³)、厘 (10⁻²)、千 (10³)、兆 (10⁶) 和吉 (10⁹)。前缀之间的换算是基本技能，但许多考生在代入公式前匆忙进行单位换算，因而失分。
When reporting a measurement, pay attention to accuracy, precision, repeatability and resolution. Accuracy describes how close a result is to the true value, while precision describes how close repeated readings are to one another. Repeatability means that the same person can obtain consistent results using the same method and equipment, and resolution is the smallest change a measuring instrument can detect.
报告测量结果时，要注意准确度、精密度、可重复性和分辨率。准确度描述结果与真实值的接近程度，而精密度描述重复读数彼此之间的接近程度。可重复性指同一人使用相同方法和仪器能得到一致结果，分辨率指测量仪器能检测到的最小变化。
Uncertainty and percentage uncertainty are also common exam topics. Absolute uncertainty has the same unit as the measured value, while percentage uncertainty compares the absolute uncertainty with the measured value. When readings are added or subtracted, absolute uncertainties add; when quantities are multiplied or divided, percentage uncertainties add.
不确定度和百分不确定度也是常见考点。绝对不确定度与测量值单位相同，而百分不确定度把绝对不确定度与测量值比较。读数相加或相减时，绝对不确定度相加；量相乘或相除时，百分不确定度相加。
Percentage uncertainty = (absolute uncertainty ÷ measured value) × 100%
Finally, treat significant figures and orders of magnitude carefully. A final answer should not contain more significant figures than the least precise measurement used in the calculation. Orders of magnitude help you check whether an answer is physically reasonable.
最后，要仔细处理有效数字和数量级。最终答案的有效数字不应多于计算中使用的最不精确测量值。数量级能帮助你检查答案在物理上是否合理。
2. Kinematics: Motion in a Straight Line | 运动学：直线运动
Kinematics describes motion without considering its causes. You must distinguish clearly between scalars and vectors. Distance and speed are scalars, while displacement, velocity and acceleration are vectors. Displacement is the straight-line distance in a specified direction, and velocity is the rate of change of displacement.
运动学描述运动而不考虑其成因。你必须清楚区分标量和矢量。距离和速率是标量，而位移、速度和加速度是矢量。位移是沿指定方向的直线距离，速度是位移的变化率。
The four equations of motion apply only when acceleration is constant. You must choose the equation that contains the three quantities given in the question and the one quantity you need to find. These equations are essential in AS mechanics and are also used in projectile questions.
四个运动方程只适用于加速度恒定的情况。你必须选择包含题目给出的三个量以及你需要求的一个量的方程。这些方程在 AS 力学中至关重要，也用于抛体问题。
nbsp; s = u t + ½ a t² v² = u² + 2 a s s = ((u + v) ÷ 2) t
In these equations, u is initial velocity, v is final velocity, a is acceleration, t is time and s is displacement. Always decide which direction is positive before substituting values. If an object moves upward and gravity acts downward, the acceleration may be negative.
在这些方程中，u 是初速度，v 是末速度，a 是加速度，t 是时间，s 是位移。代入数值前一定要确定哪个方向为正。如果物体向上运动而重力向下，加速度可能为负。
Motion graphs are another key skill. On a displacement-time graph, the gradient gives velocity. On a velocity-time graph, the gradient gives acceleration, and the area under the graph gives displacement. Areas below the time axis count as negative displacement.
运动图像是另一项关键技能。在位移-时间图像上，斜率给出速度。在速度-时间图像上，斜率给出加速度，图像下方面积给出位移。时间轴下方的面积计为负位移。
Free fall is motion under gravity alone, where air resistance is neglected. Near the Earth's surface, the acceleration of free fall is approximately 9.81 m s⁻². Projectile motion can be treated by separating horizontal and vertical motion: horizontal velocity is constant, while vertical motion has constant acceleration due to gravity.
自由落体是仅在重力作用下的运动，忽略空气阻力。在地球表面附近，自由落体加速度约为 9.81 m s⁻²。抛体运动可以通过分解水平和竖直运动来处理：水平速度恒定，而竖直运动具有恒定的重力加速度。
3. Forces and Newton's Laws | 力与牛顿定律
Newton's first law states that an object remains at rest or moves with constant velocity unless acted on by a resultant force. This law introduces the idea of inertia: mass is a measure of how difficult it is to change an object's motion.
牛顿第一定律指出，除非受到合力作用，物体保持静止或匀速直线运动。这一定律引入惯性概念：质量是衡量改变物体运动难易程度的量度。
Newton's second law states that the resultant force is directly proportional to the rate of change of momentum. For constant mass, this simplifies to force equals mass times acceleration. The direction of acceleration is always the same as the direction of the resultant force.
牛顿第二定律指出，合力与动量变化率成正比。对于恒定质量，这简化为力等于质量乘以加速度。加速度方向始终与合力方向相同。
F = m a p = m v F = Δp ÷ Δt
Newton's third law states that if object A exerts a force on object B, then object B exerts an equal and opposite force on object A. These two forces act on different objects, so they do not cancel each other when drawing a free-body diagram for one object.
牛顿第三定律指出，如果物体 A 对物体 B 施加一个力，那么物体 B 对物体 A 施加大小相等、方向相反的力。这两个力作用在不同物体上，因此在对一个物体画自由体图时不会相互抵消。
Free-body diagrams are essential for solving force problems. Draw only the forces acting on the chosen object, including weight, normal contact force, friction, tension, drag and applied forces. Then resolve forces into perpendicular components, often horizontal and vertical, and apply equilibrium or Newton's second law.
自由体图是解决力学问题的关键。只画出作用在所选物体上的力，包括重力、法向接触力、摩擦力、张力、阻力和外加力。然后把力分解为互相垂直的分量，通常为水平和竖直方向，再应用平衡条件或牛顿第二定律。
Momentum and impulse are closely linked to forces. Impulse is the product of force and contact time, and it equals the change in momentum. This explains why crumple zones and airbags reduce the force on passengers by increasing the time taken to change momentum.
动量与冲量与力密切相关。冲量是力与接触时间的乘积，等于动量变化。这解释了为什么溃缩区和安全气囊通过增加动量变化时间来减小乘客受到的力。
4. Work, Energy and Power | 功、能量与功率
Work is done when a force moves its point of application through a distance. If the force is not in the same direction as the displacement, only the component of force along the displacement does work. Work is a scalar quantity and is measured in joules (J).
当力使其作用点发生位移时，就做了功。如果力与位移方向不同，只有沿位移方向的力分量做功。功是标量，单位为焦耳 (J)。
W = F s cos θ
Kinetic energy is the energy an object has due to its motion. Gravitational potential energy is the energy an object has due to its position in a gravitational field. Near the Earth's surface, changes in gravitational potential energy depend on mass, gravitational field strength and change in height.
动能是物体由于运动而具有的能量。重力势能是物体由于在重力场中的位置而具有的能量。在地球表面附近，重力势能变化取决于质量、重力场强度和高度变化。
KE = ½ m v² ΔGPE = m g Δh
The principle of conservation of energy states that energy cannot be created or destroyed, only transferred from one form to another. In closed systems, the total energy remains constant. In real systems, some energy is dissipated to the surroundings, often as thermal energy due to friction or air resistance.
能量守恒原理指出，能量不能被创造或消灭，只能从一种形式转化为另一种形式。在封闭系统中，总能量保持不变。在真实系统中，部分能量会耗散到周围环境，通常由于摩擦或空气阻力转化为热能。
Power is the rate of doing work or the rate of energy transfer. One watt is equal to one joule per second. When a force moves an object at constant velocity, power can also be calculated as force times velocity.
功率是做功的速率或能量转移的速率。一瓦特等于一焦耳每秒。当力使物体匀速运动时，功率也可以用力乘以速度计算。
P = W ÷ t = F v
Efficiency compares useful output energy or power with total input energy or power. It is often expressed as a percentage. No real machine is 100% efficient because energy is always dissipated in some way.
效率把有用输出能量或功率与总输入能量或功率进行比较。它通常用百分比表示。没有真实机器能达到 100% 效率，因为能量总会以某种方式耗散。
5. Materials: Density, Stress and Strain | 材料：密度、应力与应变
Density is defined as mass per unit volume. It is a property of the material and can be used to identify substances or check purity. For a regular solid, volume can be calculated from dimensions; for an irregular solid, volume can be found by displacement.
密度定义为单位体积的质量。它是材料的属性，可用于识别物质或检查纯度。对于规则固体，体积可由尺寸计算；对于不规则固体，体积可用排水法测量。
ρ = m ÷ V
Hooke's law states that the extension of a spring or wire is directly proportional to the applied force, provided the elastic limit is not exceeded. The spring constant k is a measure of stiffness. The area under a force-extension graph gives the elastic potential energy stored.
胡克定律指出，只要不超过弹性极限，弹簧或导线的伸长量与所施加的力成正比。弹簧常数 k 是刚度的量度。力-伸长量图像下的面积给出储存的弹性势能。
F = k x E = ½ k x²
Stress is force per unit cross-sectional area, and strain is the fractional change in length. Young modulus is the ratio of stress to strain for a material in the linear region. It is a material property and does not depend on the dimensions of the sample.
应力是单位横截面积上的力，应变是长度的相对变化。杨氏模量是材料在线性区域内应力与应变的比值。它是材料属性，不依赖于样品的尺寸。
σ = F ÷ A ε = ΔL ÷ L E = σ ÷ ε
A stress-strain graph shows important material behaviour. The straight section obeys Hooke's law, and its gradient gives Young modulus. The elastic limit is the point beyond which the material no longer returns to its original length. Yield point, plastic deformation, ultimate tensile strength and fracture point may also be identified.
应力-应变图显示重要的材料行为。直线部分遵守胡克定律，其斜率给出杨氏模量。弹性极限是材料不再恢复原长的点。屈服点、塑性形变、极限抗拉强度和断裂点也可能需要识别。
Ductile materials can be drawn into wires and show significant plastic deformation before breaking. Brittle materials fracture with little plastic deformation. Polymeric materials may show elastic hysteresis and different loading and unloading curves, which is important in damping applications.
韧性材料可以拉成丝，并在断裂前表现出明显的塑性形变。脆性材料在很小的塑性形变后断裂。聚合物材料可能表现出弹性滞后以及不同的加载和卸载曲线，这在减震应用中很重要。
6. Electricity: Current, Charge and Resistance | 电学：电流、电荷与电阻
Electric current is the rate of flow of charge. In metallic conductors, current is carried by electrons. Conventional current is defined as the direction in which positive charge would flow, which is opposite to the direction of electron flow.
电流是电荷流动的速率。在金属导体中，电流由电子载运。常规电流定义为正电荷流动的方向，与电子流动方向相反。
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