Cambridge IGCSE Physics: Mastering Motion, Forces and Energy | 剑桥IGCSE物理：掌握运动、力与能量

📚 Cambridge IGCSE Physics: Mastering Motion, Forces and Energy | 剑桥IGCSE物理：掌握运动、力与能量

Motion, forces and energy form the backbone of the Cambridge IGCSE Physics syllabus. Understanding how objects move, what causes them to start or stop, and how energy is transferred and conserved is essential for success in your exams and beyond. This comprehensive bilingual guide covers all the key concepts from the Cambridge IGCSE Physics Coursebook, with clear explanations in both English and Chinese to support learners at every level.

运动、力与能量是剑桥 IGCSE 物理教学大纲的基石。理解物体如何运动、什么导致它们启动或停止，以及能量如何传递和守恒，对于在考试中取得成功以及更深入的学习至关重要。这份全面的双语指南涵盖了剑桥 IGCSE 物理教材中的所有关键概念，以清晰的中英双语解释为各层次的学习者提供支持。

1. Scalars and Vectors | 标量与矢量

Physical quantities are divided into scalars and vectors. A scalar has magnitude (size) only, while a vector has both magnitude and direction. Typical scalars include distance, speed, mass, energy and temperature. Vectors include displacement, velocity, force, acceleration and momentum.

物理量分为标量和矢量。标量只有大小（量值），而矢量既有大小又有方向。典型的标量包括距离、速率、质量、能量和温度。矢量包括位移、速度、力、加速度和动量。

When adding vectors, you must account for direction. For forces acting along a straight line, a positive sign indicates one direction, and a negative sign the opposite. For perpendicular vectors, use Pythagoras’ theorem to find the resultant magnitude and trigonometry to find the direction.

在矢量相加时，必须考虑方向。对于沿直线作用的力，正号表示一个方向，负号表示相反方向。对于互相垂直的矢量，使用勾股定理求合量的大小，使用三角函数求方向。

A simple example: if a boat sails 3 km east then 4 km north, the displacement is 5 km at an angle of 53° north of east.

一个简单的例子：如果一艘船向东航行 3 千米，然后向北航行 4 千米，其位移为 5 千米，方向为东偏北 53°。

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2. Speed, Velocity and Acceleration | 速率、速度与加速度

Speed is the rate at which distance is covered. It is a scalar. The average speed v = d / t, where d is total distance and t is time. Velocity, however, is speed in a given direction and is therefore a vector. Average velocity = displacement / time.

速率是走过距离的快慢程度，是一个标量。平均速率 v = d / t，其中 d 是总距离，t 是时间。而速度是给定方向上的速率，因此是一个矢量。平均速度 = 位移 / 时间。

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 taken. If an object slows down, the acceleration is negative (deceleration). The unit of acceleration is m/s².

加速度是速度变化的快慢程度。a = (v – u) / t，其中 u 是初速度，v 是末速度，t 是所用时间。如果物体减速，加速度为负（减速度）。加速度的单位是 m/s²。

Uniform acceleration means the velocity changes by equal amounts in equal time intervals. Near the Earth’s surface, all objects in free fall experience a uniform acceleration of approximately 9.8 m/s², denoted by g.

匀加速意味着在相等的时间间隔内速度的变化量相等。在地球表面附近，所有自由落体都经历约 9.8 m/s² 的匀加速度，记作 g。

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3. Motion Graphs | 运动图像

Distance–time graphs and speed–time graphs are essential tools for analysing motion. The gradient of a distance–time graph gives speed. A straight, sloping line indicates constant speed; a horizontal line indicates the object is stationary. A curved line shows changing speed (acceleration).

距离—时间图像和速率—时间图像是分析运动的基本工具。距离—时间图像的斜率表示速率。一条倾斜的直线表示匀速；水平线表示物体静止。曲线表示变速率（加速度）。

On a speed–time graph, the gradient gives acceleration. A horizontal line means constant speed, while a sloping line shows acceleration or deceleration. The area under a speed–time graph represents the distance travelled. If the line goes below the time axis, the area is considered negative for displacement, but distance is added as a positive area.

在速率—时间图像上，斜率表示加速度。水平线表示匀速，倾斜线表示加速或减速。速率—时间图像下的面积表示行驶的距离。如果线段延伸至时间轴以下，面积在位移计算中视为负值，但距离仍作为正面积相加。

For example, a car accelerating uniformly from rest to 20 m/s in 10 s will produce a triangle on the speed–time graph. The area = ½ × 10 × 20 = 100 m, which is the distance covered.

例如，一辆汽车从静止均匀加速至 20 m/s 用时 10 s，其速率—时间图像将形成一个三角形。面积 = ½ × 10 × 20 = 100 m，即行驶的距离。

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4. Forces and Newton’s Laws | 力与牛顿定律

A force is a push or a pull that can change an object’s speed, shape or direction. Forces are measured in newtons (N). Contact forces include friction, tension and normal reaction. Non‑contact forces include gravitational, magnetic and electrostatic forces.

力是一种推或拉的作用，可以改变物体的速率、形状或方向。力的单位是牛顿（N）。接触力包括摩擦力、张力和法向反作用力。非接触力包括引力、磁力和静电力。

Newton’s First Law states that an object remains at rest or in uniform motion unless acted upon by a resultant force. This is the principle of inertia. Newton’s Second Law gives: F = m × a, where F is the resultant force, m is mass, and a is acceleration. Newton’s Third Law states that for every action there is an equal and opposite reaction; these forces act on different objects.

牛顿第一定律指出，除非受到合外力的作用，物体将保持静止或匀速直线运动状态。这就是惯性的原理。牛顿第二定律给出：F = m × a，其中 F 是合外力，m 是质量，a 是加速度。牛顿第三定律指出，每一个作用力都有一个大小相等、方向相反的反作用力；这两个力作用在不同的物体上。

When forces are balanced, the resultant force is zero and the object moves with constant velocity or stays still. Unbalanced forces cause acceleration in the direction of the net force.

当力平衡时，合外力为零，物体以恒定的速度运动或保持静止。不平衡的力会产生沿合力方向的加速度。

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5. Mass, Weight and Gravity | 质量、重量与重力

Mass is the amount of matter in an object; it is a scalar and is measured in kilograms (kg). Weight is the gravitational force acting on that mass; it is a vector and is measured in newtons. The relationship is W = m × g, where g is the gravitational field strength. On Earth, g ≈ 9.8 N/kg.

质量是物体所含物质的多少；它是一个标量，单位是千克（kg）。重量是作用在该质量上的引力；它是一个矢量，单位是牛顿。关系式为 W = m × g，其中 g 是引力场强度。在地球上，g ≈ 9.8 N/kg。

The mass of an object is constant everywhere, but its weight changes depending on the gravitational field strength. For example, on the Moon, g is about 1.6 N/kg, so an astronaut weighs less, though their mass remains the same.

物体的质量在任何地方都是恒定的，但其重量会随引力场强度而变化。例如，在月球上，g 约为 1.6 N/kg，因此宇航员体重更轻，尽管其质量保持不变。

Free fall is when the only force acting on an object is its weight. In the absence of air resistance, all objects fall with the same acceleration g. Air resistance opposes motion and increases with speed; eventually, an object may reach terminal velocity, where weight equals air resistance and acceleration becomes zero.

自由落体是指作用在物体上的唯一力是其重量的情况。在没有空气阻力的情况下，所有物体都以相同的加速度 g 下落。空气阻力阻碍运动，并随速度增加而增大；最终，物体可能达到终极速度，此时重量等于空气阻力，加速度为零。

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6. Momentum | 动量

Momentum is a vector quantity defined as the product of mass and velocity: p = m × v. Its unit is kg m/s. Momentum is conserved in all collisions and explosions, provided no external forces act. This means total momentum before an event equals total momentum after.

动量是一个矢量，定义为质量与速度的乘积：p = m × v。其单位是 kg m/s。在没有外力作用的情况下，动量在所有碰撞和爆炸中守恒。这意味着事件前的总动量等于事件后的总动量。

In an elastic collision, both momentum and kinetic energy are conserved. In an inelastic collision, momentum is conserved but some kinetic energy is transformed into other forms, such as heat or sound.

在弹性碰撞中，动量和动能都守恒。在非弹性碰撞中，动量守恒，但部分动能转化为其他形式的能量，如热或声。

Force is related to the rate of change of momentum: F = Δp / Δt. This helps explain why airbags reduce injury: they increase the time over which the momentum change occurs, thereby reducing the average force exerted on the occupant.

力与动量的变化率相关：F = Δp / Δt。这有助于解释为什么安全气囊能减少伤害：它们延长了动量变化发生的时间，从而减小了施加在乘员身上的平均力。

An example: a 1500 kg car moving at 20 m/s crashes into a wall and stops in 0.1 s. The change in momentum = 1500 × 20 = 30 000 kg m/s. Average force = 30 000 / 0.1 = 300 000 N.

例如：一辆 1500 kg 的汽车以 20 m/s 的速度行驶，撞上墙并在 0.1 s 内停下。动量变化 = 1500 × 20 = 30 000 kg m/s。平均力 = 30 000 / 0.1 = 300 000 N。

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7. Energy Transfers and Work | 能量转移与功

Energy is the capacity to do work. The principle of conservation of energy states that energy cannot be created or destroyed, only transferred from one form to another. Common forms include kinetic, gravitational potential, thermal (heat), chemical, elastic, nuclear and electromagnetic.

能量是做功的本领。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。常见的形式包括动能、重力势能、热能（热）、化学能、弹性势能、核能和电磁能。

Work is done when a force moves an object in the direction of the force. Work = force × distance moved in the direction of the force: W = F × d. Work is measured in joules (J). One joule is the work done when a force of 1 N moves through 1 m.

当力使物体沿力的方向移动时，力做功。功 = 力 × 沿力方向移动的距离：W = F × d。功的单位是焦耳（J）。1 焦耳是 1 N 的力移动 1 m 所做的功。

Kinetic energy (E_k = ½ × m × v²) and gravitational potential energy (E_p = m × g × h) are frequently interconverted. In a pendulum swing, energy continuously transfers between kinetic and gravitational potential energy. Assuming no air resistance, the total mechanical energy stays constant.

动能（E_k = ½ × m × v²）与重力势能（E_p = m × g × h）经常相互转化。在摆锤摆动中，能量在动能和重力势能之间不断转移。假设没有空气阻力，总机械能保持恒定。

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8. Power and Efficiency | 功率与效率

Power is the rate of doing work or the rate of energy transfer. P = W / t, where W is work done or energy transferred, and t is time. The unit of power is the watt (W): 1 W = 1 J/s. A more powerful engine does the same work in less time or more work in the same time.

功率是做功的快慢或能量转移的速率。P = W / t，其中 W 是所做的功或转移的能量，t 是时间。功率的单位是瓦特（W）：1 W = 1 J/s。功率更大的发动机能在更短时间内做相同的功，或在相同时间内做更多的功。

Efficiency measures how much of the input energy is usefully transferred. Efficiency = (useful output energy / total input energy) × 100%. No real device is 100% efficient; some energy is always lost as heat due to friction or electrical resistance.

效率衡量有用输出能量占输入能量的比例。效率 = （有用输出能量 / 总输入能量）× 100%。没有实际设备能达到 100% 的效率；由于摩擦或电阻，总有一部分能量以热的形式损失掉。

A Sankey diagram shows energy transfers with arrows; the width of each arrow represents the amount of energy. The input arrow splits into useful output and waste energy arrows, making energy pathways clear.

桑基图用箭头表示能量转移；每个箭头的宽度代表能量的大小。输入箭头分为有用输出和浪费能量箭头，使能量传递路径一目了然。

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9. Energy Resources | 能源

Energy resources are classified as renewable or non‑renewable. Non‑renewable sources include fossil fuels (coal, oil, natural gas) and nuclear fuel (uranium). They are finite and contribute to pollution and greenhouse gas emissions. Fossil fuels release carbon dioxide when burned, intensifying the greenhouse effect.

能源分为可再生和不可再生。不可再生能源包括化石燃料（煤、石油、天然气）和核燃料（铀）。它们是有限的，会导致污染和温室气体排放。化石燃料燃烧时释放二氧化碳，加剧温室效应。

Renewable energy sources include solar, wind, wave, tidal, hydroelectric, biomass and geothermal. These are replenished naturally and generally produce lower emissions, though they often depend on weather or location. Solar panels convert light directly to electricity; wind turbines use kinetic energy of wind.

可再生能源包括太阳能、风能、波浪能、潮汐能、水力发电、生物质能和地热。这些能源可以自然补充，通常排放较低，但往往依赖天气或地理位置。太阳能电池板将光直接转化为电；风力涡轮机利用风的动能。

When choosing an energy resource, factors such as cost, reliability, environmental impact, power output and the ability to store energy must be considered. Often a mix of sources is used in a national grid.

选择能源时，必须考虑成本、可靠性、环境影响、输出功率以及能量储存能力等因素。国家电网通常使用多种能源的组合。

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10. Pressure | 压强

Pressure is the force acting per unit area. P = F / A, where F is the force perpendicular to the surface and A is the area. The unit of pressure is the pascal (Pa): 1 Pa = 1 N/m². A larger force or a smaller area results in greater pressure.

压强是作用在单位面积上的力。P = F / A，其中 F 是垂直于表面的力，A 是面积。压强的单位是帕斯卡（Pa）：1 Pa = 1 N/m²。力越大或面积越小，压强越大。

In fluids (liquids and gases), pressure acts uniformly in all directions. The pressure in a liquid at a given depth is given by P = h × ρ × g, where h is the depth, ρ is the density of the liquid, and g is gravitational field strength. This explains why dams are thicker at the bottom: to withstand greater pressure.

在流体（液体和气体）中，压强均匀地作用于所有方向。液体中给定深度的压强为 P = h × ρ × g，其中 h 是深度，ρ 是液体的密度，g 是引力场强度。这解释了为什么大坝底部更厚：为了承受更大的压强。

Atmospheric pressure is caused by the weight of air above us. It decreases with altitude. Barometers measure atmospheric pressure; a simple mercury barometer shows a column height of about 760 mm at sea level.

大气压是由上方空气的重量引起的，随海拔升高而降低。气压计用于测量大气压；一个简单的汞气压计在海平面显示约 760 mm 的柱高。

Hydraulic systems use Pascal’s principle: pressure applied to an enclosed fluid is transmitted equally in all directions. With a small input force on a small piston, a large output force can be obtained on a larger piston, since P₁ = P₂, so F₁/A₁ = F₂/A₂. This is used in car brakes and hydraulic lifts.

液压系统运用帕斯卡原理：施加在封闭流体上的压强会大小不变地向各个方向传递。利用小活塞上的小输入力，可以在大活塞上得到大的输出力，因为 P₁ = P₂，所以 F₁/A₁ = F₂/A₂。这应用于汽车刹车和液压升降机中。

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