IGCSE WJEC Physics: Dynamics Key Points | IGCSE WJEC 物理：动力学 考点精讲

📚 IGCSE WJEC Physics: Dynamics Key Points | IGCSE WJEC 物理：动力学 考点精讲

Dynamics is the branch of physics that deals with the motion of objects and the forces causing this motion. In the IGCSE WJEC Physics specification, dynamics covers essential concepts such as Newton’s laws of motion, momentum, impulse, and the conservation of momentum. Understanding these principles is crucial for explaining everyday phenomena like car crashes, sports, and rocket launches. This revision guide breaks down the key points you need to master for the exam.

动力学是物理学中研究物体运动及其成因的分支。在IGCSE WJEC物理课程中，动力学涵盖了牛顿运动定律、动量、冲量以及动量守恒等核心概念。理解这些原理对于解释汽车碰撞、体育运动和火箭发射等日常现象至关重要。本复习指南将分解你需掌握的考点。

1. Newton’s First Law of Motion (Law of Inertia) | 牛顿第一定律（惯性定律）

Newton’s first law states that an object will remain at rest or in uniform motion in a straight line unless acted upon by a resultant external force.

牛顿第一定律指出，除非受到合外力的作用，否则物体将保持静止或匀速直线运动状态。

This property of an object to resist changes in its state of motion is called inertia. The greater the mass of an object, the greater its inertia.

物体抵抗其运动状态变化的这种性质称为惯性。物体的质量越大，其惯性就越大。

For example, a passenger lurches forward when a bus brakes suddenly because their body tends to continue moving at the original velocity due to inertia.

例如，当公交车突然刹车时，乘客会向前倾，因为他们的身体由于惯性倾向于保持原来的速度运动。

In space, where there is almost no external force, a spacecraft will continue moving at constant velocity without using its engines, demonstrating the law.

在几乎没有外力的太空中，航天器无需发动机就能继续以恒定速度运动，这证明了该定律。

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2. Newton’s Second Law (F = ma) | 牛顿第二定律（F = ma）

Newton’s second law relates the resultant force acting on an object to its acceleration:

F = m a

牛顿第二定律将作用在物体上的合力与其加速度联系起来：

F = m a

Here, F is the resultant force in newtons (N), m is the mass in kilograms (kg), and a is the acceleration in metres per second squared (m/s²). The acceleration is directly proportional to the force and inversely proportional to the mass.

式中，F 是合力，单位为牛顿（N）；m 是质量，单位为千克（kg）；a 是加速度，单位为米每二次方秒（m/s²）。加速度与力成正比，与质量成反比。

Rearranging gives a = F / m. This means that for the same force, a heavier object will accelerate less.

整理后可得 a = F / m。这意味着在相同力作用下，较重的物体加速度较小。

In free fall under gravity, the force is weight (W = mg), so the acceleration due to gravity is g = W / m, which is approximately 9.8 m/s² on Earth.

在重力作用下的自由落体中，力是重力（W = mg），因此重力加速度为 g = W / m，在地球上约为 9.8 m/s²。

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3. Newton’s Third Law (Action-Reaction) | 牛顿第三定律（作用力与反作用力）

Newton’s third law states that whenever object A exerts a force on object B, object B simultaneously exerts an equal and opposite force on object A.

牛顿第三定律指出，每当物体A对物体B施加一个力时，物体B同时会对物体A施加一个大小相等、方向相反的力。

These two forces are of the same type, act on different bodies, and have the same magnitude but opposite direction. They are often called action and reaction forces.

这两个力是同种性质的力，作用在不同的物体上，大小相等但方向相反。它们通常被称为作用力和反作用力。

For example, when you push against a wall, the wall pushes back on you with an equal force. The force you apply does not cancel the reaction because they act on different objects.

例如，当你推墙时，墙会以同样大小的力回推你。你施加的力与反作用力不会相互抵消，因为它们作用在不同的物体上。

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4. Mass, Weight, and Gravitational Field Strength | 质量、重量与引力场强度

Mass is the amount of matter in an object and is measured in kilograms (kg). It is a scalar quantity and remains the same everywhere in the universe.

质量是物体所含物质的多少，以千克（kg）为单位。它是标量，在宇宙中任何地方都保持不变。

Weight is the gravitational force acting on an object and is measured in newtons (N). It depends on the gravitational field strength (g) and mass (m):

W = m g

重量是作用在物体上的重力，以牛顿（N）为单位。它取决于引力场强度（g）和质量（m）：

W = m g

Gravitational field strength g on Earth is about 9.8 N/kg. On the Moon, g ≈ 1.6 N/kg, so your weight would be much less, but your mass unchanged.

地球上的引力场强度 g 约为 9.8 N/kg。月球上 g ≈ 1.6 N/kg，因此你的体重会轻很多，但质量不变。

The table below summarises the key differences between mass and weight:

下表总结了质量和重量的主要区别：

Property	Mass	Weight
Definition	Amount of matter	Gravitational pull
Unit	kg	N
Depends on	Quantity of matter	Mass and g
Measured with	Balance	Spring scale / newton meter

Mass is measured using a balance, whereas weight is measured using a spring scale or newton meter.

质量使用天平测量，而重量使用弹簧秤或牛顿计测量。

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5. Resultant Force and Free-Body Diagrams | 合力和受力图

The resultant (net) force is the vector sum of all forces acting on an object. When forces are balanced, the resultant force is zero, and the object moves with constant velocity or stays at rest.

合力是作用在物体上所有力的矢量和。当力平衡时，合力为零，物体保持匀速运动或静止状态。

Free-body diagrams are simplified sketches showing all forces acting on a single object, using arrows to indicate magnitude and direction.

受力图是显示作用在单个物体上所有力的简图，用箭头表示力的大小和方向。

For an object on a horizontal surface, typical forces include weight (downwards), normal reaction (upwards), applied force, and friction.

对于水平面上的物体，常见的力有重力（向下）、法向反作用力（向上）、施加的力和摩擦力。

When drawing free-body diagrams, arrows must be drawn to scale and labeled correctly. The resultant force can be found by vector addition.

画受力图时，箭头必须按比例绘制并正确标注。合力可以通过矢量加法求得。

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6. Momentum and Impulse | 动量和冲量

Momentum p is the product of mass and velocity:

p = m v

动量 p 是质量与速度的乘积：

p = m v

It is a vector quantity measured in kg m/s.

动量是矢量，单位为 kg m/s。

Impulse is the product of force and the time for which it acts, and it equals the change in momentum:

F Δt = Δp = m v – m u

冲量是力与其作用时间的乘积，等于动量的变化量：

F Δt = Δp = m v – m u

where u is initial velocity and v is final velocity.

其中 u 是初速度，v 是末速度。

The impulse-momentum relationship explains why airbags are used in cars: they increase the time over which the force acts during a collision, reducing the force experienced by passengers.

冲量-动量关系解释了为什么汽车使用安全气囊：它们延长了碰撞时力作用的时间，从而减小了乘客受到的力。

In sports, the follow-through in hitting a ball increases contact time, leading to a greater change in momentum and thus a faster ball speed.

在体育运动中，击球后的顺势动作增加了接触时间，导致更大的动量变化，从而球速更快。

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7. Conservation of Momentum | 动量守恒

The law of conservation of momentum states that in a closed system (no external forces), the total momentum before a collision or explosion equals the total momentum after.

动量守恒定律指出，在封闭系统中（无外力），碰撞或爆炸前的总动量等于碰撞或爆炸后的总动量。

For a collision between two objects A and B:

m₁ u₁ + m₂ u₂ = m₁ v₁ + m₂ v₂

对于两个物体A和B之间的碰撞：

m₁ u₁ + m₂ u₂ = m₁ v₁ + m₂ v₂

In an explosion, the total momentum is zero before and after, so the two fragments move in opposite directions with equal and opposite momenta.

在爆炸中，前后总动量为零，因此两个碎片以大小相等、方向相反的动量反向运动。

Problems often involve calculating velocities after a collision or recoil speed in gun-shooting scenarios.

问题通常涉及计算碰撞后的速度或枪击情形中的反冲速度。

Inelastic collisions are those where kinetic energy is not conserved (though momentum is). Perfectly inelastic collisions result in objects sticking together.

非弹性碰撞是指动能不守恒（但动量守恒）的碰撞。完全非弹性碰撞导致物体粘在一起。

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8. Terminal Velocity and Air Resistance | 终端速度与空气阻力

When an object falls through a fluid (liquid or gas), it experiences air resistance (drag) that increases with speed.

当物体在流体（液体或气体）中下落时，会受到随速度增加而增大的空气阻力（阻力）。

Initially, the weight is greater than the drag, resulting in a downward acceleration. As speed increases, drag grows until it equals the weight.

初始时，重量

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