Forces & Movement | 力与运动

📚 Forces & Movement | 力与运动

Forces are the invisible pushes and pulls that shape every movement in the universe. Whether a cricket ball is sailing through the air or a rocket is escaping Earth’s gravity, the underlying physics follows the same simple but powerful rules. In the Edexcel IGCSE Physics course, understanding forces and movement is essential — it connects directly to Newton’s laws, momentum, stopping distances and even terminal velocity. Let’s explore these core ideas step by step.

力是塑造宇宙中一切运动的无形推拉作用。无论是一颗板球在空中飞过，还是一枚火箭挣脱地球引力，背后的物理规律都是相同且强大的。在 Edexcel IGCSE 物理课程中，理解力与运动至关重要——它直接联系着牛顿定律、动量、制动距离甚至终端速度。让我们一步步深入这些核心概念。

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1. Forces as Vectors | 力是矢量

In physics, quantities are sorted into two families. Scalar quantities have only magnitude (size), such as speed, mass and energy. Vector quantities have both magnitude and direction, such as velocity, acceleration and force. A force is therefore a vector: pushing something north is not the same as pushing it east, even if the strength of the push is identical.

在物理学中，物理量分为两大类。标量只有大小，例如速率、质量和能量。矢量同时具有大小和方向，例如速度、加速度和力。因此，力是矢量：向北推一个物体与向东推它并不相同，即使推力的大小完全相同。

In diagrams, forces are drawn as arrows. The length of the arrow represents the magnitude of the force, while the arrowhead shows its direction. When several forces act on an object, we can find the resultant force by adding them as vectors — often using a scale drawing or resolving into perpendicular components.

在示意图中，力用箭头表示。箭头的长度代表力的大小，箭头方向代表力的方向。当多个力作用在一个物体上时，我们可以通过矢量加法求出合力——通常使用比例尺作图，或将力分解为垂直分量。

Scalar | 标量	Vector | 矢量
Speed 速率	Velocity 速度
Mass 质量	Weight 重力
Energy 能量	Force 力
Distance 距离	Displacement 位移

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2. Contact and Non-Contact Forces | 接触力与非接触力

Forces can be grouped by whether the objects are physically touching. Contact forces require direct contact between objects. Friction, air resistance (drag), tension in a rope and the normal reaction from a surface are all contact forces. Without touching, no such force would exist.

力可以根据物体是否实际接触来分类。接触力要求物体直接接触。摩擦力、空气阻力（阻力）、绳中的张力和来自表面的法向反作用力都属于接触力。如果没有接触，这样的力就不存在。

Non-contact forces act at a distance, without any physical touch. Gravity, electrostatic forces and magnetism are the classic examples. An apple falling from a tree is pulled by Earth’s gravity even though there is a huge gap of empty air between them. These distant forces are fundamental to the Universe — gravity keeps planets in orbit, and electrostatic forces hold atoms together.

非接触力在远处起作用，不需要任何实际接触。引力、静电力和磁力是典型的例子。一个苹果从树上掉落，尽管它与地球之间有巨大的空气间隙，仍然被地球引力拉向地面。这些远距离力是宇宙的根本——引力使行星保持在轨道上，静电力则将原子结合在一起。

• Contact forces: friction, drag, tension, normal contact force | 接触力：摩擦力、阻力、张力、法向接触力

• Non-contact forces: gravitational force, electrostatic force, magnetic force | 非接触力：引力、静电力、磁力

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3. Newton’s First Law of Motion | 牛顿第一定律

Newton’s First Law states: an object will remain at rest, or continue to move at a constant velocity, unless acted on by a resultant (unbalanced) force. In other words, moving objects do not naturally slow down and stop — that is friction doing its job. On a frictionless surface, a sliding puck would glide forever.

牛顿第一定律指出：物体将保持静止，或保持匀速直线运动，除非受到合（不平衡）外力作用。换句话说，运动的物体并不会自动减速停下——那是摩擦力在起作用。在无摩擦的表面上，滑动的冰球将永远滑行下去。

This property of matter is called inertia. Inertia is not a force; it is the tendency of an object to resist changes in its velocity. The greater the mass of an object, the greater its inertia, and the harder it is to start it moving or to stop it once moving. That is why a loaded lorry takes much longer to reach full speed than a bicycle.

物质的这种性质称为惯性。惯性不是力，而是物体抵抗速度变化的倾向。物体的质量越大，惯性就越大，越难使它开始运动，也越难使它停下来。这就是为什么满载的卡车达到全速所需的时间远长于自行车。

When the forces on an object are balanced, the resultant force is zero. The object may be at rest, or it may be moving with constant velocity. Balanced forces do not change the motion — they only keep the current state. This is a common exam point: constant velocity does not mean no forces act; it means the forces cancel out.

当物体上的力平衡时，合力为零。物体可以处于静止状态，或者以恒定速度运动。平衡力不会改变运动状态——它们只是维持当前状态。这是一个常见的考点：匀速并不意味着没有力作用，而是意味着力相互抵消。

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4. Newton’s Second Law: F = ma | 牛顿第二定律：F = ma

Newton’s Second Law gives the quantitative link between force, mass and acceleration. The resultant force on an object equals the product of its mass and its acceleration. The equation is beautifully simple:

牛顿第二定律给出了力、质量和加速度之间的定量联系。物体所受合力等于其质量与加速度的乘积。这个公式非常简洁：

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). Because force is a vector, the direction of the acceleration matches the direction of the resultant force. If a force of 100 N acts on a 25 kg object, the acceleration is:

其中，F 是合力，单位是牛顿（N）；m 是质量，单位是千克（kg）；a 是加速度，单位是米每秒平方（m/s²）。由于力是矢量，加速度的方向与合力的方向一致。如果一个 100 N 的力作用在一个 25 kg 的物体上，加速度为：

a = F ÷ m = 100 N ÷ 25 kg = 4 m/s²

When an object is accelerating downhill, the resultant force comes from the component of gravity acting along the slope minus friction and drag. If the driving force of a car engine is exactly balanced by friction and drag, the car does not speed up – it moves at constant velocity. Acceleration only happens when forces are unbalanced.

当物体沿斜坡加速下滑时，合力来自重力沿斜坡方向的分量减去摩擦力和空气阻力。如果汽车发动机的驱动力恰好与摩擦力和阻力平衡，汽车不会加速——它以恒定速度运动。只有在力不平衡时，才会产生加速度。

Situation | 情景	Resultant force | 合力	Effect | 效果
Car cruising at 60 km/h | 汽车以 60 km/h 匀速行驶	Zero | 零	Constant velocity 匀速
Lift accelerating upward | 电梯向上加速	Upward | 向上	Acceleration upward 向上加速
Braking cyclist | 刹车中的骑车人	Backward | 向后	Deceleration 减速

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5. Mass and Weight | 质量与重力

Mass and weight are frequently confused, but they are completely different physical quantities. Mass is the amount of matter in an object. It is a scalar quantity, measured in kilograms (kg), and does not change with location. A 10 kg bag of rice is still 10 kg on the Moon, in a spaceship, or at the bottom of the ocean.

质量和重力（重量）经常被混淆，但它们是完全不同的物理量。质量是物体所含物质的多少。它是标量，单位是千克（kg），不会随位置变化而变化。一袋 10 kg 的大米，无论是在月球上、在飞船里，还是在海底，仍然是 10 kg。

Weight, on the other hand, is the gravitational force acting on an object’s mass. It is a vector quantity, measured in newtons (N), and depends on the local gravitational field strength, g. On Earth, g is approximately 9.8 N/kg; on the Moon, it is only about 1.6 N/kg. The equation linking weight and mass is:

另一方面，重力（重量）是作用在物体质量上的引力。它是矢量，单位是牛顿（N），与所在位置的重力场强度 g 有关。在地球上，g 约为 9.8 N/kg；在月球上，它只有约 1.6 N/kg。连接重力与质量的公式是：

W = m × g

So a 60 kg student on Earth has a weight of 60 × 9.8 = 588 N. If that student travelled to the Moon, their mass would still be 60 kg, but their weight would drop to 60 × 1.6 = 96 N. Mass stays the same, weight changes with gravity. Exam questions often test this exact distinction.

因此，一名 60 kg 的学生在地球上的重力为 60 × 9.8 = 588 N。如果这名学生前往月球，他的质量仍然是 60 kg，但重力会降到 60 × 1.6 = 96 N。质量保持不变，重力随引力而变化。考试题目经常考查这一区别。

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6. Newton’s Third Law | 牛顿第三定律

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. These two forces are always the same size, always opposite in direction, and always act on different objects. For every action, there is an equal and opposite reaction.

牛顿第三定律指出：每当物体 A 对物体 B 施加一个力，物体 B 同时会对物体 A 施加一个大小相等、方向相反的力。这两个力总是大小相同、方向相反，并且总是作用在不同的物体上。每一个作用力，都有一个大小相等、方向相反的反作用力。

When you jump off a small boat, you push the boat backward while the boat pushes you forward. The forces are equal in size, but because the boat has much smaller mass (compared to the Earth anyway), the effect of the reaction on you is more noticeable in everyday life. In a rocket launch, exhaust gases are pushed backward with huge force, and the gases push the rocket forward with an equal force.

当你从小船上跳下时，你把船向后推，同时船也把你向前推。这两个力大小相等，但因为船的质量较小，反作用对你的效果在日常生活中更为明显。在火箭发射中，废气以巨大的力向后喷出，气体同时也以相等的力将火箭向前推进。

Be careful not to confuse Newton’s Third Law with balanced forces. Balanced forces act on the same object and cancel out, producing no acceleration. Action–reaction pairs act on different objects, so they do not cancel — each object experiences its own force.

注意不要将牛顿第三定律与平衡力混淆。平衡力作用在同一物体上并相互抵消，不产生加速度。而作用力与反作用力作用在不同物体上，因此它们不会相互抵消——每个物体都会各自受力。

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7. Friction, Drag and Motion | 摩擦力、阻力与运动

Friction is a contact force that opposes relative motion between two surfaces in contact. It arises because surfaces, even polished ones, are microscopically rough. Friction acts along the contact surface, always opposing the direction of attempted sliding. Air resistance, also called drag, is a type of friction that acts through a fluid (gas or liquid) and increases with speed.

摩擦力是一种接触力，阻碍两个接触表面之间的相对运动。它产生的原因是表面即使经过打磨，在微观上仍然是粗糙的。摩擦力沿接触表面作用，总是阻碍试图滑动的方向。空气阻力（也称为阻力）是一种通过流体（气体或液体）作用的摩擦力，并且会随着速度的增加而增大。

Friction has both advantages and disadvantages. Without friction, we could not walk — our feet would slide backward. Car brakes rely on friction to slow the wheels, and tyres need friction to grip the road. On the other hand, friction in engines and gears wastes energy as heat, and it causes parts to wear out. Engineers reduce unwanted friction using lubricants, ball bearings and streamlined shapes.

摩擦力既有优点也有缺点。没有摩擦力，我们就无法行走——脚会向后滑。汽车刹车依靠摩擦力来减速车轮，轮胎需要摩擦力来抓地。另一方面，发动机和齿轮中的摩擦会以热的形式浪费能量，并导致部件磨损。工程师通过使用润滑油、滚珠轴承和流线型外形来减少不必要的摩擦。

For a falling object, two key forces act: weight pulls downward, and air resistance pushes upward. At the start of the fall, the object accelerates because weight is larger than drag. As speed increases, drag increases too. The gap between the two forces shrinks, so the acceleration becomes smaller. When the speed becomes high enough that drag exactly equals weight, the resultant force becomes zero and the object falls at constant velocity.

对于下落物体，两个关键力作用：重力向下拉，空气阻力向上推。下落开始时，物体加速，因为重力大于阻力。随着速度增加，阻力也不断增加。两个力之间的差距缩小，加速度随之减小。当速度达到足够高，使阻力恰好等于重力时，合力变为零，物体以恒定速度下落。

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8. Terminal Velocity | 终端速度

Terminal velocity is the constant maximum velocity achieved by an object when the drag force exactly balances the driving force (usually weight). At this point there is no resultant force, so according to Newton’s First Law the object continues to move at a steady speed. For a skydiver, terminal velocity depends on their body position and clothing; a spread-eagle pose creates more drag and gives a slower terminal velocity than a head-down dive.

终端速度是当阻力恰好平衡驱动力（通常是重力）时，物体达到的恒定最大速度。此时合力为零，因此根据牛顿第一定律，物体将继续以稳定速度运动。对于跳伞运动员来说，终端速度取决于身体姿势和服装；四肢展开的姿势会产生更大的阻力，从而比头朝下的俯冲姿势获得更慢的终端速度。

Consider the complete journey of a skydiver during free fall. Immediately after jumping, the skydiver moves slowly, so air resistance is small and the resultant force is large downward. This produces a large initial acceleration. As speed increases, air resistance increases, so the resultant force decreases and the acceleration becomes smaller. The graph of velocity against time therefore curves gradually until it flattens out at terminal velocity.

考虑跳伞者在自由落体过程中的完整旅程。刚跳出时，跳伞者速度较慢，空气阻力很小，合力方向向下且很大。这会产生较大的初始加速度。随着速度增加，空气阻力增大，合力减小，加速度变小。因此速度—时间图像逐渐弯曲，直到在终端速度处趋于平坦。

If the skydiver opens the parachute, the surface area increases dramatically. The drag force suddenly becomes much larger than the weight, creating a large upward resultant force. This causes rapid deceleration. The skydiver slows down until the drag force decreases enough to once again balance weight, reaching a new, much lower terminal velocity — usually safe for landing.

如果跳伞者打开降落伞，表面积会急剧增大。阻力突然变得比重力大得多，产生一个很大的向上合力。这会导致快速减速。跳伞者减速，直到阻力减小到再次平衡重力，达到一个更低的新的终端速度——通常足以安全着陆。

Stage 1: accelerating free fall | Stage 2: terminal velocity | Stage 3: parachute opening → deceleration → new terminal velocity

阶段 1：加速自由落体 | 阶段 2：终端速度 | 阶段 3：开伞 → 减速 → 新的终端速度

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9. Stopping Distances | 制动距离

The stopping distance of a vehicle is the total distance travelled from the moment the driver sees a hazard until the vehicle comes to a complete stop. It is the sum of two parts: the thinking distance (travelled during the driver’s reaction time) and the braking distance (travelled while the brakes are applied).

车辆的制动距离（停车距离）是从驾驶员看到危险的那一刻起，到车辆完全停下来所行驶的总距离。它由两部分组成：思考距离（驾驶员反应时间内行驶的距离）和制动距离（刹车踩下后行驶的距离）。

Stopping distance = Thinking distance + Braking distance

制动距离 = 思考距离 + 制动距离

The thinking distance is affected by the driver’s reaction time. Tiredness, alcohol, drugs and distractions such as mobile phones all increase reaction time, thus increasing thinking distance. The braking distance depends on the speed of the car, the condition of the tyres and brakes, the road surface (wet or icy), and the gradient of the road. Doubling the speed does not simply double the braking distance — it quadruples it, because kinetic energy increases with the square of speed.

思考距离受驾驶员反应时间的影响。疲劳、酒精、药物以及手机等分心因素都会增加反应时间，从而增加思考距离。制动距离取决于汽车速度、轮胎和刹车状况、路面条件（湿滑或结冰）以及道路坡度。将速度加倍并不会使制动距离简单加倍——而是使其变为原来的四倍，因为动能随速度的平方增加。

Speed (mph) | 速度（英里/小时）	Thinking distance (m) | 思考距离（米）	Braking distance (m) | 制动距离（米）
20	6	6
30	9	14
40	12	24
70	21	75

Speed limits and safety distances are designed around real stopping data. A car travelling at 30 mph has a typical overall stopping distance of 23 m; at 70 mph that rises to 96 m. On wet roads, braking distance can increase by up to 50 %, and on icy roads it can be ten times longer. Always keep a safe following distance.

限速和安全车距都是基于真实的制动数据设计的。以 30 英里/小时行驶的汽车，其典型的整体制动距离为 23 米；在 70 英里/小时时则上升到 96 米。在湿滑路面上，制动距离可能增加多达 50%，而在结冰路面上可能达到十倍以上。始终保持安全车距。

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10. Momentum and Impulse | 动量与冲量

Momentum is a vector quantity that measures the quantity of motion an object has. It is defined as the product of mass and velocity. The unit of momentum is kg·m/s. The equation is:

动量是一个矢量量，用于衡量物体运动的量。它定义为质量与速度的乘积。动量的单位是 kg·m/s。公式为：

p = m × v

Momentum is conserved in any collision or explosion, provided no external resultant force acts. This principle, called the conservation of momentum, is enormously powerful. When two objects collide head-on, the total momentum before the collision equals the total momentum after the collision. That is how engineers calculate the outcomes of vehicle collisions and how snooker players predict ball movement.

在没有任何外合力作用的情况下，任何碰撞或爆炸中动量都守恒。这一原理称为动量守恒定律，极其强大。当两个物体正面碰撞时，碰撞前的总动量等于碰撞后的总动量。工程师正是利用这一点来计算车辆碰撞的结果，斯诺克选手也以此预判球的运动。

Impulse is the product of force and the time for which it acts, and it equals the change in momentum. A large force acting for a very short time can produce the same impulse as a small force acting for a long time. This explains why safety features such as airbags and crumple zones work: they extend the time over which momentum changes, so the force on passengers is reduced.

冲量是力与其作用时间的乘积，等于动量的变化量。一个很大的力在极短的时间内作用，可以产生与一个很小的力在很长时间内作用相同的冲量。这就解释了为什么安全气囊和溃缩区等安全设计有效：它们延长了动量变化的时间，从而减小了乘客受到的力。

Impulse = F × t = Δp

Every time you catch a cricket ball, you instinctively pull your hands backward. That movement extends the stopping time, reduces the average force, and prevents pain. It is the same physics that protects a cyclist wearing a helmet: the helmet spreads the impact over a longer duration, lessening the peak force on the skull.

每次你接板球时，都会本能地将手向后收。这个动作延长了停止时间，减小了平均力，从而避免疼痛。保护骑车人的头盔利用的是同样的物理原理：头盔将撞击分散到更长的时间中，从而减小了对头骨的最大力。

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11. Applying Newton’s Laws in Everyday Life | 将牛顿定律应用于日常生活

The laws of motion are not just exam topics — they explain the world around us. A rocket launching relies on Newton’s Third Law; a car accelerating away from traffic lights relies on the Second Law; a passenger jerking forward when a bus stops suddenly demonstrates inertia from the First Law. Every safer braking system, every sports technique, and every amusement park ride is designed with these principles in mind.

运动定律不仅仅是考试内容——它们解释了我们周围的世界。火箭发射依赖牛顿第三定律；汽车从红绿灯处加速出发依赖第二定律；公交车突然刹车时乘客前倾展示了第一定律中的惯性。每一个更安全的刹车系统、每一种运动技巧、每一个游乐设施，都是围绕这些原理设计的。

In the Edexcel IGCSE examination, always remember to quote units, state whether a quantity is a vector or scalar, and mention the direction when calculating forces or acceleration. Combine the equations confidently: Newton’s Second Law, the weight equation, momentum and impulse. If you can interpret velocity–time graphs and identify balanced versus unbalanced forces, you are well prepared for the Forces & Movement section.

在 Edexcel IGCSE 考试中，务必记住注明单位，说明一个量是矢量还是标量，并在计算力或加速度时指明方向。自信地组合运用公式：牛顿第二定律、重力公式、动量和冲量。如果你能解读速度—时间图像，并能区分平衡力与不平衡力，那么你对于力与运动部分已经有了充分的准备。

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