GCSE WJEC Physics: Kinetics – Key Revision Points | GCSE WJEC 物理：动力学考点精讲

📚 GCSE WJEC Physics: Kinetics – Key Revision Points | GCSE WJEC 物理：动力学考点精讲

In the WJEC GCSE Physics specification, the topic of kinetics – covering motion, forces, and momentum – forms a core part of Unit 2 (Forces, Space and Radioactivity). Mastering these concepts is essential for success in the examination, as they underpin many real-world applications and link to other topics such as energy and waves.

在 WJEC GCSE 物理考试大纲中，动力学主题——涵盖运动、力和动量——是第二单元（力、空间与放射性）的核心内容。掌握这些概念对于考试成功至关重要，因为它们支撑着许多实际应用，并与能量和波等其他主题相联系。

1. Distance and Displacement | 距离与位移

Distance is a scalar quantity that measures how much ground an object has covered during its motion, regardless of direction. Displacement is a vector quantity that measures the shortest straight-line distance from the initial position to the final position, including direction.

距离是一个标量，测量物体在运动过程中经过的路径长度，不考虑方向。位移是一个矢量，测量从初始位置到最终位置的最短直线距离，并包含方向。

If a runner completes one full lap on a 400 m track, the distance travelled is 400 m, but the displacement is 0 m because the start and end points coincide.

如果一名跑步者在 400 米圆形跑道上完成一整圈，所经过的距离是 400 米，而位移为 0 米，因为起点和终点重合。

2. Speed and Velocity | 速率与速度

Speed is a scalar quantity defined as the rate of change of distance. It can be calculated using: speed = distance / time. Velocity is a vector quantity defined as the rate of change of displacement. It has both magnitude and direction.

速率是一个标量，定义为距离随时间的变化率。可以用公式：速率 = 距离 ÷ 时间计算。速度是一个矢量，定义为位移随时间的变化率，既有大小也有方向。

An object moving at constant speed in a circle has a changing velocity because its direction is constantly changing, even though the speed is constant.

一个物体以恒定速率做圆周运动时，其速度一直在变化，因为方向在不断改变，尽管速率保持不变。

3. Acceleration | 加速度

Acceleration is the rate of change of velocity. It is a vector quantity, measured in metres per second squared (m/s²). The formula is: acceleration = change in velocity / time taken, or a = (v – u) / t, where u is initial velocity and v is final velocity.

加速度是速度的变化率，是一个矢量，单位为米每二次方秒 (m/s²)。计算公式为：加速度 = 速度变化量 ÷ 所用时间，即 a = (v – u) / t，其中 u 为初速度，v 为末速度。

Deceleration (negative acceleration) indicates that an object is slowing down. The magnitude of deceleration is simply the magnitude of negative acceleration.

减速（负加速度）表示物体在减慢。减速的大小就是负加速度的绝对值。

4. Equations of Motion (SUVAT) | 运动方程

For objects moving with constant acceleration in a straight line, the five SUVAT equations link displacement (s), initial velocity (u), final velocity (v), acceleration (a) and time (t).

对于在直线上以恒定加速度运动的物体，五个运动方程将位移 s、初速度 u、末速度 v、加速度 a 和时间 t 联系起来。

Equation	Missing variable
v = u + a t	s
s = ½ (u + v) t	a
s = u t + ½ a t²	v
s = v t − ½ a t²	u
v² = u² + 2 a s	t

These equations can only be used when acceleration is uniform. If acceleration changes, you must use graphical methods or break the motion into segments of constant acceleration.

这些方程只适用于加速度恒定的情况。如果加速度变化，则必须使用图像法或将运动分成若干恒定加速度段来处理。

5. Forces and Newton’s First Law | 力与牛顿第一定律

A force is a push or a pull that can change an object’s shape, speed, or direction. Newton’s First Law states that an object will remain at rest or move with constant velocity unless acted upon by a resultant external force.

力是一种推或拉，可以改变物体的形状、速度或方向。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 law introduces the concept of inertia – the tendency of an object to resist changes in its motion. The greater the mass, the greater the inertia.

该定律引入了惯性的概念——物体抵抗运动状态变化的趋势。质量越大，惯性越大。

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

Newton’s Second Law explains what happens when a resultant force acts on a mass: the acceleration produced is directly proportional to the resultant force and inversely proportional to the mass.

牛顿第二定律解释了当合力作用在质量上时会发生什么：产生的加速度与合力成正比，与质量成反比。

F = m a

The unit of force is the newton (N). 1 N is the force required to give a mass of 1 kg an acceleration of 1 m/s².

力的单位是牛顿 (N)。1 牛顿是使 1 kg 质量的物体产生 1 m/s² 加速度所需的力。

In WJEC exams, you will often need to apply F = ma in problems involving cars, falling objects, or balanced and unbalanced forces.

在 WJEC 考试中，你经常需要将 F = ma 用于涉及汽车、落体或平衡与不平衡力的问题。

7. Newton’s Third Law | 牛顿第三定律

Newton’s Third Law states that for every action, there is an equal and opposite reaction. The forces are always of the same type, act on different objects, and are equal in magnitude but opposite in direction.

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。这两个力总是同种类型，作用在不同物体上，大小相等方向相反。

Common examples include the thrust of a rocket (gas pushed backward, rocket pushed forward) and the support force of a table on a book (book pushes down, table pushes up).

常见例子包括火箭的推力（气体向后推，火箭向前推）以及桌子对书本的支持力（书本向下压，桌子向上支撑）。

8. Resultant Forces and Free-body Diagrams | 合力与受力分析图

The resultant force is the single force that has the same effect as all the individual forces acting on an object. It determines the acceleration according to F = ma. A free-body diagram shows all the forces acting on one object, often with arrows representing their magnitudes and directions.

合力是一个单独力，其作用效果与作用在物体上所有力的总和相同。它根据 F = ma 决定加速度。受力分析图显示作用在一个物体上的所有力，通常用箭头表示大小和方向。

When forces are balanced, the resultant force is zero, and the object either remains at rest or continues at constant velocity (Newton’s First Law). When unbalanced, the object accelerates in the direction of the resultant force.

当力平衡时，合力为零，物体保持静止或匀速直线运动（牛顿第一定律）。当力不平衡时，物体沿合力的方向加速。

9. Terminal Velocity | 终极速度

When an object falls through a fluid (air or liquid), it experiences drag force that opposes its motion. Initially, weight is greater than drag, so the object accelerates downward. As speed increases, drag increases until it balances the weight. At this point, the resultant force becomes zero and the object falls at a constant speed – the terminal velocity.

当物体在流体（空气或液体）中下落时，会受到与其运动方向相反的阻力。初始时重力大于阻力，物体向下加速。随着速度增加，阻力增大直至与重力平衡。此时合力为零，物体以恒定速度下落——这就是终极速度。

Terminal velocity explains why skydivers reach a maximum speed before opening their parachutes and why a feather falls slower than a stone in air.

终极速度解释了跳伞者在打开降落伞前为何会达到最大速度，以及为何在空气中羽毛比石头下落得慢。

10. Stopping Distance | 停车距离

The total stopping distance of a vehicle is the sum of the thinking distance (distance travelled while the driver reacts) and the braking distance (distance travelled while the brakes slow the car).

车辆的总停车距离等于思考距离（驾驶员反应时间内行驶的距离）与制动距离（刹车使汽车减速过程中行驶的距离）之和。

Thinking distance can be affected by tiredness, alcohol, drugs, or distractions. Braking distance depends on speed, road conditions, tyre conditions, and the mass of the vehicle.

思考距离受疲劳、酒精、药物或分心等因素影响。制动距离取决于车速、路面状况、轮胎状况和车辆质量。

With doubled speed, the braking distance increases by a factor of four, because the kinetic energy is proportional to speed squared.

当速度加倍时，制动距离增加为原来的四倍，因为动能与速度的平方成正比。

11. Momentum | 动量

Momentum is a vector quantity defined as the product of an object’s mass and velocity. It is measured in kilogram metres per second (kg m/s).

动量是一个矢量，定义为物体质量与速度的乘积，单位为千克米每秒 (kg m/s)。

p = m v

The larger the mass or the velocity, the greater the momentum. An object at rest has zero momentum.

质量或速度越大，动量越大。静止的物体动量为零。

12. Conservation of Momentum | 动量守恒

In a closed system (no external forces), the total momentum before a collision or explosion equals the total momentum after the event. This principle allows us to calculate unknown velocities.

在一个封闭系统（无外力）中，碰撞或爆炸前的总动量等于事件后的总动量。这条原理使我们能够计算未知的速度。

For example, when two trolleys collide and stick together (inelastic collision): m₁ u₁ + m₂ u₂ = (m₁ + m₂) v. For an explosion where a stationary object breaks apart: 0 = m₁ v₁ + m₂ v₂.

例如，两辆小车相撞并粘在一起（非弹性碰撞）：m₁ u₁ + m₂ u₂ = (m₁ + m₂) v。而对于一个静止物体爆炸分离的情况：0 = m₁ v₁ + m₂ v₂。

Momentum is also conserved in elastic collisions where kinetic energy is conserved as well, though GCSE usually focuses on momentum conservation only.

在弹性碰撞中动量也守恒，同时动能也守恒，不过 GCSE 通常只关注动量守恒。

Published by TutorHao | Physics Revision Series | aleveler.com

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GCSE WJEC Physics: Kinetics – Key Revision Points | GCSE WJEC 物理：动力学 考点精讲