📚 IGCSE WJEC Science: Forces and Motion Key Points | IGCSE WJEC 科学：力与运动 考点精讲
Welcome to this focused revision guide on Forces and Motion for the WJEC IGCSE Science syllabus. Understanding how objects move and interact through forces is fundamental to physics. We will break down the key concepts, definitions, graphs, and equations you need to know for your exam.
欢迎阅读这篇针对 WJEC IGCSE 科学大纲的力与运动考点精讲。理解物体如何运动以及如何通过力相互作用是物理学的基础。我们将分解你需要掌握的考试关键概念、定义、图表和方程式。
1. Scalars and Vectors | 标量与矢量
A scalar is a physical quantity that has magnitude (size) only, such as speed, distance, mass, and time. A vector is a quantity that has both magnitude and direction, such as velocity, displacement, force, and acceleration.
标量是只有大小（量值）的物理量，例如速率、路程、质量和时间。矢量是既有大小又有方向的量，例如速度、位移、力和加速度。
When adding vectors, you must consider their directions. For parallel vectors, add or subtract; for perpendicular vectors, use Pythagoras’ theorem or scale diagrams.
当矢量相加时，必须考虑它们的方向。平行矢量可相加或相减；垂直矢量需要使用勾股定理或比例图。
2. Speed and Velocity | 速度与速率
Speed is the distance travelled per unit time. It is a scalar. The formula is: average speed = total distance ÷ total time.
速率是单位时间内行进的路程，是标量。公式为：平均速率 = 总路程 ÷ 总时间。
Velocity is the speed in a given direction. It is a vector. The magnitude of velocity can be calculated as displacement ÷ time.
速度是给定方向上的速率，是矢量。速度的大小可由位移除以时间计算。
v = s ÷ t
v = s ÷ t
In exams, remember that velocity can be negative if the object moves in the opposite direction to the defined positive.
在考试中，如果物体运动方向与定义的正方向相反，速度可以为负。
3. Acceleration | 加速度
Acceleration is the rate of change of velocity. It is a vector quantity measured in m/s².
加速度是速度的变化率，是矢量，单位为 m/s²。
a = (v − u) ÷ t
a = (v − u) ÷ t
If an object slows down, it experiences negative acceleration or deceleration. Uniform acceleration means constant rate of change of velocity.
如果物体减速，则经历负加速度或减速度。匀加速度表示速度变化率恒定。
4. Distance-Time Graphs | 距离-时间图
A distance-time graph shows how distance changes with time. The gradient (slope) of the line represents speed.
距离-时间图展示距离随时间的变化。线的斜率代表速率。
A flat horizontal line means the object is stationary. A straight sloping line indicates constant speed. A curved line means changing speed, hence acceleration.
水平直线表示物体静止。倾斜直线表示恒定速率。曲线表示速率在变化，即存在加速度。
The steeper the line, the greater the speed. A curve with increasing gradient indicates acceleration. To find speed at a point on a curve, draw a tangent and calculate its gradient.
线越陡，速率越大。梯度增大的曲线表示加速度。要找出曲线上某点的速率，可画切线并计算其斜率。
5. Velocity-Time Graphs | 速度-时间图
A velocity-time graph illustrates how velocity changes over time. The gradient of the line gives acceleration.
速度-时间图展示速度随时间的变化。线的斜率给出加速度。
The area under the graph between two times represents the displacement (distance in a straight line with direction). A horizontal line indicates constant velocity; a sloping straight line shows uniform acceleration. For changing acceleration, the graph is curved and the gradient changes.
两个时间点之间曲线下的面积代表位移（有方向的直线距离）。水平线表示恒定速度；倾斜直线表示匀加速度。对于变化的加速度，图形是弯曲的，斜率随之改变。
If the graph falls below the time axis, the object is moving in the opposite direction. The total area gives net displacement.
若图像落到时间轴以下，表示物体反向运动。总面积表示净位移。
6. Equations of Motion | 运动方程
For uniform acceleration, you can use the following equations (provided in the exam but must be understood). They link u (initial velocity), v (final velocity), a (acceleration), t (time) and s (displacement).
对于匀加速度，可以使用以下方程（考试提供，但必须理解）。它们联系了 u（初速度）、v（末速度）、a（加速度）、t（时间）和 s（位移）。
v = u + a t
v² = u² + 2 a s
s = u t + ½ a t²
The first equation, v = u + at, relates final velocity to initial velocity, acceleration, and time.
第一个方程 v = u + at 建立了末速度与初速度、加速度和时间的关系。
The second, v² = u² + 2as, allows calculation of final velocity without knowing time.
第二个方程 v² = u² + 2as 可以在不知道时间的情况下计算末速度。
The third, s = ut + ½at², gives displacement when initial velocity, acceleration and time are known.
第三个方程 s = ut + ½at² 当已知初速度、加速度和时间时，可计算位移。
7. Forces and Free-Body Diagrams | 力与受力图
A force is a push or pull that can change an object’s shape, speed, or direction. It is a vector measured in newtons (N).
力是能改变物体形状、速度或方向的推或拉作用，是矢量，单位为牛顿 (N)。
Free-body diagrams show all the forces acting on a single object using arrows. The length of each arrow represents the magnitude, and the direction shows the direction of the force.
受力图用箭头表示作用在单个物体上的所有力。箭头长度代表大小，方向表示力的方向。
Examples of forces: weight (downwards), normal contact force (perpendicular to surface), tension, friction, air resistance, thrust. If an object is in equilibrium, the forces are balanced and the resultant force is zero.
力的例子：重力（向下）、法向接触力（垂直于表面）、张力、摩擦力、空气阻力、推力。如果物体处于平衡状态，则力相互平衡，合力为零。
8. Newton’s Laws of Motion | 牛顿运动定律
Newton’s First Law: An object remains at rest or moves with constant velocity unless acted upon by a resultant (net) force.
牛顿第一定律：除非受到合力作用，否则物体保持静止或匀速直线运动状态。
Newton’s Second Law: The acceleration of an object is directly proportional to the resultant force and inversely proportional to its mass. F = m × a.
牛顿第二定律：物体的加速度与合力成正比，与质量成反比。F = m × a。
F = m a
F = m a
Newton’s Third Law: When two objects interact, they exert equal and opposite forces on each other. Action-reaction pairs act on different objects.
牛顿第三定律：当两个物体相互作用时，它们彼此施加大小相等、方向相反的力。作用力与反作用力作用在不同物体上。
9. Mass, Weight and Gravity | 质量、重量与重力Published by TutorHao | IGCSE Science Revision Series | aleveler.com
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