📚 OCR Science: Forces and Motion - Key Points Explained | OCR科学：力与运动考点精讲
Mastering forces and motion is essential for OCR GCSE Science. This revision guide summarises the key concepts, equations, and graphs you need to know, from scalars and vectors to momentum and braking.
掌握力与运动是 OCR GCSE 科学考试的关键。这份复习指南总结了必须掌握的核心概念、方程和图像，从标量和矢量到动量和制动。
1. Scalar and Vector Quantities | 标量与矢量
A scalar quantity has only magnitude (size), with no direction.
标量只有大小，没有方向。
Examples of scalar quantities include distance, speed, mass, time and energy.
标量的例子包括距离、速率、质量、时间和能量。
A vector quantity has both magnitude and direction.
矢量既有大小，又有方向。
Examples of vector quantities are displacement, velocity, acceleration, force and momentum.
矢量的例子有位移、速度、加速度、力和动量。
In calculations, vector directions are often indicated by positive and negative signs.
在计算中，矢量方向通常用正负号表示。
2. Distance, Displacement, Speed and Velocity | 距离、位移、速率与速度
Distance is a scalar measure of the total path length travelled. Displacement is a vector that describes the straight-line difference between the starting and finishing points, together with a direction.
距离是标量，度量运动路径的总长度。位移是矢量，描述从起点到终点的直线距离，并带有方向。
Speed is the rate of change of distance: speed = distance / time (scalar).
速率是距离的变化率：速率 = 距离 / 时间 (标量)。
Velocity is the rate of change of displacement: velocity = displacement / time (vector).
速度是位移的变化率：速度 = 位移 / 时间 (矢量)。
Average speed can be calculated using the total distance and total time. Instantaneous speed is the speed at a given moment.
平均速率可以用总距离和总时间计算。瞬时速率是某个时刻的速率。
Velocity can be expressed as:
v = s / t
where s represents displacement.
其中 s 表示位移。
3. Acceleration and Equations of Motion | 加速度与运动方程
Acceleration is the rate of change of velocity. It is a vector quantity and can be positive (speeding up) or negative (slowing down, often called deceleration).
加速度是速度的变化率。它是矢量，可以为正（加速）或负（减速，常称为减速度）。
Acceleration a is given by:
a = (v − u) / t
where u is initial velocity and v is final velocity.
其中 u 是初速度，v 是末速度。
Acceleration is measured in metres per second squared (m/s²).
加速度的单位是米每二次方秒 (m/s²)。
For uniform acceleration in a straight line, the SUVAT equations are used:
对于匀加速直线运动，可以使用以下 SUVAT 方程：
v = u + a t
s = u t + ½ a t²
v² = u² + 2 a s
In these equations, s represents displacement.
在这些方程中，s 表示位移。
4. Distance-Time and Velocity-Time Graphs | 距离-时间图与速度-时间图
A distance-time graph shows how distance changes with time. The gradient represents speed. A straight, sloping line means constant speed; a horizontal line means the object is stationary.
距离-时间图显示距离随时间的变化。斜率代表速率。倾斜直线表示匀速；水平线表示物体静止。
A steeper gradient indicates a higher speed. A curved line shows a changing speed (acceleration).
斜率越陡表示速率越高。曲线表示速率在变化（加速度）。
A velocity-time graph displays velocity over time. The gradient gives acceleration. A horizontal line indicates constant velocity; an upward sloping line indicates positive acceleration.
速度-时间图反映速度随时间的变化。斜率给出加速度。水平线表示匀速；向上倾斜的线表示正加速度。
The area under a velocity-time graph represents the displacement travelled. For a straight-line graph, this area is often calculated using rectangles and triangles.
速度-时间图下的面积表示位移。对于直线图，这个面积通常用矩形和三角形计算。
5. Newton's First Law and Inertia | 牛顿第一定律与惯性
Newton's First Law states that an object remains 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，除非受到外力作用，物体将保持静止或匀速直线运动状态。
This tendency to resist changes in motion is called inertia. Inertia depends on the mass of the object – the greater the mass, the greater the inertia.
这种抵抗运动状态变化的性质叫做惯性。惯性与物体的质量有关——质量越大，惯性越大。
If the resultant force on an object is zero, its velocity will not change.
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