📚 Forces and Motion | 力与运动
Understanding how objects move and why they move the way they do is a central theme of Cambridge Lower Secondary Science Stage 9. In this article, we will explore the key concepts of motion, including speed, velocity, acceleration, and Newton’s laws of motion. We will also look at how forces such as friction and air resistance affect movement, and how we can represent motion using graphs. Through clear explanations and practical examples, you will build a solid foundation in forces and motion.
理解物体如何运动以及为何如此运动，是剑桥初中科学第9阶段的核心主题。在本文中，我们将探讨运动的关键概念，包括速率、速度、加速度以及牛顿运动定律。我们还会考察摩擦力和空气阻力等力如何影响运动，以及如何用图像表示运动。通过清晰的解释和实际例子，你将建立起牢固的力与运动基础。
1. Describing Motion | 描述运动
To describe motion, we need a reference point. The position of an object is described relative to a fixed point. The total path length covered is called distance, which is a scalar quantity – it has magnitude only. Displacement, on the other hand, is a vector quantity; it measures the shortest distance from the starting point to the final position in a specific direction.
要描述运动，我们需要一个参照点。物体的位置是相对于一个固定点来描述的。所经过的路径总长度叫做距离，它是一个标量——只有大小。而位移是一个矢量；它测量从起点到最终位置的最短距离，并带有方向。
Speed is a scalar quantity that tells us how fast an object is moving, while velocity is a vector that includes the direction of motion. For example, if a car travels 100 km north in 2 hours, its speed is 50 km/h, but its velocity is 50 km/h north.
速率是标量，告诉我们物体运动有多快，而速度是矢量，包含运动的方向。例如，如果一辆汽车在2小时内向北行驶了100公里，它的速率是50公里/小时，但它的速度是向北50公里/小时。
2. Speed and Velocity | 速率与速度
The average speed of an object is calculated using the equation:
average speed = total distance ÷ total time
物体的平均速率用以下公式计算：
平均速率 = 总距离 ÷ 总时间
In symbols, we often write v = s / t, where s is distance and t is time. The SI unit of speed is metres per second (m/s), but kilometres per hour (km/h) is also commonly used. To convert from km/h to m/s, divide by 3.6.
用符号表示，我们常写为 v = s / t，其中 s 是距离，t 是时间。速度的国际单位是米每秒（m/s），但公里每小时（km/h）也常用。要将 km/h 转换为 m/s，除以 3.6。
Velocity is similar but includes direction. When an object moves in a straight line without turning, the magnitude of its velocity equals its speed. However, in circular motion, the speed may be constant while the velocity continuously changes because the direction changes.
速度类似但包含方向。当物体沿直线运动不转弯时，其速度的大小等于速率。然而，在圆周运动中，速率可能恒定，但由于方向不断变化，速度在持续改变。
3. Acceleration | 加速度
Acceleration is the rate of change of velocity. It is a vector quantity. If an object speeds up, slows down or changes direction, it is accelerating. The equation for average acceleration is:
acceleration = change in velocity ÷ time taken
加速度是速度变化的速率。它是一个矢量。如果物体加速、减速或改变方向，它就在加速。平均加速度的公式为：
加速度 = 速度变化量 ÷ 所用时间
In symbols, a = (v – u) / t, where v is final velocity, u is initial velocity and t is time. The SI unit of acceleration is metres per second squared (m/s²). A negative acceleration (deceleration) means the object is slowing down.
用符号表示，a = (v – u) / t，其中 v 是末速度，u 是初速度，t 是时间。加速度的国际单位是米每二次方秒（m/s²）。负加速度（减速）意味着物体在减慢。
For example, if a cyclist increases velocity from 2 m/s to 8 m/s in 3 seconds, the acceleration is (8 – 2) / 3 = 2 m/s². This means the cyclist’s velocity increases by 2 m/s every second.
例如，如果一名自行车手在3秒内速度从2 m/s增加到8 m/s，加速度为 (8 – 2) / 3 = 2 m/s²。这意味着自行车手的速度每秒增加2 m/s。
4. Distance–Time Graphs | 距离–时间图
A distance–time graph shows how the distance of an object from a starting point changes over time. The gradient (slope) of the line represents the speed. A horizontal line indicates the object is stationary. A straight, sloping line shows constant speed, and a curved line shows acceleration or deceleration.
距离–时间图显示物体离起点的距离如何随时间变化。线的斜率代表速率。一条水平线表示物体静止不动。一条倾斜的直线表示匀速运动，而曲线表示加速或减速。
To calculate speed from a distance–time graph, pick two points on the line and divide the change in distance by the change in time. If the graph is curved, you can find the instantaneous speed by drawing a tangent and calculating its gradient.
要从距离–时间图计算速率，在线上选取两点，用距离的变化量除以时间的变化量。如果图是曲线，可以通过画一条切线并计算其斜率来求得瞬时速率。
5. Speed–Time Graphs | 速度–时间图
A speed–time graph (or velocity–time graph) displays how speed changes with time. The gradient of the line gives the acceleration. A horizontal line means constant speed; an upward sloping line indicates constant acceleration; a downward sloping line shows deceleration.
速度–时间图（或速率–时间图）显示速度如何随时间变化。线的斜率给出加速度。水平线表示速度恒定；向上的斜线表示匀加速；向下的斜线表示减速。
The area under the line of a speed–time graph represents the distance travelled. To find the total distance, calculate the area between the line and the time axis, splitting the shape into rectangles and triangles if necessary.
速度–时间图线下的面积代表行驶的距离。要求得总距离，计算线与时间轴之间的面积，必要时将形状拆分为矩形和三角形。
For instance, a car accelerates uniformly from rest to 20 m/s in 10 s, shown by a straight line. The area under this line is a triangle of base 10 s and height 20 m/s, giving an area of ½ × 10 × 20 = 100 m. So the car travels 100 m.
例如，一辆汽车从静止开始匀加速，10秒内速度达到20 m/s，用一条直线表示。此线下的面积是一个底为10 s、高为20 m/s的三角形，面积为 ½ × 10 × 20 = 100 m。因此汽车行驶了100米。
6. What is a Force? | 什么是力？
A force is a push or a pull that can change the shape, speed or direction of an object. Forces are vector quantities, meaning they have both magnitude and direction. The SI unit of force is the newton (N). A force of 1 N is roughly the weight of a small apple.
力是一种推或拉，可以改变物体的形状、速度或方向。力是矢量，意味着既有大小又有方向。力的国际单位是牛顿（N）。1 N的力大约是一个小苹果的重量。
Forces can be contact forces, like friction and tension, or non-contact forces, such as gravitational, magnetic and electrostatic forces. When more than one force acts on an object, they combine to produce a resultant (net) force.
力可以是接触力，如摩擦力和张力，也可以是非接触力，如引力、磁力和静电力。当多个力作用在一个物体上时，它们会合成一个合力（净力）。
7. Balanced and Unbalanced Forces | 平衡力与非平衡力
When the forces acting on an object are balanced, the resultant force is zero. This means the object will remain stationary or continue moving at a constant velocity. This is a direct consequence of Newton’s first law.
当作用在物体上的力平衡时，合力为零。这意味着物体将保持静止或继续以恒定速度运动。这是牛顿第一定律的直接结果。
When forces are unbalanced, there is a net force in a particular direction, causing the object to accelerate in that direction. The acceleration depends on the size of the net force and the mass of the object, as described by Newton’s second law.
当力不平衡时，就会在
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导