📚 Forces and Motion | 力与运动
In this chapter you will learn about forces, how they act on objects, and how they cause motion. You will study balanced and unbalanced forces, friction, air resistance, gravity, mass and weight. You will also learn to calculate speed and interpret distance-time graphs.
在本章中，你将学习力、力如何作用于物体以及力如何引起运动。你将研究平衡力与不平衡力、摩擦力、空气阻力、重力、质量与重量。你还将学习计算速度并解释距离-时间图。
1. What Is a Force? | 什么是力？
A force is a push or a pull acting on an object. Forces can make an object start moving, speed up, slow down, change direction, or change shape. Some forces are contact forces, such as friction and tension, because they only act when objects touch. Other forces are non-contact forces, such as gravity, magnetic force, and electrostatic force, because they can act at a distance.
力是作用在物体上的推力或拉力。力可以使物体开始运动、加速、减速、改变方向或改变形状。有些力是接触力，例如摩擦力和张力，因为它们只在物体接触时才会作用。其他力是非接触力，例如重力、磁力和静电力，因为它们可以在一定距离外作用。
Force is a vector quantity, meaning it has both size and direction. The SI unit of force is the newton (N), named after the scientist Isaac Newton. When several forces act on an object, we can add them together to find the resultant force.
力是矢量，既有大小也有方向。力的国际单位是牛顿（N），以科学家艾萨克·牛顿的名字命名。当多个力作用在一个物体上时，我们可以把它们合成来求合力。
A force can change the shape of an object. For example, a spring stretches when it is pulled, and a ball squashes when it is kicked. Forces can also change the direction of a moving object, such as when a hockey stick changes the path of a ball.
力可以改变物体的形状。例如，弹簧被拉时会伸长，球被踢时会变形。力还可以改变运动物体的方向，例如曲棍球棍改变球运动的路径。
Forces are usually shown on diagrams as arrows. The length of the arrow represents the size of the force, and the arrowhead shows the direction in which the force acts. The point where the arrow starts shows where the force is applied.
力通常在图上用箭头表示。箭头的长度代表力的大小，箭头指向表示力的作用方向，箭头起点表示力的作用点。
2. Common Types of Forces | 常见的力类型
You need to recognise these common forces for Cambridge Science: gravity, friction, air resistance, upthrust, tension, normal contact force, magnetic force, and electrostatic force. Each type has a different origin and acts in a particular way.
在剑桥科学中你需要认识这些常见的力：重力、摩擦力、空气阻力、浮力、张力、法向接触力、磁力和静电力。每种类型的力来源不同，作用方式也不同。
| Force | Description | 力 | 说明 |
|---|---|---|---|
| Gravity | Pulls objects towards the centre of the Earth; gives an object its weight | 重力 | 将物体拉向地球中心；使物体具有重量 |
| Friction | Opposes motion between two surfaces in contact | 摩擦力 | 阻碍接触面之间的相对运动 |
| Air resistance | A drag force that opposes motion through the air | 空气阻力 | 阻碍物体在空气中运动的阻力 |
| Upthrust | An upward push from a liquid or gas on an object in it | 浮力 | 液体或气体对其中物体向上的托力 |
| Tension | A pulling force in a stretched rope, string or cable | 张力 | 被拉伸的绳、线或缆中产生的拉力 |
| Normal contact force | The push from a surface, acting at right angles to the surface | 法向接触力 | 表面施加的推力，方向垂直于该表面 |
| Magnetic force | A non-contact force between magnets or magnetic materials | 磁力 | 磁体或磁性材料之间的非接触力 |
| Electrostatic force | A non-contact force between electric charges; like charges repel, unlike charges attract | 静电力 | 电荷之间的非接触力；同种电荷相斥，异种电荷相吸 |
When you name a force in an answer, always be specific. Writing "the force of the surface" is weaker than writing "the normal contact force from the table on the book".
在答题时命名力一定要具体。写"表面的力"不如写"桌面对书的法向接触力"准确。
3. Contact and Non-contact Forces | 接触力与非接触力
A contact force only acts when two objects are touching. Friction, air resistance, upthrust, tension and the normal contact force are all contact forces. If the objects stop touching, the force disappears immediately.
接触力只在两个物体相互接触时才起作用。摩擦力、空气阻力、浮力、张力和法向接触力都是接触力。一旦物体不再接触，这些力立即消失。
A non-contact force acts even when the objects are not touching. Gravity, magnetic force and electrostatic force are non-contact forces. For example, a magnet can pull a steel paper clip across a gap, and a charged balloon can attract a thin stream of water without touching it.
非接触力即使在物体不接触时也能起作用。重力、磁力和静电力都是非接触力。例如，磁体可以隔着一段距离吸引钢制回形针，带电的气球可以不用接触就吸引细水流。
A useful test question is: "Is the force still there if the objects are separated?" If the answer is yes, the force is a non-contact force.
一个有用的判断题是："如果两个物体分开了，这个力还存在吗？"如果答案是"存在"，那么这就是非接触力。
4. Balanced and Unbalanced Forces | 平衡力与不平衡力
Forces acting on an object are balanced when the total force in one direction equals the total force in the opposite direction. The resultant force is then zero. A stationary object stays stationary, and a moving object keeps moving at a constant speed in a straight line.
当某个方向上的总力等于相反方向上的总力时，作用在物体上的力就是平衡的。此时合力为零。静止的物体保持静止，运动的物体会以恒定速度沿直线继续运动。
Forces are unbalanced when the total force in one direction is larger than in the other. The resultant force is not zero. An unbalanced force makes a stationary object start to move, makes a moving object speed up, slow down, or change direction.
当某个方向上的总力大于相反方向的力时，力就是不平衡的，合力不为零。不平衡的力会使静止的物体开始运动，使运动的物体加速、减速或改变方向。
- Balanced forces → resultant force = 0 N → no change in motion (平衡力 → 合力 = 0 N → 运动状态不变)
- alanced forces → resultant force ≠ 0 N → acceleration, deceleration or change of direction (不平衡力 → 合力 ≠ 0 N → 加速、减速或改变方向)
Typical examples of balanced forces include a book resting on a table and a car cruising at a steady speed on a straight road. In both cases the driving forces and the opposing forces cancel out.
平衡力的典型例子包括静止在桌上的书，以及在直路上匀速行驶的汽车。这两种情况下，驱动力与阻碍力相互抵消。
5. Calculating the Resultant Force | 计算合力
The resultant force is the single force that has the same effect as all the forces acting on the object added together. To find it, decide on a positive direction first, then add the forces as vectors.
合力是所有作用在物体上的力相加后效果相同的那一个力。求合力时，先规定一个正方向，然后按矢量相加。
- Forces in the same direction: add them together. (同方向的力：相加)
- Forces in opposite directions: subtract the smaller from the larger. (相反方向的力：用较大的力减去较小的力)
- The resultant force always acts in the direction of the larger force. (合力方向始终与较大的力相同)
Example: a box is pushed to the right with 30 N while friction acts to the left with 12 N.
例如：用 30 N 的力向右推箱子，同时摩擦力以 12 N 向左作用。
resultant force = 30 N − 12 N = 18 N to the right
Because the resultant force is not zero, the box accelerates to the right.
由于合力不为零，箱子向右加速。
6. Friction | 摩擦力
Friction is a contact force that acts between two surfaces in contact and always opposes relative motion. It acts in the opposite direction to the way the object is moving, or to the way it is trying to move.
摩擦力是作用在两个接触面之间的接触力，总是阻碍相对运动。它的方向与物体运动的方向相反，或与物体想要运动的方向相反。
Friction can be useful or a nuisance. It lets us walk without slipping, allows brakes to stop a bicycle, and lets car tyres grip the road. However, it also wears away surfaces and produces unwanted heat in engines.
摩擦力可能有用，也可能带来麻烦。它让我们走路时不会滑倒，让刹车能停下自行车，让汽车轮胎抓住路面。但它也会磨损表面，并在发动机中产生多余的热量。
- To increase friction: use rougher surfaces, add treads or grooves, use wider tyres. (增大摩擦：使用更粗糙的表面、增加花纹或沟槽、使用更宽的轮胎)
- To reduce friction: add a lubricant such as oil or grease, use smooth surfaces, use rollers or ball bearings, use a streamlined shape in air or water. (减小摩擦：加入润滑油或油脂、使用光滑表面、使用滚轮或滚珠轴承、在空气或水中采用流线型外形)
Friction also transfers energy. When you rub your hands together, kinetic energy is transferred to thermal energy and your hands feel warmer. This is why friction is described as a dissipative force.
摩擦还会传递能量。当你搓手时，动能转化为热能，手会变热。这就是摩擦力被称为耗散力的原因。
7. Air Resistance and Drag | 空气阻力与阻力
Air resistance is a type of friction that acts on an object moving through air. It is also called drag. It acts in the opposite direction to the motion and increases as the object moves faster.
空气阻力是一种摩擦力，作用在空气中运动的物体上，也称为阻力。它的方向与运动方向相反，并且随物体速度增大而增大。
Air resistance depends on the speed of the object, its shape and its cross-sectional area. Streamlined shapes, such as the body of a racing car or a cyclist leaning over the handlebars, reduce air resistance. A parachute works because its large area creates a very large air resistance force.
空气阻力取决于物体的速度、形状和横截面积。流线型外形（如赛车车身，或俯身握把的自行车手）可以减小空气阻力。降落伞之所以有效，是因为其巨大的面积产生了极大的空气阻力。
When a falling object is released, gravity is larger than air resistance, so the object accelerates. As it speeds up, air resistance increases until the two forces balance. The object then falls at a constant maximum speed, called terminal velocity.
物体刚开始下落时，重力大于空气阻力，所以物体会加速。随着速度增大，空气阻力增大，直到两个力平衡。此后物体以恒定的最大速度下落，这个速度称为终端速度。
8. Gravity, Mass and Weight | 重力、质量与重量
Mass is the amount of matter in an object, measured in kilograms (kg). Mass does not change when you move to a different place, so your mass on the Moon is the same as your mass on Earth.
质量是物体所含物质的多少，用千克（kg）测量。质量不会因为你到了不同地方而改变，所以你在月球上的质量与在地球上相同。
Weight is the force of gravity acting on an object, measured in newtons (N). Weight is a force, so it is a vector quantity. Weight changes with the strength of gravity, so you weigh less on the Moon than on Earth.
重量是重力作用在物体上的力，用牛顿（N）测量。重量是一种力，因此是矢量。重量随重力强度变化，所以你在月球上比在地球上轻。
weight = mass × gravitational field strength W = m × g
On Earth, g is about 9.8 N/kg, which is often rounded to 10 N/kg in calculations. On the Moon, g is about 1.6 N/kg, so the same object has a much smaller weight there.
在地球上，g 约为 9.8 N/kg，计算中常取 10 N/kg。在月球上，g 约为 1.6 N/kg，所以同一物体在那里的重量小得多。
Example: a pupil of mass 45 kg stands on the Earth.
例如：一名质量为 45 kg 的学生站在地球上。
W = m × g = 45 kg × 10 N/kg = 450 N
Gravity also keeps the planets in orbit around the Sun, causes the tides, and gives every object a downward pull towards the centre of the Earth.
重力还使行星围绕太阳运行，引起潮汐，并使每个物体都受到指向地球中心的向下拉力。
9. Calculating Speed | 速度的计算
Speed is how far an object travels in a given time. It is a scalar quantity, so it has size but no direction. Velocity is speed in a stated direction, so it is a vector quantity.
速度是物体在给定时间内运动的路程。速度（速率）是标量，只有大小没有方向。速度（速度矢量）是带有明确方向的速率，因此是矢量。
speed = distance ÷ time v = s ÷ t
The SI unit of speed is metres per second (m/s). Other common units are km/h and miles per hour. To convert m/s into km/h, multiply by 3.6; to convert km/h into m/s, divide by 3.6.
速度的国际单位是米每秒（m/s）。其他常用单位有 km/h 和英里每小时。把 m/s 换算为 km/h 要乘以 3.6；把 km/h 换算为 m/s 要除以 3.6。
Average speed is the total distance divided by the total time, even if the object moved at different speeds during the journey. Instantaneous speed is the speed at one particular moment, shown by a car's speedometer.
平均速度等于总路程除以总时间，即使物体在途中速度不断变化。瞬时速度是某一时刻的速度，由汽车的速度表显示。
Example: a runner covers 400 m in 50 s.
例如：一名跑步者在 50 s 内跑完 400 m。
v = s ÷ t = 400 m ÷ 50 s = 8 m/s
Always check that the distance is in metres and the time in seconds before you calculate, and always give the unit in your answer.
计算前一定要检查路程用米、时间用秒，并务必在答案中写出单位。
10. Distance-Time Graphs | 距离-时间图
A distance-time graph plots distance travelled on the vertical axis against time on the horizontal axis. The gradient (slope) of the line tells you the speed of the object.
距离-时间图把运动的路程画在纵轴上，时间画在横轴上。直线的斜率（梯度）告诉物体的速度。
- A horizontal line means the object is stationary. (水平线表示物体静止)
- A straight sloping line means constant speed. (倾斜直线表示匀速运动)
- A steeper line means a greater speed. (线越陡，速度越大)
- A curve getting steeper means the object is accelerating. (越来越陡的曲线表示物体在加速)
- A curve getting flatter means the object is decelerating. (越来越平的曲线表示物体在减速)
- A line sloping back towards the time axis means the object is returning to its starting point. (向时间轴方向回落的线表示物体返回起点)
speed = gradient = change in distance ÷ change in time
tween 20 s and 60 s, the speed is (500 m − 100 m) ÷ (60 s − 20 s) = 400 m ÷ 40 s = 10 m/s.
例如，若一名骑行者在 20 s 到 60 s 之间从 100 m 行驶到 500 m，则速度为（500 m − 100 m）÷（60 s − 20 s）= 400 m ÷ 40 s = 10 m/s。
11. Forces and Shape: Springs and Extension | 力与形状：弹簧与伸长
A force can change the shape of an object, and this change can be temporary or permanent. A spring, rubber band or elastic band returns to its original shape when the force is removed, so the change is elastic. A plastic ruler or a ball of clay stays bent or squashed, so the change is plastic.
力可以改变物体的形状，这种改变可以是暂时的，也可以是永久的。弹簧、橡皮筋或弹性带在力撤去后恢复原状，这种改变是弹性的。塑料尺或橡皮泥则会保持弯曲或被压扁的状态，这种改变是塑性的。
For a spring, the extension is the increase in length compared with its original length. The extension is found by subtracting the original length from the new length.
对弹簧而言，伸长量是与其原长相比增加的长度。伸长量等于新长度减去原长。
extension = new length − original length
For many springs, the extension is directly proportional to the force applied, as long as the elastic limit is not exceeded. This relationship is called Hooke's law, and it produces a straight-line graph through the origin when force is plotted against extension.
对许多弹簧来说，只要不超过弹性限度，伸长量与所加的力成正比。这一关系称为胡克定律，当以力对伸长量作图时会得到一条过原点的直线。
If the force is too large, the spring passes its elastic limit. It will no longer return to its original length, and the graph becomes curved.
如果力过大，弹簧会超过弹性限度。它将不再恢复原长，图像也会变成曲线。
12. Exam Tips and Common Mistakes | 考试技巧与常见错误
Most marks in forces questions are lost for small, avoidable reasons. Read the question carefully, decide which forces are acting, and always state the direction of a force as well as its size.
力学习题中丢分大多是因为一些可以避免的小错误。认真读题，判断有哪些力在作用，并且一定要同时写出力的大小和方向。
- Never write "the object moves because there is a force" without saying whether the forces are balanced. (不要只写"因为有所以物体运动"，要说明力是否平衡)
- Do not mix up mass (kg) and weight (N). (不要混淆质量 kg 与重量 N)
- Do not forget the unit, and do not write N/kg as N kg. (不要漏掉单位，也不要把 N/kg 写成 N kg)
- In speed calculations, convert units before calculating. (速度计算前先统一单位)
- On distance-time graphs, the gradient is speed, not the height of the line. (在距离-时间图上，斜率才是速度，不是线的高度)
- When describing a graph, quote figures from the axes to support your answer. (描述图像时，引用坐标轴上的数据来支持你的答案)
A good habit is to draw a quick free-body diagram before you start any calculation. Mark every force with an arrow, label the size in newtons, then work out the resultant force step by step.
一个好习惯是在开始任何计算前先快速画一个受力图。用箭头标出每一个力，用牛顿标注大小，然后逐步求出合力。
Finally, remember the key idea of this whole chapter: a force does not cause motion by itself, but an unbalanced force causes a change in motion. Balanced forces mean no change at all.
最后，记住整章的核心思想：力本身并不会产生运动，但不平衡的力会引起运动状态的变化。平衡力意味着完全没有变化。
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