📚 Forces and Motion: KS3 Physics | 力与运动：KS3 物理
Forces are pushes or pulls that can change the shape, speed or direction of an object. In KS3 Physics, we explore how forces act around us every day, from a book resting on a table to a rocket launching into space.
力是能够改变物体形状、速度或方向的推或拉。在KS3物理中，我们探索力如何在日常生活中起作用，从放在桌上的书到发射升空的火箭。
1. What is a Force? | 什么是力？
A force is a push or a pull acting on an object. Forces can be contact forces, where objects touch, or non-contact forces, where objects do not touch. The unit of force is the newton (N).
力是作用在物体上的推或拉。力可以是物体相互接触的接触力，也可以是物体不接触的非接触力。力的单位是牛顿（N）。
Forces can change an object’s speed, direction or shape. When you kick a ball, the force of your foot changes the ball’s motion and deforms it slightly.
力可以改变物体的速度、方向或形状。当你踢球时，脚施加的力改变了球的运动并使其轻微变形。
2. Types of Forces | 力的类型
Common forces include gravity, friction, air resistance, tension, normal contact force, magnetic force and electrostatic force.
常见的力包括重力、摩擦力、空气阻力、张力、法向接触力、磁力和静电力。
Gravity pulls objects towards the centre of the Earth. Friction acts between two surfaces that slide past each other, while air resistance acts on objects moving through air.
重力将物体拉向地球中心。摩擦力作用在两个相互滑动的表面之间，而空气阻力作用在空气中运动的物体上。
| Force (力) | Description (描述) | Example (例子) |
|---|---|---|
| Gravity (重力) | Attracts objects towards each other (吸引物体彼此靠近) | An apple falling from a tree (苹果从树上掉落) |
| Friction (摩擦力) | Opposes motion between two surfaces (阻碍两个表面之间的运动) | Brakes slowing a bicycle (刹车使自行车减速) |
| Air resistance (空气阻力) | Friction from air particles (空气粒子产生的摩擦) | A parachute slowing a skydiver (降落伞使跳伞者减速) |
3. Balanced and Unbalanced Forces | 平衡力与不平衡力
When forces on an object are equal in size and opposite in direction, they are balanced. Balanced forces do not change the motion of the object; it remains at rest or continues at constant speed.
当作用在物体上的力大小相等、方向相反时，它们就是平衡力。平衡力不会改变物体的运动状态；物体会保持静止或继续匀速运动。
Unbalanced forces cause an object to speed up, slow down or change direction. The resultant force is the overall force left after combining all forces acting on the object.
不平衡力会使物体加速、减速或改变方向。合力是所有作用在物体上的力合成后剩余的总体力。
4. Resultant Force | 合力
The resultant force is the single force that has the same effect as all the forces acting on an object. If forces act along the same straight line, add them if they point in the same direction, or subtract them if they point in opposite directions.
合力是一个单独的力，其作用效果与作用在物体上所有力相同。如果力沿同一直线作用，方向相同则相加，方向相反则相减。
F_resultant = F₁ + F₂ (same direction)
F_resultant = F₁ − F₂ (opposite direction)
For example, if a 10 N force pushes right and a 4 N force pushes left, the resultant force is 6 N to the right.
例如，如果一个10 N的力向右推，一个4 N的力向左推，合力为向右6 N。
5. Newton’s First Law | 牛顿第一定律
Newton’s First Law states that an object remains at rest or moves at a constant speed in a straight line unless acted on by an unbalanced force.
牛顿第一定律指出，除非受到不平衡力的作用，否则物体保持静止或沿直线匀速运动。
This is why a passenger moves forward when a car brakes suddenly: the body continues moving forward until a force, such as a seatbelt, stops it.
这就是为什么汽车突然刹车时乘客会向前倾：身体继续向前运动，直到有安全带等力使其停止。
6. Speed and Velocity | 速率与速度
Speed tells us how fast an object is moving. It is the distance travelled divided by the time taken. Speed is a scalar quantity, while velocity is a vector that includes direction.
速率告诉我们物体运动有多快。它等于通过的距离除以所用的时间。速率是标量，而速度是包含方向的矢量。
v = d ÷ t
where v is speed in metres per second (m/s), d is distance in metres (m), and t is time in seconds (s). A car covering 100 metres in 5 seconds has a speed of 20 m/s.
其中 v 是速率，单位为米/秒（m/s）；d 是距离，单位为米（m）；t 是时间，单位为秒（s）。一辆汽车在5秒内行驶100米，其速率为20 m/s。
7. Distance-Time Graphs | 距离-时间图
A distance-time graph shows how far an object has travelled over time. A straight sloping line represents constant speed; a horizontal line represents rest; a steeper slope means faster movement.
距离-时间图显示物体随时间通过的距离。倾斜直线代表匀速运动；水平线代表静止；斜率越大表示运动越快。
The gradient of a distance-time graph is equal to the speed of the object. A curved line means the speed is changing, so the object is accelerating or decelerating.
距离-时间图的斜率等于物体的速率。曲线表示速率在变化，因此物体正在加速或减速。
8. Friction and Air Resistance | 摩擦与空气阻力
Friction is a force that opposes motion between two surfaces. Air resistance is a type of friction that acts on objects moving through air.
摩擦力是阻碍两个表面相对运动的力。空气阻力是物体在空气中运动时受到的摩擦阻力。
Lubricants such as oil reduce friction, while streamlined shapes reduce air resistance. This helps vehicles move more efficiently and saves energy.
油等润滑剂可以减少摩擦，而流线型外形可以减少空气阻力。这有助于车辆更高效地行驶并节省能源。
9. Gravity and Weight | 重力与重量
Gravity is a non-contact force that attracts objects to each other. Near the Earth’s surface, every kilogram of mass experiences a gravitational force of about 10 newtons per kilogram.
重力是一种吸引物体的非接触力。在地球表面附近，每千克质量受到约10牛顿每千克的重力。
W = m × g
where W is weight in newtons (N), m is mass in kilograms (kg), and g = 10 N/kg on Earth. Weight is a force, measured in newtons, while mass is the amount of matter, measured in kilograms.
其中 W 是重量，单位为牛顿（N）；m 是质量，单位为千克（kg）；地球上 g = 10 N/kg。重量是力，以牛顿为单位；而质量是物质的多少，以千克为单位。
10. Pressure | 压强
Pressure is the force acting per unit area. It is measured in pascals (Pa), where 1 Pa = 1 N/m².
压强是单位面积上作用的力，单位为帕斯卡（Pa），其中 1 Pa = 1 N/m²。
P = F ÷ A
where P is pressure in pascals (Pa), F is force in newtons (N), and A is area in square metres (m²). A sharp knife cuts better than a blunt one because the same force acts over a smaller area, creating higher pressure.
其中 P 是压强，单位为帕斯卡（Pa）；F 是力，单位为牛顿（N）；A 是面积，单位为平方米（m²）。锋利的刀比钝刀更容易切割，因为相同的力作用在更小的面积上，产生更大的压强。
11. Moments and Levers | 力矩与杠杆
A moment is the turning effect of a force. It is calculated by multiplying the force by the perpendicular distance from the pivot.
力矩是力的转动效果。它等于力乘以力到支点的垂直距离。
M = F × d
where M is moment in newton metres (N·m), F is force in newtons (N), and d is distance in metres (m). Levers use moments to make work easier; a longer distance from the pivot means a smaller force is needed to produce the same moment.
其中 M 是力矩，单位为牛米（N·m）；F 是力，单位为牛顿（N）；d 是距离，单位为米（m）。杠杆利用力矩使工作更省力；离支点越远，产生相同力矩所需的力越小。
12. Forces in Everyday Life | 日常生活中的力
Understanding forces helps us design safer cars, build stronger bridges and improve sports performance. Engineers use ideas such as friction, gravity and moments every day.
理解力有助于我们设计更安全的汽车、建造更坚固的桥梁并提高运动表现。工程师每天都会运用摩擦、重力和力矩等概念。
From the grip of your shoes on the ground to the flight of a football, forces explain how and why objects move. Learning to describe them clearly is the first step in physics.
从鞋子在地面上的抓地力到足球的飞行，力解释了物体如何以及为何运动。学会清楚地描述它们，是学习物理的第一步。
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