Friction | 摩擦力 - aleveler.com

📚 Friction | 摩擦力

Friction is a contact force that opposes the relative motion between two surfaces. It acts along the surface and always tries to prevent sliding. In KS3 science, understanding friction helps explain why objects slow down, how brakes work, and why rough surfaces grip better than smooth ones.

摩擦力是一种接触力，它阻碍两个表面之间的相对运动。摩擦力沿接触面作用，总是试图阻止滑动。在 KS3 科学课程中，理解摩擦有助于解释物体为什么会减速、刹车如何工作，以及为什么粗糙表面比光滑表面更有抓地力。

1. What Is Friction? | 什么是摩擦力？

Friction is a force that appears when two surfaces slide or try to slide past each other. It is measured in newtons (N) and always acts parallel to the contact surface. Friction only exists as a response to an applied force or to motion.

摩擦力是当两个表面相互滑动或试图滑动时产生的一种力。它的单位是牛顿（N），方向总是平行于接触面。摩擦只是对施加的力或运动作出响应，不会凭空产生。

Friction is a contact force, which means the two objects must touch for friction to act. It is different from non-contact forces such as gravity and magnetism. The rougher or more pressed the surfaces are, the stronger the friction tends to be.

摩擦力是一种接触力，这意味着两个物体必须接触才会产生摩擦。它不同于重力和磁力等非接触力。表面越粗糙或压得越紧，摩擦力往往就越大。

2. Direction of Friction | 摩擦力的方向

Friction always acts in the opposite direction to the motion or attempted motion of an object. If a book is pushed to the right across a table, friction acts to the left. If an object is pulled downwards by gravity on a slope, friction acts upwards along the slope.

摩擦力的方向总是与物体的运动方向或运动趋势方向相反。如果把一本书在桌面上向右推，摩擦力就向左作用。如果物体在斜面上因重力向下滑，摩擦力就沿斜面向上作用。

Even when an object is not moving, friction can still oppose the applied force. This is why a stationery box stays still when you push it gently: static friction balances your push, so there is no resultant force and no movement.

即使物体没有移动，摩擦仍然可以抵抗所施加的力。这就是为什么轻轻推一个静止的箱子时它不动：静摩擦力与推力平衡，因此合力为零，物体不运动。

3. Measuring Friction | 如何测量摩擦力

To measure the friction force on a block, attach a spring balance or newton meter to the block and pull it steadily across a surface. When the block moves at a constant speed, the reading on the meter equals the friction force. This is because the forces are balanced.

要测量木块所受的摩擦力，可以在木块上系一个弹簧测力计或牛顿计，然后沿表面匀速拉动木块。当木块匀速运动时，测力计的读数就等于摩擦力。这是因为此时力相互平衡。

If the block accelerates, the pulling force is greater than friction. If it does not move at all, the reading shows the static friction, which may be less than the maximum possible static friction. Constant speed is therefore the key condition for an accurate measurement.

如果木块加速，拉力就大于摩擦力。如果木块完全不动，读数显示的是静摩擦力，它可能小于最大静摩擦力。因此，匀速运动是准确测量摩擦力的关键条件。

4. Factors That Affect Friction | 影响摩擦大小的因素

Three main factors affect the size of the friction force: surface roughness, the normal force pressing the surfaces together, and the materials involved. Rougher surfaces have more interlocking bumps, so they produce more friction. A heavier object presses the surfaces together more strongly, increasing friction.

影响摩擦力大小的主要因素有三个：表面粗糙程度、把表面压在一起的法向力以及材料种类。表面越粗糙，凸起相互咬合得越多，因此摩擦力越大。物体越重，表面被压得越紧，摩擦力也会增大。

The relationship can be written as F = μN, where F is the friction force, μ is the coefficient of friction for the two surfaces, and N is the normal contact force. Doubling the normal force roughly doubles the friction if the surfaces do not change.

这种关系可以写成 F = μN，其中 F 是摩擦力，μ 是两种表面之间的摩擦系数，N 是法向接触力。如果表面不变，法向力加倍，摩擦力也大约加倍。

Factor	Effect on Friction
Rougher surface	Increases friction
Smoother surface	Decreases friction
Greater normal force	Increases friction
Lubricant added	Decreases friction

中文对照：表面越粗糙，摩擦力越大；表面越光滑，摩擦力越小；法向力越大，摩擦力越大；加入润滑剂后，摩擦力减小。

5. Static Friction and Sliding Friction | 静摩擦与滑动摩擦

Static friction acts on an object that is not moving, preventing it from starting to slide. Sliding friction, also called kinetic friction, acts once the object is already moving. Static friction is usually slightly greater than sliding friction for the same two surfaces.

静摩擦力作用在静止的物体上，阻止它开始滑动。滑动摩擦，也叫动摩擦，在物体已经开始运动时起作用。对于相同的两个表面，静摩擦力通常略大于滑动摩擦力。

This explains why it often feels harder to push a heavy box from rest than to keep it moving. Once the box starts moving, the surfaces do not have time to settle into every tiny bump, so the friction drops slightly.

这解释了为什么把重箱子从静止推起来往往比继续推着它走更费力。箱子一旦开始运动，表面来不及嵌入每一个微小凸起，所以摩擦力会略微下降。

6. Friction Produces Heat | 摩擦与生热

Friction transfers useful kinetic energy into thermal energy. When you rub your hands together, the work done against friction warms your skin. The same effect makes car brake discs hot after braking and can cause sparks when metal is ground.

摩擦将有用的动能转化为热能。搓手时，克服摩擦做的功使皮肤变暖。同样的效应会让汽车刹车盘在制动后发烫，也会在金属打磨时产生火花。

This energy transfer is why machines with moving parts need cooling or lubrication. If too much thermal energy builds up, parts can expand, wear out, or even melt. In some cases, such as starting a fire with wood, useful friction heat is actually desirable.

这种能量转移说明了为什么带有运动部件的机器需要冷却或润滑。如果热能积累过多，部件可能膨胀、磨损甚至熔化。在某些情况下，例如用木头摩擦生火，有用的摩擦热反而是人们想要的。

7. Useful Effects of Friction | 有益的摩擦

Friction is essential for walking. When you push backwards on the ground, friction from the ground pushes you forwards. Without friction, your feet would slip and you could not move forwards. Tyres also rely on friction to grip the road and accelerate, brake, or turn safely.

摩擦力对行走至关重要。当你向后蹬地时，地面的摩擦力把你向前推。没有摩擦，脚会打滑，人就无法向前移动。轮胎也依靠摩擦抓住路面，才能安全地加速、刹车或转弯。

Pencils use friction to leave graphite marks on paper, and matches use friction to ignite. Brakes use friction between brake pads and wheels to slow a bicycle or car. Even holding a glass or tying shoelaces depends on friction.

铅笔依靠摩擦在纸上留下石墨痕迹，火柴依靠摩擦点燃。刹车利用刹车片与车轮之间的摩擦使自行车或汽车减速。甚至握住杯子、系鞋带也离不开摩擦。

8. Reducing Friction | 减小摩擦

Friction can be reduced by using lubricants such as oil or grease. A lubricant fills the tiny gaps between surfaces and separates them with a thin film, so the surfaces slide over the lubricant rather than directly over each other. Polishing surfaces also reduces interlocking.

使用油或润滑脂等润滑剂可以减小摩擦。润滑剂填满表面之间的微小空隙，并用一层薄膜把它们隔开，使表面在润滑剂上滑动，而不是直接相互摩擦。抛光表面也能减少相互咬合。

Ball bearings and rollers replace sliding with rolling. Rolling friction is usually much smaller than sliding friction, which is why wheeled suitcases, trolleys, and skateboards are easier to move. Streamlining reduces fluid friction in air or water.

滚珠轴承和滚轮用滚动代替滑动。滚动摩擦通常远小于滑动摩擦，这就是为什么带轮的行李箱、手推车和滑板更容易移动。流线型设计则可以减小空气或水中的流体摩擦。

9. Increasing Friction | 增大摩擦

Sometimes we need more friction for safety or control. Tyre treads and shoe soles have deep patterns to channel water away and increase grip on wet roads. Chalk or grip powder on hands absorbs sweat and gives athletes better contact with equipment.

有时为了安全或控制，我们需要更大的摩擦力。轮胎花纹和鞋底纹路可以排开积水，增加湿滑路面上的抓地力。手上的滑石粉或防滑粉能吸收汗水，让运动员与器材接触得更牢。

Sand or grit is spread on icy paths to increase friction and prevent slipping. Rough tapes on stairs, textured handles, and rubber grips all use the same principle: increasing surface roughness or contact area improves grip and reduces accidents.

在结冰的路面上撒沙子或沙砾可以增大摩擦并防止滑倒。楼梯上的防滑带、带纹理的把手和橡胶握把都使用相同的原理：增加表面粗糙度或接触面积，从而增强抓地力、减少事故。

10. Air Resistance and Water Resistance | 空气阻力和水阻力

Fluid friction is the force a liquid or gas exerts on an object moving through it. Air resistance is a type of friction that opposes objects moving through air, while water resistance opposes motion through water. Both are types of drag.

流体摩擦是液体或气体对在其中运动的物体施加的力。空气阻力是一种阻碍物体在空气中运动的摩擦，水阻力则阻碍物体在水中运动。两者都属于阻力。

Drag increases with speed and with the cross-sectional area facing the flow. A parachute creates a huge air resistance because of its large area, slowing a falling person. A streamlined shape reduces drag by allowing fluid to flow smoothly around the object.

阻力随速度增大和迎流截面积增大而增大。降落伞因面积很大而产生巨大空气阻力，使下落的人减速。流线型外形使流体能够平顺地绕过物体，从而减小阻力。

11. Friction and Stopping Distances | 摩擦与刹车距离

A moving car needs friction between the tyres and the road to stop. When the brakes are applied, friction in the brakes slows the wheels, and friction between tyres and road stops the car. On wet or icy roads, friction is reduced, so the stopping distance becomes much longer.

行驶中的汽车需要轮胎与路面之间的摩擦才能停下来。刹车时，制动器内的摩擦使车轮减速，轮胎与路面之间的摩擦使汽车停下。在湿滑或结冰的路面上，摩擦力减小，刹车距离会大大增加。

Tyre tread patterns pump water away from the contact patch, helping the rubber touch the road. Drivers must leave a larger gap in wet weather because the reduced friction means the car cannot decelerate as quickly as on a dry road.

轮胎花纹会把水从接触面排走，帮助橡胶接触路面。在潮湿天气，驾驶员必须保持更大的车距，因为摩擦力减小意味着汽车不能像在干燥路面上那样迅速减速。

12. Quick Check: Friction in Action | 快速测验：摩擦现象

Question 1: Why does a block pulled at constant speed show a spring balance reading equal to friction?

问题 1：为什么匀速拉木块时，弹簧测力计的读数等于摩擦力？

Answer: At constant speed the forces are balanced, so the pulling force equals the friction force. If the block accelerated, the pulling force would be greater than friction.

答案：匀速运动时力处于平衡状态，所以拉力等于摩擦力。如果木块加速，拉力就会大于摩擦力。

Question 2: Why do we put sand on an icy path?

问题 2：为什么要在结冰的小路上撒沙子？

Answer: Sand increases surface roughness, which increases friction and reduces the risk of slipping.

答案：沙子增加了表面粗糙度，从而增大摩擦力并降低滑倒的风险。

Question 3: Name one useful effect and one harmful effect of friction.

问题 3：举出一个摩擦的有益作用和有害作用。

Answer: Useful: walking or braking a car. Harmful: wearing out shoe soles or heating machine parts, wasting energy.

答案：有益作用：走路或汽车刹车。有害作用：磨损鞋底或使机器部件发热，浪费能量。

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Friction | 摩擦力