IGCSE Physics: Force and Changes in Motion | IGCSE物理：力与运动状态的改变

📚 IGCSE Physics: Force and Changes in Motion | IGCSE物理：力与运动状态的改变

In IGCSE Physics, one of the most important ideas is that a force can change the state of motion of an object. Whether an object stays at rest, speeds up, slows down, or changes direction, these changes are all caused by forces. In this article, we will explore how forces affect motion, the three laws of motion, momentum, and real-life applications such as car safety.

在IGCSE物理中，一个最重要的观点是力可以改变物体的运动状态。无论是物体保持静止、加速、减速还是改变方向，这些变化都是由力引起的。在本文中，我们将探讨力如何影响运动、牛顿三大运动定律、动量以及汽车安全等实际应用。

1. What is a Force? | 什么是力？

A force is a push or a pull acting on an object. It is a vector quantity, so it has both magnitude and direction. The SI unit of force is the newton (N).

力是作用在物体上的推或拉。它是一个矢量，既有大小也有方向。力的SI单位是牛顿（N）。

Forces can change the speed, direction, or shape of an object. For example, a football is accelerated when a player kicks it, and a rubber band changes its shape when stretched.

力可以改变物体的速度、方向或形状。例如，球员踢球时足球加速，橡皮筋被拉伸时改变形状。

Effects of forces:

力的作用效果：

• Change the speed of an object / 改变物体的速度
• Change the direction of motion / 改变运动方向
• Change the shape of an object / 改变物体的形状

A force is measured with a spring balance or a force meter. The extension of the spring is used as a measure of the force, based on Hooke’s law for elastic materials.

力可以用弹簧测力计或测力计测量。根据弹性材料的胡克定律，弹簧的伸长量被用作力的量度。

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2. Balanced and Unbalanced Forces | 平衡力与不平衡力

When several forces act on an object, they combine into a resultant (net) force. If the resultant force is zero, the forces are balanced.

当多个力作用在一个物体上时，它们合成为一个合力（净力）。如果合力为零，则这些力是平衡的。

Newton’s first law is closely related: an object with balanced forces has no acceleration; it remains at rest or moves with constant velocity.

牛顿第一定律与此密切相关：受平衡力作用的物体没有加速度；它保持静止或做匀速直线运动。

When the resultant force is not zero, the forces are unbalanced. The object then accelerates in the direction of the resultant force.

当合力不为零时，力是不平衡的。物体随后在合力方向上加速。

For example, a car moving at a steady speed on a flat road has balanced forces: the forward engine thrust equals the total friction and air resistance.

例如，在平直道路上匀速行驶的汽车受到平衡力：发动机向前的推力等于总摩擦力和空气阻力。

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3. Newton’s First Law of Motion | 牛顿第一定律

Newton’s first law states: an object will remain at rest or continue to move in a straight line at constant velocity unless acted upon by an external resultant force.

牛顿第一定律指出：除非受到外合力作用，否则物体将保持静止或沿直线做匀速运动。

This property of matter is called inertia. Inertia is the reluctance of an object to change its state of motion.

物体的这种属性称为惯性。惯性是物体不愿改变其运动状态的特性。

Mass is a measure of inertia: the larger the mass, the harder it is to change its velocity.

质量是惯性大小的量度：质量越大，越难改变其速度。

Situation	Inertia effect
Car suddenly stops / 汽车突然停下	Passengers continue moving forward / 乘客继续向前运动
Car suddenly accelerates / 汽车突然加速	Passengers lurch backward / 乘客向后倾

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4. Newton’s Second Law of Motion | 牛顿第二定律

Newton’s second law states that the acceleration of an object is directly proportional to the resultant force and inversely proportional to its mass.

牛顿第二定律指出：物体的加速度与所受合力成正比，与物体的质量成反比。

F = ma

Where F is the resultant force in newtons (N), m is the mass in kilograms (kg), and a is the acceleration in metres per second squared (m/s²).

其中 F 是合力，单位牛顿（N）；m 是质量，单位千克（kg）；a 是加速度，单位米每二次方秒（m/s²）。

1 N = 1 kg m/s²

For example, a resultant force of 10 N acting on a 2 kg object gives an acceleration of 5 m/s².

例如，10 N的合力作用在2 kg的物体上，会产生5 m/s²的加速度。

Here is a useful triangle to rearrange F = ma:

下面是一个用于变换 F = ma 的公式三角形：

To find F: F = m × a / 求F：F = m × a

To find m: m = F ÷ a / 求m：m = F ÷ a

To find a: a = F ÷ m / 求a：a = F ÷ m

Important: “a” is the acceleration of the whole object caused by the unbalanced force. If several forces act, first find the resultant force.

重要：“a” 是整个物体在合力作用下产生的加速度。如果有多个力作用，要先求合力。

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5. Weight and Mass | 重量与质量

Mass is the amount of matter in an object and is the same everywhere in the universe. Its unit is the kilogram (kg).

质量是物体所含物质的多少，在宇宙任何地方都不变。其单位是千克（kg）。

Weight is the gravitational force pulling an object towards the centre of the Earth. Weight is a force, so its unit is the newton (N).

重量是把物体拉向地球中心的重力。重量是一种力，因此其单位是牛顿（N）。

W = mg

Here g is the gravitational field strength. On Earth, g ≈ 9.8 N/kg; on the Moon, g ≈ 1.6 N/kg.

这里的 g 是重力场强度。在地球上，g ≈ 9.8 N/kg；在月球上，g ≈ 1.6 N/kg。

So a 10 kg object has a mass of 10 kg on both Earth and Moon, but its

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