Momentum and Impulse for Edexcel IGCSE Science | Edexcel IGCSE 科学：动量与冲量

📚 Momentum and Impulse for Edexcel IGCSE Science | Edexcel IGCSE 科学：动量与冲量

Momentum is a fundamental concept in physics that helps explain collisions, explosions, and the effectiveness of safety features such as airbags and crumple zones. In the Edexcel IGCSE Science (Physics) specification, the topic of momentum is covered primarily under points 4.19P to 4.22P. This article will take you through all the essential ideas, from the basic definition of momentum to the conservation law and real-world applications. Whether you are preparing for the Double Award or the separate Physics qualification, a solid understanding of momentum will boost your confidence in tackling calculation and explanation questions.

动量是物理学中的一个核心概念，它能帮助我们解释碰撞、爆炸以及安全气囊和溃缩区等安全装置的工作原理。在 Edexcel IGCSE 科学（物理）课程大纲中，动量相关内容主要集中在 4.19P 至 4.22P 知识点。本文将带你掌握所有关键概念，从动量的基本定义到动量守恒定律以及实际应用。无论你是在准备双证书科学考试还是单独的物理科目，扎实掌握动量知识都将极大提高你解答计算题和解释题的信心。

1. What is Momentum? | 什么是动量？

Momentum is the product of an object’s mass and its velocity. It describes the quantity of motion an object has and, importantly, it depends on both how much matter is moving and how fast it is moving. An object with a large mass moving at high speed has a large momentum, making it harder to stop.

动量是物体质量与速度的乘积。它描述了物体所具有的运动量，重要的是，动量取决于物体的质量和运动速度。一个质量大且高速运动的物体会具有很大的动量，因此更难使其停下来。

The SI unit of momentum is kilogram metre per second (kg m/s). Since mass is a scalar and velocity is a vector, momentum is also a vector quantity, meaning it has both magnitude and direction.

动量的国际单位是千克米每秒（kg m/s）。因为质量是标量而速度是矢量，所以动量也是矢量，既有大小又有方向。

momentum = mass × velocity
p = m × v

The symbol p is used for momentum. In the IGCSE exam, you must be able to use this equation to calculate any one of the three quantities when the other two are given.

符号 p 表示动量。在 IGCSE 考试中，你必须能够使用该方程，在已知其中两个量的情况下计算第三个量。

2. Calculating Momentum | 动量计算

To find an object’s momentum, simply multiply its mass (in kilograms) by its velocity (in metres per second). For example, a 1500 kg car travelling at 20 m/s has a momentum of 1500 × 20 = 30 000 kg m/s. A bicycle of mass 15 kg moving at 8 m/s has a much smaller momentum of 120 kg m/s.

要计算物体的动量，只需将其质量（以千克为单位）乘以速度（以米每秒为单位）。例如，一辆质量为 1500 kg 的汽车以 20 m/s 的速度行驶，其动量为 1500 × 20 = 30 000 kg m/s。一辆质量为 15 kg 的自行车以 8 m/s 的速度行驶，动量仅有 120 kg m/s。

You may also be asked to rearrange the formula. For instance, if you know the momentum and velocity, the mass is p / v. Always check that the units are consistent – use kg for mass and m/s for velocity.

考试中你也可能需要变换公式。例如，已知动量和速度，质量就等于 p / v。务必确保单位一致——质量用 kg，速度用 m/s。

3. Vector Nature of Momentum | 动量的矢量性

Because velocity has direction, momentum also has direction. When describing momentum, you must state its direction or assign a positive or negative sign according to a chosen convention, such as ‘to the right is positive’. This is crucial when two objects move towards each other – one will have a positive momentum and the other a negative momentum.

由于速度具有方向，动量也同样具有方向。在描述动量时，你必须指明方向，或者根据选定的正方向（例如“向右为正”）赋予其正负号。这一点在两个物体相向运动时至关重要——一个物体的动量为正，另一个则为负。

For example, a 2 kg ball moving right at 3 m/s has momentum +6 kg m/s, while a 1 kg ball moving left at 4 m/s has momentum –4 kg m/s (if right is positive). The total momentum of the system is the algebraic sum, +2 kg m/s to the right.

例如，一个 2 kg 的小球以 3 m/s 的速度向右运动，动量为 +6 kg m/s；而一个 1 kg 的小球以 4 m/s 的速度向左运动，动量则为 –4 kg m/s（假设向右为正）。系统的总动量是代数和，即向右的 +2 kg m/s。

4. Impulse and Change in Momentum | 冲量与动量变化

When a force acts on an object for a certain amount of time, it changes the object’s momentum. This change in momentum is called impulse. The impulse is equal to the force multiplied by the time for which it acts. In equation form: impulse = F × t, and impulse also equals the change in momentum, Δp.

当力作用于物体并持续一定时间时，物体的动量会发生改变。这种动量的变化量称为冲量。冲量等于力乘以作用时间。公式为：冲量 = F × t，同时冲量也等于动量的变化量 Δp。

This relationship helps explain why a large force applied for a very short time (such as a hit with a bat) can produce the same change in momentum as a smaller force applied over a longer time (such as gently catching a ball).

这一关系可以解释为什么短时间施加的大力（如球棒击球）与较长时间施加的小力（如轻轻接球）可以产生相同的动量变化。

impulse = F × t = Δ(mv) = final momentum – initial momentum

You will often see this written as F × t = mv – mu

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