📚 IGCSE AQA Physics: Dynamics Key Points | IGCSE AQA 物理：动力学考点精讲
Dynamics is a core topic in the IGCSE AQA Physics syllabus, explaining how objects move and what causes changes in their motion. It links forces, momentum, and energy, and the skills you build here are essential for tackling exam questions on motion graphs, calculations, and real-world safety applications. This article distils every key point you need to master Dynamics for your IGCSE AQA exam.
动力学是 IGCSE AQA 物理大纲的核心专题，用于解释物体如何运动以及什么导致运动状态改变。它将力、动量与能量联系在一起，你在此处建立的能力对于处理运动图像、计算和实际安全应用类的考试题目至关重要。本文提炼了你在 IGCSE AQA 动力学考试中需要掌握的每一个关键考点。
1. Scalar and Vector Quantities | 标量与矢量
A scalar quantity has only magnitude (size) and no direction. Common examples are distance, speed, mass, and energy. A vector quantity has both magnitude and direction. Force, velocity, displacement, and momentum are all vectors.
标量只有大小（量值）而没有方向，常见的例子有距离、速率、质量和能量。矢量既有大小又有方向，力、速度、位移和动量都是矢量。
When you add vectors, you must consider their directions. Two forces of 3 N and 4 N acting in the same direction give a resultant of 7 N. If they act at right angles, the resultant is 5 N, calculated using Pythagoras’ theorem: √(3² + 4²) = 5 N.
当你对矢量进行加法时，必须考虑方向。两个力 3 N 和 4 N 沿同一方向作用，其合力为 7 N；若互成直角作用，合力为 5 N，可用勾股定理计算：√(3² + 4²) = 5 N。
2. Speed and Velocity | 速率与速度
Speed is a scalar that tells you how fast an object is moving. Average speed = total distance ÷ total time. The SI unit is metres per second (m/s). Speed can also be measured in km/h or mph, but always convert to m/s in calculations.
速率是标量，表示物体运动的快慢。平均速率 = 总距离 ÷ 总时间。SI 单位是米每秒 (m/s)。速率有时也用 km/h 或 mph 度量，但在计算中必须转化为 m/s。
Velocity is speed in a given direction, so it is a vector. A car turning a corner at constant speed may change its velocity because the direction changes. The instantaneous velocity is the gradient of a displacement–time graph.
速度是特定方向上的速率，因而是矢量。汽车以恒定速率转弯时，速度会因方向变化而改变。瞬时速度等于位移–时间图像的斜率。
3. Acceleration | 加速度
Acceleration is the rate of change of velocity. It is a vector. The average acceleration a is given by: a = (v – u) / t, where u is initial velocity, v is final velocity, and t is time taken. The SI unit is m/s².
加速度是速度的变化率，是矢量。平均加速度 a 由下式给出：a = (v – u) / t，其中 u 为初速度，v 为末速度，t 为所用时间。SI 单位是 m/s²。
Negative acceleration (deceleration) means the object is slowing down. In a velocity–time graph, acceleration equals the gradient. Uniform acceleration produces a straight sloping line.
负加速度（减速）表示物体正在慢下来。在速度–时间图像中，加速度等于斜率。匀加速度产生一条倾斜的直线。
4. Motion Graphs | 运动图像
Distance–time graphs: The gradient gives speed. A horizontal line means the object is stationary. A straight sloping line shows constant speed; a curve shows changing speed (acceleration or deceleration).
距离–时间图像：斜率表示速率。水平线代表物体静止；倾斜直线表示恒定速率；曲线表示速率正在改变（加速或减速）。
Velocity–time graphs: The gradient gives acceleration. The area under the graph gives the displacement. A horizontal line means constant velocity. The area of a trapezium or triangle is often needed to find displacement in exams.
速度–时间图像：斜率给出加速度。图像下的面积代表位移。水平线表示速度恒定。在考试中常需通过梯形或三角形面积求出位移。
5. Equations of Motion | 运动方程
For uniform acceleration, you can use the SUVAT equations. v = u + at; s = ut + ½at²; v² = u² + 2as; s = ½(u + v)t. Here, s is displacement, u initial velocity, v final velocity, a acceleration, t time.
对于匀加速度，你可以使用 SUVAT 方程。v = u + at；s = ut + ½at²；v² = u² + 2as；s = ½(u + v)t。其中 s 是位移，u 初速度，v 末速度，a 加速度，t 时间。
Always identify the known quantities and choose the equation that omits the one you don’t need. These equations only work when acceleration is constant. If acceleration is not constant, use graph area methods.
要始终先识别已知量，再选择不含所需未知量的方程。这些方程仅在加速度恒定时有效。若加速度不恒定，应使用图像面积法。
6. Newton’s Laws of Motion | 牛顿运动定律
Newton’s First Law: An object remains at rest or in uniform motion in a straight line unless acted upon by a resultant external force. This is the principle of inertia.
牛顿第一定律：如果不受合外力的作用，物体将保持静止或沿直线匀速运动状态。这就是惯性原理。
Newton’s Second Law: The acceleration of an object is directly proportional to the resultant force and inversely proportional to its mass. F = m × a, where F is resultant force in newtons, m is mass in kg, a is acceleration in m/s².
牛顿第二定律：物体的加速度与合外力成正比，与物体的质量成反比。F = m × a，其中 F 是合力 (N)，m 是质量 (kg)，a 是加速度 (m/s²)。
When several forces act, you must find the resultant force before applying F = ma. The direction of the acceleration is the same as the direction of the resultant force.
当多个力作用时，必须先求出合力再应用 F = ma。加速度的方向与合力的方向相同。
7. Mass and Weight | 质量与重量
Mass is a scalar quantity that measures the amount of matter in an object. It does not change with location and is measured in kilograms (kg). Weight is the gravitational force on the mass and is a vector. Weight = mass × gravitational field strength (g).
质量是标量，衡量物体所含物质的多少，不随位置改变，单位是千克 (kg)。重量是作用在物体上的引力，是矢量。重量 = 质量 × 重力场强度 (g)。
On Earth, g ≈ 9.8 N/kg (or 10 N/kg for exam estimates). Weight is measured in newtons. The mass of an object is the same on the Moon, but its weight is less because the Moon’s gravitational field strength is smaller.
在地球表面，g ≈ 9.8 N/kg（考试中常取 10 N/kg）。重量的单位是牛顿。同一物体在月球上的质量不变，但重量较小，因为月球的重力场强度更小。
8. Momentum | 动量
Momentum p is a vector quantity defined as the product of mass and velocity: p = m v. The unit is kg m/s. Momentum depends on both how much matter is moving and how fast it is moving in a particular direction.
动量 p 是矢量，定义为质量与速度的乘积：p = m v。单位是 kg m/s。动量既取决于运动物体的质量，也取决于它在特定方向上的运动快慢。
In a closed system, the total momentum before a collision or explosion equals the total momentum after, provided no external forces act. This is the principle of conservation of momentum.
在封闭系统中，只要没有外力作用，碰撞或爆炸前的总动量等于碰撞或爆炸后的总动量。这就是动量守恒原理。
9. Conservation of Momentum | 动量守恒
For two objects colliding, momentum conservation is written as: m₁ u₁ + m₂ u₂ = m₁ v₁ + m₂ v₂, where u represents initial velocities and v represents final velocities. The equation works with positive and negative directions.
两个物体碰撞时，动量守恒写作：m₁ u₁ + m₂ u₂ = m₁ v₁ + m₂ v₂，其中 u 表示初速度，v 表示末速度。公式处理时需考虑正负方向。
Momentum problems often ask you to find an unknown velocity after a collision. Always define a positive direction, list masses and velocities with signs, then solve. In explosions, the total momentum before is zero, so the pieces move apart with equal and opposite momentum.
动量问题常要求你求碰撞后的未知速度。总是先定义一个正方向，列出带符号的质量和速度，然后求解。在爆炸问题中，爆炸前总动量为零，因而碎片以大小相等方向相反的动量分开。
10. Vehicle Safety | 车辆安全
Car safety features use impulse and momentum. Impulse = change in momentum = force × time. For the same change in momentum, increasing the stopping time reduces the average force experienced by occupants.
车辆安全特性应用了冲量与动量。冲量 = 动量的变化量 = 力 × 时间。对于相同的动量改变，延长停止时间可以减少乘员所承受的平均作用力。
Seat belts stretch, airbags inflate, and crumple zones deform in a controlled way. All these increase the time over which the passenger’s momentum is reduced to zero, lowering the force and reducing injury.
安全带会拉伸，安全气囊会充气，溃缩区则会以受控方式变形。这些设计都延长了乘员动量减至零的过程时间，从而减小了作用力，降低了伤害风险。
When working out forces in collisions, use: force = change in momentum ÷ time. This direct relationship is frequently tested in written and calculation questions.
在分析碰撞力时，使用：力 = 动量的变化量 ÷ 时间。这一直接关系在文字题和计算题中经常被考到。
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