Edexcel Physics: Dynamics | 动力学考点精讲

📚 Edexcel Physics: Dynamics | 动力学考点精讲

Dynamics is a core topic in Edexcel Physics, focusing on the relationship between force and motion. Understanding Newton’s laws, momentum, and circular motion is essential for tackling both theoretical and practical problems. This guide summarises the key concepts and common exam pitfalls.

动力学是Edexcel物理的核心内容，聚焦力与运动的关系。理解牛顿定律、动量与圆周运动对于处理理论和实际问题至关重要。本指南总结核心概念与常见考试陷阱。

1. Mass and Weight | 质量与重量

Mass is a scalar quantity measuring the amount of matter in an object, measured in kilograms (kg). It does not change with location.

质量是标量，衡量物体所含物质的多少，单位是千克(kg)，不随位置改变。

Weight is a vector quantity, the gravitational force acting on an object, measured in newtons (N). The relationship is:

重量是矢量，是作用在物体上的重力，单位是牛顿(N)。二者的关系为：

W = mg

On Earth, g = 9.81 N/kg. In exams, always distinguish clearly between mass and weight and use the correct formula.

在地球表面，g = 9.81 N/kg。考试中务必清晰区分质量与重量，并正确使用公式。

2. Newton’s First Law | 牛顿第一定律

An object will remain at rest or move with constant velocity unless acted upon by a resultant force. This property is called inertia.

物体将保持静止或匀速直线运动状态，除非受到合外力的作用。这一性质称为惯性。

If the resultant force is zero, the object is in equilibrium and its velocity is constant (or zero).

若合外力为零，则物体处于平衡状态，速度恒定(或为零)。

3. Newton’s Second Law | 牛顿第二定律

Resultant force is proportional to acceleration: F = ma, where F is resultant force (N), m is mass (kg), a is acceleration (m/s²).

合外力与加速度成正比：F = ma，其中F为合外力(N)，m为质量(kg)，a为加速度(m/s²)。

More generally, force equals the rate of change of momentum:

更普遍的表述为力等于动量的变化率：

F = Δp / Δt

For constant mass, this reduces to F = ma. Always find the net force by considering all forces acting.

若质量恒定，则简化为F = ma。必须考虑所有作用力以求出合外力。

4. Newton’s Third Law | 牛顿第三定律

If body A exerts a force on body B, body B exerts an equal and opposite force on body A. These forces are of the same type and act on different bodies; they never cancel out.

若物体A对物体B施加力，则B对A施加大小相等、方向相反的力。这两个力同种类型，作用在不同物体上，永远不会相互抵消。

A common exam mistake is confusing action–reaction pairs with balanced forces on the same object.

常见考试错误是将作用力–反作用力对与同一物体上的平衡力混淆。

5. Free-Body Diagrams | 受力分析图

A free-body diagram shows all forces acting on a single object. Use labelled arrows for weight, normal reaction, tension, friction, etc.

受力分析图展示作用在单一物体上的所有力。用标注的箭头表示重力、法向反力、拉力、摩擦力等。

For an object on an inclined plane, include weight (vertically down), normal reaction (perpendicular to the surface), and friction (parallel, opposing motion).

对于斜面上的物体，应包含重力(竖直向下)、法向反力(垂直斜面)以及摩擦力(平行斜面，与运动方向相反)。

6. Resolving Forces | 力的分解

When forces act at angles, resolve them into perpendicular components (horizontal/vertical or parallel/perpendicular to a slope). Use trigonometry: F_x = F cosθ, F_y = F sinθ.

当力存在夹角时，需将其分解为垂直分量(水平/竖直或平行/垂直于斜面)。使用三角函数：F_x = F cosθ, F_y = F sinθ。

For equilibrium, the sum of force components in each direction must be zero.

平衡状态下，各方向合力分量必须为零。

7. Momentum and Impulse | 动量与冲量

Momentum p is a vector: p = mv, measured in kg m/s. Impulse is the product of force and time, equal to the change in momentum: J = FΔt = Δp.

动量p是矢量：p = mv，单位为kg m/s。冲量是力与时间的乘积，等于动量的变化量：J = FΔt = Δp。

The area under a force–time graph represents impulse. This is key for collisions, where increasing impact time reduces average force (e.g., crumple zones).

力-时间图下的面积表示冲量。这点对碰撞问题很关键，延长作用时间可减小平均力(如溃缩区)。

8. Conservation of Momentum | 动量守恒

In a closed system with no external resultant force, total momentum before an event equals total momentum after: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂.

在没有外合力的封闭系统中，事件前后总动量守恒：m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂。

Momentum is a vector; assign positive and negative directions consistently in calculations.

动量为矢量，计算时须统一规定正负方向。

9. Elastic and Inelastic Collisions | 弹性与非弹性碰撞

In an elastic collision, both momentum and kinetic energy are conserved. In an inelastic collision, momentum is conserved but kinetic energy is not – some energy is transferred to heat, sound or deformation.

弹性碰撞中动量和动能均守恒。非弹性碰撞中动量守恒但动能不守恒，部分动能转化为热能、声能或形变能。

In a perfectly inelastic collision, objects stick together and move with a common velocity.

完全非弹性碰撞中物体粘合在一起，以共同速度运动。

10. Circular Motion | 圆周运动

An object moving in a circle at constant speed is still accelerating because its direction changes continuously

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Edexcel Physics: Dynamics | 动力学 考点精讲