Newton’s Three Laws and Their Applications | 牛顿三定律及其应用

📚 Newton’s Three Laws and Their Applications | 牛顿三定律及其应用

Newton’s three laws of motion are the foundation of classical mechanics and an essential topic in A-level Physics. They explain how forces change motion, why moving objects continue moving, and how interacting objects exert forces on each other. Exam questions often connect these laws with lifts, pulleys, inclined planes, projectiles, friction, and momentum.

牛顿三大定律是整个经典力学的基石，也是 A-level 物理的核心考点。它们解释了力如何改变运动、物体为什么能继续运动，以及相互作用的物体之间如何施力。考试题常把这些定律与电梯、滑轮、斜面、抛体运动、摩擦力和动量结合起来考查。

1. Newton’s First Law: The Law of Inertia | 牛顿第一定律：惯性定律

Newton’s first law states that, in an inertial reference frame, a body will remain at rest or continue to move in a straight line at constant velocity unless acted upon by a resultant external force.

牛顿第一定律指出：在惯性参考系中，除非受到合外力的作用，否则物体将保持静止状态，或沿直线做匀速运动。

This law tells us that force is not needed to keep an object moving; force is needed to change its velocity. If the resultant force is zero, the acceleration is zero. This is true whether the object is at rest or moving with a constant velocity.

这条定律告诉我们：维持运动并不需要力，改变运动速度才需要力。如果合外力为零，加速度就为零；无论物体是静止还是做匀速运动，这一点都成立。

• The tendency of an object to resist changes in its motion is called inertia.

  物体抵抗运动状态改变的性质叫作惯性。

• For an object in equilibrium, the vector sum of all forces must be zero: ΣF = 0.

  物体处于平衡状态时，所有力的矢量和必须为零：ΣF = 0。

• Constant velocity and rest both satisfy the first law because they both describe zero acceleration.

  匀速直线运动和静止都满足第一定律，因为它们的加速度都为零。

2. Newton’s Second Law: F = Δp/Δt and F = ma | 牛顿第二定律：F = Δp/Δt 与 F = ma

Newton’s second law says that the rate of change of momentum of a body is proportional to the resultant force acting on it, and the change in momentum takes place in the direction of the resultant force.

牛顿第二定律指出：物体动量的变化率与作用在它上面的合外力成正比，且动量变化的方向与合外力的方向相同。

In symbols, if momentum is p = mv, then the law can be written as:

用公式表示，若动量 p = mv，第二定律可写为：

ΣF = Δp/Δt

For an object with constant mass, this reduces to the familiar equation:

当物体质量不变时，上式就简化为我们熟悉的方程：

ΣF = m a

The acceleration that appears here is always in the same direction as the resultant force. One newton is the force that gives a mass

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