Forces and Energy | 力与能量

2026-08-21 13:35 · 本文部分内容由AI辅助创作

📚 Forces and Energy | 力与能量

Forces are pushes or pulls that can change an object's speed, shape or direction. Energy is needed to make forces do useful jobs, and this unit links the two ideas through work, power, motion and turning effects.

力是能够改变物体速度、形状或方向的推或拉作用。能量则用于让力完成有用的工作，本单元通过功、功率、运动以及转动效应将这两个概念联系起来。

1. Understanding Forces | 理解力

A force is a vector quantity, so it has both magnitude and direction. Forces are measured in newtons (N). Contact forces include friction, tension and normal reaction; non-contact forces include gravity, magnetism and electrostatic force.

力是矢量，既有大小又有方向。力的单位是牛顿（N）。接触力包括摩擦力、张力和支持力；非接触力包括重力、磁力和静电力。

When drawing forces, use arrows from the centre of mass. The length of an arrow represents the size of the force, while the direction of the arrow shows the direction in which the force acts. Always label each force clearly.

画力时，应从物体的质心画出箭头。箭头的长度表示力的大小，箭头方向表示力的作用方向。每个力都应清楚地标注。

2. Resultant Forces | 合力

When several forces act on an object, they combine to give a single resultant force. If the forces act in the same straight line, add those pointing in one direction and subtract those pointing in the opposite direction.

当几个力同时作用于一个物体时，它们会合成为一个合力。如果力作用在同一直线上，将同一方向的力相加，再减去相反方向的力。

A zero resultant force does not mean no forces are present; it means the forces cancel each other. The object remains at rest or keeps moving at constant velocity.

合力为零并不意味着没有力的作用，而是各力相互抵消。此时物体保持静止或匀速直线运动。

resultant force = forward force − backward force

3. Balanced and Unbalanced Forces | 平衡力与非平衡力

Balanced forces are equal in size and opposite in direction. They do not change the motion of an object. Unbalanced forces produce a resultant force that causes acceleration, deceleration or a change of direction.

平衡力大小相等、方向相反，不会改变物体的运动状态。非平衡力产生合力，使物体加速、减速或改变运动方向。

This idea is summarised by Newton's first law: an object keeps its velocity unless a resultant force acts on it. This is also called the law of inertia.

这一思想总结为牛顿第一定律：除非有合力作用，否则物体保持原来的速度。这也叫惯性定律。

4. Force, Mass and Acceleration | 力、质量与加速度

Newton's second law relates unbalanced force, mass and acceleration. For the same resultant force, a larger mass gives a smaller acceleration; for the same mass, a larger force gives a larger acceleration.

牛顿第二定律联系了非平衡力、质量和加速度。合力相同时，质量越大，加速度越小；质量相同时，力越大，加速度越大。

resultant force = mass × acceleration (F = m × a)

Acceleration is measured in metres per second squared (m/s²) and force in newtons (N). This equation is used to predict how an object will speed up or slow down.

加速度的单位是米每二次方秒（m/s²），力的单位是牛顿（N）。这个公式可以用来预测物体会如何加速或减速。

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