📚 GCSE WJEC Physics: Circular Motion | 圆周运动 考点精讲
In GCSE WJEC Physics, circular motion is a fundamental topic that describes objects moving along a circular path at constant speed. Although the speed may not change, the direction is constantly changing, which means the velocity is changing and there must be an acceleration and a resultant force towards the centre of the circle. Understanding these concepts helps explain how satellites orbit, how cars turn corners, and why objects on a roundabout experience a force pulling them outward.
在GCSE WJEC物理中，圆周运动是一个基础课题，描述物体以恒定速率沿圆形路径运动。尽管速率不变，但方向不断改变，这意味着速度在变化，因此必定存在指向圆心的加速度和合力。理解这些概念有助于解释卫星如何绕轨道运行、汽车如何转弯，以及为什么旋转平台上的物体会感到向外拉的力。
1. What is Circular Motion? | 什么是圆周运动？
An object in circular motion travels along a circular path. In WJEC GCSE Physics, we consider uniform circular motion, where the speed (magnitude of velocity) remains constant but the direction changes continuously. This is not constant velocity because velocity is a vector – it has both magnitude and direction. A change in direction means a change in velocity, hence acceleration.
做圆周运动的物体沿圆形路径运动。在WJEC GCSE物理中，我们考虑匀速圆周运动，即速率（速度的大小）保持不变但方向持续变化。这不是恒定速度，因为速度是矢量——既有大小又有方向。方向改变意味着速度改变，从而存在加速度。
Examples: a car turning a corner at steady speed, a planet orbiting the Sun, a stone whirled on a string, and a spinning fairground ride are all examples of circular motion.
实例：汽车匀速转弯、行星绕太阳运行、用绳子挥动的小石块以及旋转的游乐设施，都是圆周运动的例子。
2. Velocity in Circular Motion | 圆周运动中的速度
The speed v of an object moving in a circle of radius r can be calculated if we know the time for one complete revolution (the period T). The distance travelled in one revolution is the circumference, 2πr. Therefore, speed v = distance/time = 2πr / T. Since speed is constant in uniform circular motion, this formula gives the constant speed. The direction of velocity at any instant is tangential to the circle.
如果知道物体沿半径为 r 的圆运动一周所需时间（周期 T），便可计算其速率 v。一圈的距离为圆周长 2πr。因此速率 v = 距离/时间 = 2πr / T。由于匀速圆周运动中速率恒定，该公式给出恒定速率。任一瞬间速度的方向沿圆的切线方向。
v = 2πr / T
v = 2πr / T
The frequency f is the number of revolutions per second, f = 1/T. So speed can also be written as v = 2πrf. In WJEC, you might be given the frequency in hertz (Hz) or revolutions per second.
频率 f 是每秒转动的圈数，f = 1/T。因此速率也可写作 v = 2πrf。在WJEC考试中，可能会给出以赫兹（Hz）或每秒转数给出的频率。
3. Acceleration Towards the Centre | 指向圆心的加速度
Although the speed is constant, the direction of motion changes continuously in circular motion, resulting in a change in velocity. Hence, the object experiences acceleration. This acceleration is directed towards the centre of the circle and is called centripetal acceleration (a). It can be calculated using the formula:
尽管速率恒定，但圆周运动中运动方向连续变化，导致速度变化。因此，物体存在加速度。该加速度指向圆心，称为向心加速度（a）。可通过以下公式计算：
a = v
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