IB Physics: Space, Time and Motion | IB 物理：空间、时间与运动

📚 IB Physics: Space, Time and Motion | IB 物理：空间、时间与运动

Space and time are the stage on which all physical events unfold. Motion is the change of an object’s position with respect to a reference point and to time. In IB Physics, this theme is not only about memorising equations; it is about learning to model the physical world with vectors, graphs and fields.

空间与时间是所有物理事件展开的舞台，而运动则是物体相对于参考点和时间的位置变化。在 IB 物理中，这一主题不仅仅要求熟记公式，更要求学会用矢量、图像和场来建立物理世界的模型。

1. Frames of Reference | 参考系

Motion must always be measured relative to a chosen frame of reference. A frame of reference consists of a coordinate system and a clock. The same event can have different positions and velocities in different frames.

运动必须相对于选定的参考系来测量。参考系由坐标系统和时钟组成。同一个事件在不同参考系中可以有不同的位置和速度。

An inertial frame is one that is not accelerating; in such a frame, Newton’s first law applies. A non-inertial frame, such as a rotating or braking train, introduces fictitious forces such as the centrifugal effect. When doing mechanics problems on Earth, we often approximate the ground as an inertial frame, even though it is strictly accelerating.

惯性参考系是不加速度的参考系；在这样的参考系中，牛顿第一定律成立。非惯性参考系，例如旋转或刹车的列车，会引入像离心作用这样的惯性力。在解决力学问题时，我们常把地面近似为惯性参考系，尽管它实际上是有加速度的。

2. Distance, Displacement, Speed and Velocity | 距离、位移、速率与速度

Distance is a scalar that measures the total path length; displacement is a vector that points from the initial position to the final position. The SI unit of both is the metre, but they are conceptually very different.

距离是标量，表示路径总长度；位移是矢量，从初位置指向末位置。两者的国际单位都是米，但它们在概念上有很大区别。

Speed is the rate of change of distance, while velocity is the rate of change of displacement. Average velocity therefore depends on the straight-line separation, not on how much ground was covered.

速率是距离的变化率，而速度是位移的变化率。因此，平均速度取决于起点到终点的直线分离量，而不取决于实际走了多少路程。