Within a vehicle in free fall, such as a spacecraft orbiting the Earth, everything appears as if there was no force of gravity: objects float and remain stationary with respect to the walls, a situation commonly described as “weightlessness.” In relativity, such a vehicle is considered an inertial reference system. In fact, this is not exactly true because of the non-uniformity of the gravitational field. Tidal effects and forces appear, and complicated motions are generated. This article examines these effects, limiting the use of complicated mathematical formalism in favor of physical reasoning. Two particular cases are treated: a spacecraft that rotates about its center of mass in sync with its orbit, so it always faces the Earth the same way, and a spacecraft that maintains the same orientation with respect to the fixed stars throughout its orbit. The static problem is solved by finding the tidal forces acting on an object at different positions within the spacecraft. The trajectories of objects released from various positions within the spacecraft are then calculated. Some concluding conceptual observations are given.

中文翻译：

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航天器中的潮汐效应

在自由落体的飞行器中，例如绕地球运行的航天器，一切似乎都没有重力：物体漂浮并相对于墙壁保持静止，这种情况通常被称为“失重”。在相对论中，这样的车辆被认为是惯性参考系统。事实上，由于引力场的不均匀性，这并不完全正确。潮汐效应和力出现，并产生复杂的运动。本文研究了这些影响，限制了复杂数学形式主义的使用，以支持物理推理。处理两种特殊情况：航天器围绕其质心与轨道同步旋转，因此它始终以相同的方式面向地球，以及在整个轨道上相对于固定恒星保持相同方向的航天器。静态问题是通过找到作用在航天器内不同位置的物体上的潮汐力来解决的。然后计算从航天器内不同位置释放的物体的轨迹。给出了一些结论性的概念观察。