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<blockquote data-quote="Lylandra" data-source="post: 7285268" data-attributes="member: 6816692">nah, you don't necessarily need general relativity. Still, rather advanced physics thoughts. What you discuss is defining a frame of reference for the rod. One could argue that an inertial frame of reference (a system onto which no external force is applied) could be seen as &quot;not moving&quot; by an observer in the system. So if you had a ship which was moving at a constant speed into the same direction, the immovable rod could be placed inside a cabin in the ship. Now if the ship stopped moving or changed its direction, the rod would leave indeed a rod-sized hole in the cabin. (Thinking about it, any immovable force field must have an indefinite mass equivalent. At least in a world where &quot;our&quot; physics apply. Which would also lead to indefinite kinetic energy and a definite violation of energy conversion. Ouch.)A spinning planet is different indeed. While the speed of rotation is constant, rotating systems itself can never be inertial as the diretion of the movement constantly changes (albeit only a bit. Otherwise our &quot;linear&quot; movements would go nuts and we'd have coriolis forces etc everywhere in our small scale everyday lives). But I guess one can approximate the radial acceleration to zero in smaller timeframes <img src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f642.png" class="smilie smilie--emoji" loading="lazy" width="64" height="64" alt=":)" title="Smile&nbsp; &nbsp; :)"&nbsp; data-smilie="1"data-shortname=":)" /></blockquote>

[QUOTE="Lylandra, post: 7285268, member: 6816692"] nah, you don't necessarily need general relativity. Still, rather advanced physics thoughts. What you discuss is defining a frame of reference for the rod. One could argue that an inertial frame of reference (a system onto which no external force is applied) could be seen as "not moving" by an observer in the system. So if you had a ship which was moving at a constant speed into the same direction, the immovable rod could be placed inside a cabin in the ship. Now if the ship stopped moving or changed its direction, the rod would leave indeed a rod-sized hole in the cabin. (Thinking about it, any immovable force field must have an indefinite mass equivalent. At least in a world where "our" physics apply. Which would also lead to indefinite kinetic energy and a definite violation of energy conversion. Ouch.) A spinning planet is different indeed. While the speed of rotation is constant, rotating systems itself can never be inertial as the diretion of the movement constantly changes (albeit only a bit. Otherwise our "linear" movements would go nuts and we'd have coriolis forces etc everywhere in our small scale everyday lives). But I guess one can approximate the radial acceleration to zero in smaller timeframes :) [/QUOTE]

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