地球的旋转速度是由它的角动量。角动量守恒是一个非常严重的物理定律(可能更严格的质量守恒定律)。以同样的方式,让地球失去质量,质量必须去某个地方。地球失去角动量,它必须去某个地方。地球自转可以通过重新排列不同在一个小范围内的分布质量。这就是为什么在冰河时代是几秒钟时间,因为大量的水,在海洋堆用于大冰原,就像一个旋转的经典例子冰溜冰者,通过移动质量(地球上的冰或溜冰者)的怀抱远离转动轴,旋转速度减慢。地球的唯一途径明显改变其转速是失去角动量,和任何可能的事件,可以显著改变地球的角动量的15年将是灾难性的,旋转速度的变化是次要的。此类事件必须像一个巨大的小行星的碰撞,行星大小的天体的飞行,或大量的弹射质量巨大的天然核反应堆的爆炸(所有事情可能发生在地球早期的历史)。有但是今天在工作过程慢慢偷地球的角动量。最重要的是潮汐。 Tides produce bulges of water around Earth that rotate slower than Earth, as in this picture (from [Wikipedia][1]): [![enter image description here][2]][2] Therefore, to keep the bulges in place the water have to be constantly flowing into those bulges moving against Earth's rotation and slowing it down by friction. The lost angular momentum goes mostly to the moon that accelerate on its orbit, therefore moving away from us [3.8 centimeters per year][3]. This process makes the day longer [1.7 milliseconds per century][3], something that is not a lot even at geological times scales. It is equivalent to 17 seconds per million of years. But even that small amount adds up to almost one full hour since the start of the Jurassic (201 Ma ago). But the change is so slow that animals would have plenty of time to adapt. So I would say that any explanation of how the dinosaurs supported their own weight have nothing to do with changes in Earth rotation speed. Also I think you are overestimating the impact of the rotation speed on the perceived weight. If we make the numbers, the centripetal acceleration due to rotation at the equator is $$a_\mathrm c=\frac{v^2}r=\frac{(462\ \mathrm{m/s})^2}{6\,378\,100\ \mathrm m}=0.0336\ \mathrm{m/s^2}$$ Equivalent to a 0.3% of the acceleration of gravity. Therefore even if the Earth were to stop completely, a dinosaur of 1000 kg would feel only ~3 kg heavier. Said that, if we go to the very hypothetical case you say, a slow down wouldn't make much (ignoring any environmental impacts of the day one year long). And for how fast the can Earth spin and support life? I think nobody can really answer that question, as we don't know what conditions life can endure. But if you make the Earth spin faster, the centripetal acceleration grows as the speed squared, therefore it wouldn't take too much to make the centripetal acceleration equal to the acceleration of gravity, in which case objects in the equator would be weightless. In that case we would loose the atmosphere and the oceans, and further speed-up would start ejecting rocks into space. by rearranging the above equation we can get that critical speed and it would be $$v_\mathrm c=\sqrt{rg}=\sqrt{6\,378\,100\mathrm m\times9.8\ \mathrm{m/s^2}}=7\,906\ \mathrm{m/s}$$ That is 17 times the current speed, equivalent to a day of one hour and 24 minutes. Finally, for your question: What is the fastest the Earth has ever spun? It depends. If we consider it after the main change in angular momentum associated to the formation of the Moon [4.51 billion years ago][4], we can just extrapolate those 17 seconds per million years (this is not the most precise as there were no oceans then, but it is a fair first approximation), and we get about 21 hours, so that the day at that time was only about **three hours**. As an anecdotal note, the fact that this number is close to the critical velocity, have made some people think that the explosion of a natural nuclear reactor could have been enough to give the extra kick to eject the whole Moon from the surface. Something that would explain why the composition of the Moon is so remarkably similar to that of Earth by recent analysis, challenging the theory of formation by a collision with "Thea" a hypothetical Mars-sized body. [1]: https://en.wikipedia.org/wiki/Tide [2]: https://i.stack.imgur.com/0pRS7.png [3]: https://www.scientificamerican.com/article/earth-rotation-summer-solstice/ [4]: https://en.wikipedia.org/wiki/Moon#Formation