Weightlessness at the Equator (Whiteboard Sketch #1)
At the present time, our Earth does a full rotation every 24 hours, which results in day and night. Just like on a carrousel, its inhabitants (and, by the way, all the other stuff on and of the planet) are pushed “outwards” due to the centrifugal force. So we permanently feel an “upwards” pulling force thanks to the Earth’s rotation. However, the centrifugal force is much weaker than the centripetal force, which is directed towards the core of the planet and usually called “gravitation”. If this wasn’t the case, we would have serious problems holding our bodies down to the ground. (The ground, too, would have troubles holding itself “to the ground”.)
Especially on the equator, the centrifugal and the gravitational force are antagonistic forces: the one points “downwards” while the other points “upwards”.
How fast would the Earth have to spin in order to cause weightlessness at the equator?
This question can be put another way just as well: At which rotational speed are the absolute values of the centrifugal and the centripetal force the same?
The answer to the question is: If the Earth rotated about 17 times faster than it currently does, equator inhabitants would be weightless indeed. A day-night-cycle would last only 1.4 hours instead of the familiar 24 hours – in other words: 1 hour and 24 minutes. The equatorial speed around the Earth’s center would increase from normal 1668 km/h to almost 28500 km/h (from 1036 mph to 17700 mph). Phew!
Astronauts aboard the International Space Station (ISS) experience this kind of day-night-rhythm every day (or rather every …umm…seventeenth-day) and see a sun rise every one and a half hours. And of course, they are weightless in their orbit.
(It’s quite reasonable to apply our conditions for weightlessness on earth surface to the ISS since its orbital height of approximately 400 kilometers is numerically small compared to the earth radius of 6400 kilometers – thus, the uncertainty in our statements is in the region of just a few minutes.)