## Gravity And Earth's Rotation

 Quote by D H That is all you feel. Even though the gravitational acceleration on astronauts on board the ISS is about 90% of that on the surface of the Earth, astronauts on board the ISS feel weightless.
In fact I can only imagine the 'feel' (I do not know any of this 4rum ever been on board the ISS). Anyway that is not difficult to imagine and I have once experience a free fall when in a plane for about at least 5 seconds.
In the surface of the earth, the gravity is stronger than when you are up 360km, sure, but then suppose the earth rotate a bit faster (not too fast that you are thrown away) and you will be in the same condition as an astronaut in space.
 Mentor If the Earth rotated once per 1.40699 hours (as opposed to once per 23.93447 hours), the normal force needed to keep an object at the equator stationary with respect to the rotating Earth would be zero. A person at the equator on such a hyper-rotating Earth would feel weightless. They would feel weightless precisely because the normal force is zero.
 So let me get this straight. As the Earth spins faster, the value of g (9.8m/s^2) remains the same? And it feels like it's decreasing?
 Since g is defined as the force of gravity (given by Newtons law of Gravitation) divided by the mass of the test object, the value of g has no dependance on the rotational speed. Therefore your statement is correct!
 Mentor That is wrong in two ways. Firstly, g is defined as 9.80665 m/s2, exactly. Secondly, that definition does reflect the Earth's rotation rate. We live on a rotating Earth. Suppose you time the fall of an object initially at rest with respect to the rotating Earth in vacuum (this is exactly what a gravimeter does). The acceleration of the toward the Earth most definitely does depend on the Earth's rotation rate.
 The Earth wouldn't need to rotate as fast as the ISS is orbitting for the gravity to cancel out because it would undergo significant plastic deformation before becoming unstable at the equator. It would deform into an oblate spheroid and start losing mass at the equator once the force of gravity and the centrifugal force were balanced (i.e. weight = 0.) I can't remember the exact rotation rate, but it depends strongly on the density-radius profile of the Earth's interior. In some simulations of the Earth-Moon separating from each other just after the Big Whack they're rotating once every 6 hours and that might be close to the mass-loss limit.