MadSci Network: Neuroscience

Re: why don't we feel the earth rotating physically?ie...dizziness,nausea etc.

Date: Mon Oct 4 15:04:44 1999
Posted By: David Hackos, Post-doc/Fellow, Molecular Biophysics, National Institutes of Health
Area of science: Neuroscience
ID: 938793014.Ns

The part of our body that "feels" acceleration is called the vestibular 
system of the inner ear - which is basically a u-shaped tube that is 
partially filled with fluid.  By measuring movements of this fluid, your 
body can detect very small accelerations.

Dizziness results from activities that cause you to experience 
accelerations at lots of different angles.  On a roller-coaster, for 
example, you experience accelerations in one direction and then another in 
quick succession.  This causes the vestibular to fail to "reset" itself 
correctly, which causes it to become confused, and dizziness results.  You 
can also experience dizziness by being exposed to no forces at all - like 
if you jump off a bridge or if you are in space.  This is because the 
vestibular system requires at least some force to be present in order to 

However, when you are standing still on earth (i.e. not on a 
roller-coaster), you only feel a constant force pulling you downward (i.e. 
gravity).  Thus you are only experiencing acceleration in one constant 
direction, and you don't get dizzy.

One would think that while standing on a spinning earth, you would feel 
lots of different angles of acceleration.  But this is not the case.  The 
fact that the earth spins results in another force called the centrifugal 
force, which points in the direction away from the earth's axis of 
rotation.  It is greatest the farther you are away from the rotation axis 
(i.e. at the equator), and non-existent at the north and south poles.  
However, even at the equator, this force is very tiny (only 0.3% of the 
gravitational force) - so you don't notice it at all.  The reason it is so 
small is simply that the earth doesn't rotate fast enough.  

Now let's say that we had the power to speed up the earth's rotation.  
Then, the centrifugal force would become quite large.  Let's say, for the 
sake of argument, that the earth was spinning so fast that the centrifugal 
force became 50% of the gravitational force at the equator.  If you do the 
calculations you will find that the earth is now spinning 13 times faster 
and the day lasts for about 1.8 hours rather than the usual 24.  Then, at 
the equator, you would weigh 50% what you do now - so you could jump very 
far!  But the direction the force you feel is still directly downward and 
is constant, so you will not become dizzy (unless perhaps if you try to do 
a long-jump).  

Interestingly, if you were in the northern hemisphere (say in the US), the 
direction of the "apparent" force of gravity would not be simply 
downward-pointing, but it would have a sideways component as well.  This is 
because the centrifugal force is perpendicular to the axis of rotation of 
the earth, not the ground.  Thus, gravity would appear to point 
significantly tilted toward the equator and you would have to tilt yourself 
toward the north in order to avoid falling over!  You would then have to 
walk around this way, always tilted to the north!  But you still wouldn't 
"feel" the motion of the earth or feel dizzy since the direction of this 
force is constant. 

Now let's say we speed up the earth's rotation even more - to about 18 
times faster than normal.  Now the day is about 1.3 hours long.  Under 
these conditions, there will not be any downward force at the equator at 
all because the centrifugal force will be large enough to completely cancel 
out gravity.  Thus you would be floating like an astronaut at the equator. 
 In these zero-g conditions, you become terribly dizzy.  Astronauts have a 
terrible time with this and usually have to take drugs (like scopolamine) 
and do a lot of practice in these conditions in order to avoid constant 
dizziness and nausea while in space. 

If we speeded up the earth's rotation even more, you would have to hang on 
to the earth in order to avoid being sucked out into space!  So under these 
conditions, you would definitely "feel" the earth's rotation - even though 
the forces are always constant and pointing in the same direction (away 
from the earth's surface).  Of course the earth would disintegrate under 
these conditions - so this is completely fictitious.

So the main reason we don't notice the effects of the rotating earth is 
that it is not rotating fast enough.  

I hope this answers all of your questions!

Your MAD scientist,

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