Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Wessel Jurrius on 20/12/2016 09:49:45
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Wessel Jurrius asked the Naked Scientists:
If you take a very wet tennis ball for example, and you throw it away from you in any direction… as it bounces the water is sprayed in a vertical direction/plane. Or another way of looking at it is the water is prayed off of the ball at the top and centre of the ball due to the centrifugal forces and the shape of the ball.
With this in mind, why is there little to no effect on the water masses at the earth's equator in terms of (ocean) water tides? The earth rotates about its own axis and should (according to this simple experiment as above) show some kind of displacement of the water, should it not?
I do realize that the earth is rotating at a constant angular velocity – could this mean that the tides remain fairly unchanged because the effect of the centrifugal forces on the water masses, does not allow the water levels to decrease, hence remaining at a high level?
I am a mechanical engineer, so I do know something about physics, but I'm not schooled in geology or other studies involving the earth.
I would appreciate it if you could help me understand this, otherwise I will probably be speculating about this for years to come!
Best Regards
Wessel Jurrius
What do you think?
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The spin of the earth on it's axis does create a centrifugal force but this is even around the circumference and very small compared to gravity so there is a very, very, slight increase in the earth's diameter at the equator, but no effect on the tides.
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The earth rotates about its own axis and should ... show some kind of displacement of the water, should it not?
Yes, all rotating stars and planets have an equatorial bulge (https://en.wikipedia.org/wiki/Equatorial_bulge), caused by centrifugal force. This causes them to form the shape of an oblate spheroid, instead of the sphere that you would expect for a non-spinning planet or star.
In the case of the Earth, this equatorial bulge represents a 43 km difference in diameter between the equator and poles (compared to 12,000 km for Saturn).
For a planet, gravity holds the water close; for a tennis ball it is the surface tension of the water.
why is there little to no effect on the water masses at the earth's equator in terms of (ocean) water tides?
Earth's tides have a daily amplitude of a few meters change in diameter, superimposed on this static equatorial bulge of 43 km diameter.
If the rotational speed of a planet or star were sufficiently fast so that gravity were almost entirely counteracted at the equator, matter would spin off at the equator, angular momentum would be lost, and the rotational speed would slow down. It is thought that this effect would prevent the formation of millisecond pulsars (https://en.wikipedia.org/wiki/Millisecond_pulsar) spinning at more than 1500 revolutions per second.
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If you go to here (http://www.thenakedscientists.com/forum/index.php?topic=69402.0) you will find two pdf:s that I 'linked' on Newton. There's a nice description of tidal forces in them too.