# Equatorial Bulge

by Sheneron
Tags: bulge, equatorial
 P: 360 What causes of the equatorial bulge? I have tried looking it up online but I am having some trouble finding a good explanation.
 Mentor P: 14,242 The Earth is spinning at one revolution per day.
 P: 360 So then is it because the tangential velocity at the equator is greater than of that at the poles?
Mentor
P: 14,242

## Equatorial Bulge

Think about how people make a big flat pizza starting from a ball of dough.
 P: 360 So its just because its spinning?
 Mentor P: 14,242 I have two questions I would like you to answer honestly before I answer any further: (1) Is this homework, and (2) how much physics/math have you taken in school?
 P: 360 1. No its not homework 2. I am just starting physics
 P: 360 however, if you would like it to be a homework problem i can certainly make it one
 Mentor P: 14,242 OK. I'll keep this simple. The simple answer is that the Earth is spinning and is not perfectly solid. Like the pizza dough, the Earth tends to flatten out a bit because of this spinning. What the Earth does (and what the spinning pizza does) is to change shape in a manner that minimizes the total energy of the Earth (or pizza). If the Earth were not spinning, the only source of energy would be the Earth's gravity. Gravity radiates spherically, so this minimum energy configuration would be a sphere. Any deviations from purely spherical would create an Earth with more potential energy than the purely spherical form. The spinning adds another energy source, kinetic energy. This extra source of energy makes it so the shape that minimizes the energy and conserves angular momentum is no longer a sphere. It is instead an oblate ellipsoid. This represents two very key concepts in physics. The concept of a system moving toward the minimum energy configuration is called Hamilton's principle. Conservation of angular momentum, along with conservation of linear momentum and conservation of energy, are cornerstones of a lot of physics and engineering.
P: 11
 Quote by D H What the Earth does (and what the spinning pizza does) is to change shape in a manner that minimizes the total energy of the Earth (or pizza).
DH, just trying to get my head around this quote of yours. If the spinning results in a minimizing of the total energy, then there must have been more energy at an earlier time. If so, where does this energy go?
Secondly, isn't any change in the shape of a spinning object just the result of the centripetal, or opposite, forces? And thus energy is conserved?
 P: 5 Part of an answer to this question is in another related question that my mother recently asked me. Why are so many heavenly bodies so close to being perfect spheres? It is important to realize that when you are talking about objects as large as planets, the pressure exerted on subsurface rock causes it to behave in a ductile manner. Not to say that everything beneath the earth's crust is liquid magma, the earth is internally very rigid on human timescales, but over millions of years the earth (and other planets)behaves like a huge drop of liquid. It doesnt take much rotation to cause distortion. Another interesting tidbit! Say, for example, the earth WAS a perfect sphere. Forces on the surface would be asymmertrical and free objects like parked cars (without the E brake on) would role towards the equator.

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