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为什么地球不是一个球?

我们都在学校学习,地球是一个球体。实际上,它更近一个稍扁球体——一个扁椭球面,也称为一个扁球。这是一个椭圆的短轴旋转。这一现象的物理原因是什么?

回答

地球大约是一个扁球是最好的解释为能量。放置一个大理石碗里。不管你在哪里的地方,它最终会在碗的底部。这个位置的总能量最小化大理石在碗中受到的约束。暂停两个职位之间的链。链来的时候需要在一个著名的形状,悬链线的曲线。这是最小化的能量链的形状,受约束的两篇文章之间的暂停。如果你把大理石远离底部将前一段时间来休息。如果你拉链式远离悬链线形状会来回摆动一段时间来休息之前的稳定形状。不平衡的大理石和平面外链比他们做的更大的潜在能量,稳定的配置。 If at all possible, nature will attempt to minimize total potential energy. It's a consequence of the second law of thermodynamics. In the case of the Earth, that minimum energy configuration is a surface over which the sum of the gravitational and centrifugal potential energies are constant. Something that makes the Earth deviate from this equipotential surface will result in an increase in this potential energy. The Earth will eventually adjust itself back into that minimum energy configuration. This equipotential surface would be an oblate spheroid were it not for density variations such as thick and light continental crust in one place, thin and dense oceanic crust in another. In terms of force, the quantity we call <i>g</i> is the gradient of the gravitational and centrifugal potential energies (specifically, $\vec g = -\nabla \Phi$). Since the Earth's surface is very close to being an equipotential surface and since that surface in turn is very close to being an oblate spheroid, gravitation at the poles is necessarily slightly more than it is at the equator. This gravitational force will not be normal to the surface at places where the surface deviates from the equipotential surface. The tangential component of the gravitational force results in places where water flows downhill and in stresses and strains in the Earth's surface. The eventual responses to these tangential forces are erosion, floods, and sometimes even earthquakes that eventually bring the Earth back to its equilibrium shape. ---- **Update: Why is this the right picture?** Based on comments elsewhere, a number of people don't understand why energy rather than force is the right way to look at this problem, or how the second law of thermodynamics comes into play. There are a number of different ways to state the second law of thermodynamics. One is that a system tends to a state that maximizes its entropy. For example, put two blocks at two different temperatures in contact with one another. The cooler block will get warmer and the warmer block will get cooler until both blocks are at the same temperature, thanks to the second law of thermodynamics. That uniform temperature is the state that maximizes the entropy of this two block system. Those two blocks only have thermal energy. What about a system with non-zero mechanical energy? Friction is almost inevitably going to sap kinetic energy from the system. That friction means the system's mechanical energy will decrease until it reaches a global minimum, if any. For a rotating, dissipative, self-gravitating body, that global minimum does exist and it is a (more or less) oblate spheroid shape.

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\ begingroup美元你有地震是因为地壳的偏差的例子的等势面而非构造应力?这个例子听起来奇怪的我……别的东西:重力可以正常表面即使偏离了大地水准面(甚至不正常不偏离)。 \ endgroup美元
- - - - - -Gaialogist
2014年4月29日17点
\ begingroup美元 @Gaialogist——关于你提到的第二个问题,大地水准面是等势面接近平均海平面。由于重力加速度是引力势的梯度,重力加速度向量必然是正常的大地水准面。它在数学。这是一个有关math.stackexchange.com回答:math.stackexchange.com/questions/122222/…。 \ endgroup美元
- - - - - -大卫Hammen
2014年4月29日17:14
\ begingroup美元你的第一个问题而言,这些构造应力是地球的一个直接结果远离流体静力学平衡或平衡形状。例如,岭推和板拉。 \ endgroup美元
- - - - - -大卫Hammen
2014年4月29日17:22
\ begingroup美元没关系,重力大地水准面但表面不正常有与大地水准面有重力正常或相反地。考虑表面接近和大地水准面平行但不叠加:它可能有一个正常的重力;考虑一个表面穿越大地水准面:跨越两个表面匹配但重力是地球表面的不正常。 \ endgroup美元
- - - - - -Gaialogist
2014年4月29日,在十七34
\ begingroup美元为我的第一个问题,我同意平衡参数解释(或在)地球的任何运动。我只是认为这是大胆的切向分量的重力向量之间的联系和地震。也许这种观点甚至可能错误地逆转原因和后果(大地构造对重力异常的影响,而不是相反)…… \ endgroup美元
- - - - - -Gaialogist
2014年4月29日

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