在太阳系，大多数的行星都各自拥有统一的圆周轨道。
In our solar system, planets for the most part have fairly uniform circular orbits.
克普勒圆周轨道第三定律；利用积分推导球壳的引力；弹簧。
Newton's law of gravity; Gravitational force from a shell; Hooke's 'law'.
开普勒圆周轨道第三定律；利用积分推导球壳的引力；弹簧。
Kepler's 3rd law for circular orbits; Building up the shell-force by integration; Spring gun.
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