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Lecture4 - Tides A tidal force arises from the differential...

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Tides A tidal force arises from the differential gravitational force on different sides of the same object. ¾ E.g. the pull of the Moon is stronger on the side of the Earth that faces it, than on the other side ¾ This causes the oceans to “bulge” The differential force is 3 r dF ¾ This raises a tidal bulge, which itself exerts a force 6 r dF ¾ Thus tidal forces diminish quickly as objects are separated Rotation causes the tidal bulges to advance, and the “frictional” tidal force changes the orbits and induces synchronous rotation ¾ the Moon always show the same face to Earth, because the drag force caused by the tidal bulge has caused the Moon’s rotation period and orbital period to be the same ¾ This effect causes Pluto and Charon to always show the same face to one another (as Earth and Moon will do eventually) ¾ Similarly, Mercury rotates exactly 3 times for every two orbits of the Sun, which ensures Mercury’s tidal bulge always aligns with Sun at perihelion.
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