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Q & A: tides and gravity

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Most recent answer: 03/26/2015
Q2. At noon, the sun and the earth pull the objects on the earth’s surface in opposite directions. At midnight, the sun and the earth pull these objects in same direction. Is the weight of an object, as measured by a spring balance on the earth’s surface, more at midnight as compared to its weight at noon?
- dhanashree shahane (age 19)

It's true that the Sun's gravity has effects here on measured weights. However, the average gravity from the Sun has no effects on those weights. That's because it causes equal accelerations of the Earth and the object being weighed, giving no effect on their relative positions. That absence of effects from uniform gravitational fields is called the Equivalence Principle, and forms the basis of General Relativity.  

The Sun's effects aren't quite uniform, however, since one side of the Earth is closer and the other farther from the Sun. The parts closest to the Sun (noon) are pulled more than average toward the Sun, reducing their weight. The parts farthest from the Sun (midnight) are pulled less than average, which also reduces their weight! The parts in between (dawn, dusk) weigh more. These effects are what causes tides. The tidal effects from the Moon are even bigger from the Sun, because the Moon is so much closer. Even though it has less gravitational pull, the difference in strengths between different places on Earth are larger for it.

Mike W.

(published on 03/26/2015)

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