Question: what is the specific gravity of grvel, sand? 

Answer: Gravity affects every single object on Earth in the same way, by a pull of approximately 9.8 meters/sec squared, of course you have to take into account wind resistance, therefore an object such as a truck going off a building will drop slower than that of a baseball because the wind resistance will affect the truck more. So sand, gravel, the specific gravity is 9.8 meters/sec ( in a vaccuum, meaning where there is absolutely no air resistance but since places that this aren't normal you have to assume wind resistance.)
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