Easy! the formula is Gravity x Time(seconds) = Acceleration, gravity pull is basically 9.8m/s^2 (im not sure how to put squared so i used "^2" to symbolize it) and so basically if you drop a rock on...
The 'quick' part of the word quicksand comes from a very old meaning of the word quick - alive, living - because the sand moved and was not like normal sand.