Second only to oxygen, silicon is the most abundant element in Earth's crust. It is found in rocks, sand, clays and soils, combined with either oxygen as silicon dioxide, or with oxygen and other elements as silicates.
Gravity is a force that attracts objects. It is also the force that makes objects fall down when you drop them. Because of gravity things on earth have a certain weight. Gravity on Earth pulls objects to the centre of the planet. Every time you throw a ball up into the air it is pulled back to Earth. This occurs in nature all the time.
The force that is pulling the particle down is gravity, f = m*a. As the particle gets larger and denser, m (mass) increases. The opposing force is the friction of the water, which also increases with the size of the particle and its shape (with more friction as its surface area increases) . So for example, with two particles of the same mass and density, the one with the larger surface area (thus more friction) will settle slower. Two particles of the same size and shape, but different density, the one with the higher density (more mass) will settle faster.