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.
Gravity can be thought of as a downward force because it pushes and keeps everything down onto the ground. For this reason, any force that goes in the opposite direction of gravity, such as buoyancy, is an upward force..