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, or gravitation, is one of the fundamental forces of the universe. In everyday use, it describes the force which causes objects to fall onto the ground. Newton's laws discusses how gravity keeps the Solar System together and Einstein's theory of general relativity is about the role of gravity in the universe.