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.
An upward force is any force that counteracts the force of gravity. Buoyancy is an example of an upward force because it pushes an object away from the Earth's center of mass. Force is any sort of push or pull interaction for an object. Forces tend to make objects change direction, change motion or stay in motion.