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.
Four Forces Affect Things That Fly: 1  Weight is the force of gravity. It acts in a downward directionâtoward the center of the Earth. 2  Lift is the force that acts at a right angle to the direction of motion through the air. Lift is created by differences in air pressure. 3  Thrust is the force that propels a flying machine in the direction of motion.