Gravity is one of the most obvious forces that exists. Isaac Newton famously discovered that matter attracts all other matter with a force directly proportunate to the mass of the objects involved and inversely proportunate to the square of the distance between them.
The acceleration of a falling object on Earth due to gravity is 9.81 Meters per Second; this is constant in a vacuum for all particles, but when testing this fact on Earth, air-resistance should be taken into account. Then Einstein created a ground-breaking theory for Neoconservatives and Conservapedia to whine of. Gravity is different on different planets and in space. For example, on the moon it is about 6 times less. Also, being in a rapidly falling closed vessel can give a feeling of 0 gravity.
Einstein improved on Newton's theory of gravity with his Theory of Relativity.