Gravity is the force of attraction between all matter. Although gravity exists for even small amounts of matter, the force is usually not significant enough to detect or generate pull. Gravity is usually referred to in relation to planets or astronomical bodies -- massive objects that generate significant gravitational pull; enough to condense matter to form planets and keep them in orbit. In addition to mass, gravity also depends on the distance between two bodies, which is the reason why the Earth's gravity affects humans more than more massive bodies, such as the sun or Jupiter.
Effect of Distance on Gravity
Gravity is exerted between two bodies of mass -- on Earth, the Earth's gravity is the strongest and most significant to humans. This is due to the close distance between an individual and the Earth and is the reason why all bodies on Earth are not being pulled to the nearby, more massive sun. Because gravity depends on distance -- its strength is inversely proportional to the square of the distance from the object -- the force of gravity is also affected by altitude. At the top of a mountain, a person will experience less gravity than someone below sea level -- although for purposes of human sensory abilities, the difference will be undetectable.
Effect of Centripetal Force
On Earth, the gravitational pull is also depends on latitude. Because of the rotation of the Earth, there is a centripetal force that affects the weight of humans and the gravitational pull people experience. The centripetal force makes it such that someone on the Earth's equator, who is "traveling" at greater speed than someone at one of the poles, weighs less and hence experiences less gravity -- 9.789 meters per second squared vs. 9.832 meters per second squared.
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Gravity and the Earth's Atmosphere
Gravity is not only responsible for keeping humans and objects on the ground, but it is essential to life. Without gravity, the Earth would have ceased to form in the first place, and crucial elements to life would also cease to exist. Supposing that Earth was formed but that gravity was not present, life would not be able to survive because there would be no atmosphere and no concentration of crucial elements, such as water. All in all, gravity plays a significant part in the delicate balance of life on Earth -- it is the reason that humans are able to walk, build structures and live in a pressure-controlled environment.
Law of Universal Gravitation
The effect of gravity was first explained mathematically by Isaac Newton -- the famous story being that the Universal Law of Gravitation is the product of Newton watching an apple fall from a tree. While this may not be the exact circumstance under which he developed his Universal Law of Gravitation, it is certainly the reason people are able to understand the force of gravity and its relationship to the masses of bodies and distances between them.