Gravity is an attractive force responsible for the formation and movement of celestial bodies. Newton’s law of universal gravitation is widely used, but Einstein’s theory of general relativity and string theory provide more accurate explanations. Weightlessness is achieved through free fall, not by being outside a gravitational force field.
In essence, gravity is an attractive force between objects. Most people are familiar with gravity as the reason things stay on the earth’s surface, or “what goes up must come down,” but gravity actually has a much broader meaning. Gravity is responsible for the formation of our Earth and all other planets and for the movement of all celestial bodies. It is gravity that makes our planet go around the Sun and the Moon around the Earth.
While humans have always been aware of gravity, there have been many attempts to explain it accurately over the years, and theories need to be regularly improved to account for aspects of gravity not previously considered. Aristotle was one of the first thinkers to postulate the reason for gravity, and his and other early theories were based on a geocentric model of the universe, with the Earth at its center. Galileo, the Italian physicist who made the first telescopic observations in support of a heliocentric model of the solar system, with the Sun at its centre, also made great strides in the theory of gravity towards the end of the 17th century. He discovered that objects of varying weights fall towards the Earth at the same speed.
In 1687, English scientist Sir Isaac Newton published his law of universal gravitation, which is still used to describe the forces of gravity in most everyday contexts. Newton’s first law states that the force of gravity between two masses is directly proportional to the product of the two masses and inversely proportional to the square of the distance between them, or mathematically: F=G(m1m2/d2), where G is a constant .
Newton’s second law states that gravitational force is equal to the product of a body’s mass and its acceleration, or F=ma. This means that two masses that are gravitationally attracted to each other experience the same force, but that results in a much greater acceleration for a smaller object. Therefore, when an apple falls towards the Earth, both the Earth and the apple experience the same force, but the Earth accelerates towards the apple at negligible speed, as it is much more massive than the apple.
Towards the end of the 19th century, astronomers began to notice that Newton’s law did not fully account for the gravitational phenomena observed in our solar system, particularly in the case of Mercury’s orbit. Albert Einstein’s theory of general relativity, published in 19, solved the problem of Mercury’s orbit, but has since been found to be incomplete as well, since it cannot explain the phenomena described in quantum mechanics. String theory is one of the main modern theories to explain quantum gravity. While Newton’s law is not perfect, it is still widely used and taught due to its simplicity and close approximation to reality.
Since the gravitational force is proportional to the masses of the two objects that undergo it, different celestial bodies exert a more or less strong gravitational force. Because of this an object will have different weights on different planets, being heavier on more massive planets and lighter on less massive planets. This is why humans are much lighter on the Moon than on Earth.
It is a popular misconception that astronauts experience weightlessness during space travel because they are outside the gravitational force field of a large body. Indeed, weightlessness during space travel is actually achieved thanks to free fall: the astronaut and the space shuttle or rocket are falling (or accelerating) at the same speed. The speed itself gives the notion of weightlessness or buoyancy. This is the same concept as a person on a “free fall” ride at an amusement park. Both the rider and the carousel are falling at the same rate, making it appear that the rider is falling regardless of the ride. The same sensation can be experienced while riding an airplane or an elevator that suddenly breaks out of its normal pace of decent.
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