Newton’s law of universal gravitation states that all objects are attracted to each other by gravity, with the force of attraction depending on mass and distance. It was detailed in his book Principia Mathematica in 1687 and used to predict the position of celestial bodies. Einstein’s theory of general relativity replaced it, but it remains accurate for practical applications.
The law of universal gravitation is an essential principle of physics. It was first codified by Sir Isaac Newton in the 1600s. It states that all objects are attracted to each other by gravity; the force of attraction depends on the mass of the objects and decreases with the distance between them. Newton’s discovery was replaced by Einstein’s theory of general relativity. It’s still accurate, however, for most practical applications.
Newton did not discover gravity, as popular belief holds, but he expanded on the work of earlier scientists like Galileo. Newton was referring to these scientists when he famously wrote, “If I have seen further, it is by standing on the shoulders of giants.” The fall of an apple inspired Newton to study the subject of gravity; however, the apple did not bring immediate understanding by hitting its head. Instead, he used the moon’s orbit around the Earth to check and confirm his calculations over a 20-year span. The law of universal gravitation was detailed in his groundbreaking book Principia Mathematica, published in 1687.
Newton’s book included mathematical formulas describing the law of universal gravitation. In essence, the law states that all objects exert a gravitational pull on all other objects. Objects with large masses have stronger gravitational areas, or gravitational fields, which is why objects and people are attracted to Earth, but not noticeably to each other. The gravitational pull decreases with increasing distance; this decrease can be measured precisely and is known in physics as the inverse square law. Universal gravitation is the force that keeps planets and satellites locked in orbit, rather than traveling freely through the universe.
In the centuries following Newton’s life, the law of universal gravitation was used to predict the position of natural planets and satellites that had not yet been discovered. Any discoveries of these celestial bodies confirmed that the law was correct. One aspect of the law that Newton could not explain was how gravitational force is transmitted between objects. Other fundamental forces, such as electromagnetism, work because subatomic particles travel between objects, pulling them towards each other. A similar gravity-carrying particle, the graviton, was described in theory, but remains unknown more than 300 years after Newton’s work.
By the 20th century, scientists had discovered minor inconsistencies in the law of universal gravitation. These inconsistencies were explained by Einstein’s theory of general relativity. Einstein realized that, in fact, the masses of celestial bodies affect not only each other, but the fabric of space-time around them. These effects are only evident in very precise measurements and calculations. For practical applications like rocket launches, the law of universal gravitation is still accurate and much easier to calculate than the effects of relativity.
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