There are some people who worry that when they’re outside, if they don’t keep a good grip on the ground, they’ll just go flinging off into space. They needn’t really worry about this, because gravity generally keeps that sort of thing from happening. The thing is, no one is really sure what causes gravity, but the effects have been studied by many physicists and astronomers. Three of the more obvious effects of gravity are things falling down, weight, and the the moon and planets staying in their orbits. Things fall down.
People have generally grown to accept that if one lets go of ne’s prized and valuable textbook when walking through a mud puddle, the book will invariably end up in the puddle and therefore be stripped of all value and even legibility. Things fall down because there is a strong gravitational attraction between things of great mass, like the Earth, and things of little mass, like a book. The only problem with this relatively simple explanation is that no one really knows why it’s like that. What people have figured out so far is that gravity is a force, and a force is anything that changes the state of rest or motion of an object.
In the absence of outside forces, the momentum of a system remains constant. This means that if there was no gravity, when one would relinquish one’s hold on the textbook, it would remain at rest in the air. If a force acts on a body, the body accelerates in the direction of the force. In the example of the force of gravity, small things like textbooks are pulled downward toward the center of the large mass of the Earth, not up into space, even if some people think that this might happen. Torgerson 2 Isaac Newton was the first to conceive of weight as the gravitational attraction between a body and the Earth.
The force that results from the gravitational attraction of the Earth on bodies at its surface is what we call weight. Science has chosen to measure the mass of objects in units that are roughly equivalent to the weight of those objects on Earth. For example, if a textbook weighs four pounds on Earth, it would have a mass of four pounds in an orbiting spaceship. The textbook would be “weightless” because it does not feel the gravitational attraction of the Earth, but, even in outer space, to push the textbook from one place to another, someone would have to exert a force sufficient to vercome the inertial mass of four pounds.
If that same textbook which weighs four pounds on Earth, was placed on the surface of the much more massive planet Jupiter, the book would weigh 15. 76 pounds, because of Jupiter’s stronger gravitational attraction. Newton was also the first to assume correctly that the same force of gravity that causes objects to fall and to have weight, also explains the movement of astronomical bodies. Newton stated that every particle is attracted to every other particle with a force directly proportional to the product of their masses and inversely proportional to the quare of the distance between them.
It was natural for him to turn his attention to celestial bodies, since, on Earth, the distance between a body and Earth could not be varied greatly, and the force of attraction between two different bodies on Earth was too feeble to be detected by methods available at that time. The astronomer Johannes Kepler knew that planets moved in ellipses, but he didn’t know why. He sensed that planetary bodies and the sun probably had a natural affinity for each other, and that possibly magnetism was involved.
By studying Kepler’s observations, Newton derived laws that escribed how the force of gravity acted. He was the first to declare that the pull of the Torgerson 3 Earth extended to infinity. He showed that a projectile launched at sufficiently great speed, in a direction parallel to the Earth’s surface, would not fall down to Earth, but would fall around and around the Earth. So, too, the moon continuously falls around the Earth, and the planets fall around the Sun.
Each of these objects balances the Sun’s force of gravity with its own momentum, that is, with its innate tendency to keep moving in the same irection at the same speed. Take away the Sun’s force of gravity, and the planets would sail into space, in whatever dircetion at whatever speed they happen to have. Take away a planet’s momentum, perhaps with a giant hand that stops it in orbit, and it would head straight for the Sun, drawn by the Sun’s gravitational force.
But, because both gravity and momentum are at work, the planets “fall”, not down, but around, forever as they move in elliptical, but nearly circular, orbits. Working with insight that has never been exceeded since, Isaac Newton showed hat by making one simple assumption, that all objects in the universe attract one another in accordance with a simple relationship, he could explain the fall of objects to the ground, the moon’s orbit around the Earth, and the planets’ motions around the Sun.
The relationship Newton conceived was this: Every object attracts every other object with an amount of force that varies in proportion to the product of the two objects’ masses, and in inverse proportion to the square of the distance between the centers of the objects. It is plain to see that gravity is quite important. The effects of gravity have been studied by many brilliant physicists and astronomers, but still the cause of gravity remains unknown.
The law of gravitation plays a very prominent role in our view of the universe; it is the force of gravity which causes a book to fall in a mud puddle, which holds our planet in its orbit around the Sun, which provides a reason for the existence of weight, yet the law of Torgerson 4 gravity was unknown to humankind until rather recently in history. Isaac Newton is the genius who hypothesized that the heavens and the Earth are united by this single, all encompassing principle.