Forces and Motion




Whirlpool Galaxy, M51, image from NASAAll things that have mass….ANY mass… have gravity. Gravity, as we now understand it, is one of the four fundamental forces in nature; the other three being electromagnetic force, strong nuclear force, and weak nuclear force; more about these later. Together, these forces bind the matter in the universe together, indeed, bind the universe together, and make the formation of galaxies, planets, and people possible. Gravity is an attractive force that accelerates objects. In this way, gravity is an acceleration.


So far, we have not found an anti-gravity (repulsive) force like you may have read about in science fiction stories. Galaxies have gravity, a LOT of gravity, so they often orbit each other in clusters called (duh!) galactic clusters. The gravity to hold them together comes from their huge masses. A typical spiral galaxy might have a mass of around 100 billion times the mass of our sun (said as “100 billion solar masses”). You have gravity too, just not very much. All things that have mass have gravity, so all thing are pulled towards each other, but not by much unless the gravity is huge.


The Earth has gravity. When you jump up, you do not continue to travel upward into space, never to be heard of again. Thankfully, the Earth’s gravity pulls you back. Early philosophers and scientists did not know about gravity. They wondered that if the Earth is spinning, why don’t we fly off? This argument was used for over a thousand years to support the idea that the heavens revolved around a fixed and motionless Earth. Sir Isaac Newton thought a lot about gravity. He saw that things (including apples) fall toward the Earth with increasing velocity. The longer they fall, the faster they go. Newton theorized that if the force of gravity reached to the top of an apple tree, then maybe it reached well beyond the tree tops. Maybe, it reaches all the way to the Moon and so is responsible for keeping the Moon in orbit around the Earth! Of course, he was right.

(Right: Newton from Wikipedia)

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