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ASTRO101: Newtonian gravity
- Course
- ASTRO101 (ASTRO101)
- Institution
- University Of Alberta (UofA)
Understanding Newtonian gravity
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Gravity is the force that keeps us standing on Earth's surface. It's the reason that a ball thrownupwards falls back down towards the ground. It was Newton who first realized that this force,
gravity, doesn't just affect physical objects here on earth, but is also responsible for the motion of
the stars and planets. Gravity keeps the earth moving in orbit around the sun and the sun in orbit
around the supermassive black hole at the center of the Milky Way galaxy. Gravity is a central
principle in black hole physics because it's gravity that gives black holes their extreme properties.
Until the year 1687, the year that Isaac Newton put forth his vision of gravity, no one had a clear
understanding of what causes the attraction of objects towards the ground. A similarly mysterious
force was also keeping the Earth moving around the sun. Even in antiquity, humans understood that
something held objects in place, but lacked the mathematical description. It was Newton who
provided the first empirical description of how gravity works. Although Newton was the first to
explain gravity mathematically, almost exactly 100 years earlier in 1589, Galileo Galilei was busy
investigating gravity and his observations greatly advanced our understanding of the interaction
between objects and their masses. Galileo theorized that falling objects of different masses would
fall at the same rate contrary to the Aristotelian belief that heavy objects fall faster than light objects.
It's famously claimed that to prove this idea, Galileo climbed up the Leaning Tower of Pisa and
dropped two cannon balls with different masses one heavier and lighter. He observed that if both
cannonballs were dropped simultaneously they hit the ground at precisely the same time
independent of their weights. Galileo made the mistake of assuming that the gravitational force was
a constant between two objects with no relationship to the distance between them. Historians
disagree whether this experiment really took place because it's first mentioned almost 65 years
after it's supposedly took place in a biography of Galileo by Vincenzo Viviane.
One experiment done during Apollo 15's mission to the moon demonstrates the principle that
Galileo addressed. At the end of the last moon walk, astronaut David Scott performed the same
demonstration that Galileo did with a hammer and a feather in the vacuum of space. The result of
course is visible in this famous video.
In my left hand I have a feather, in my right hand, a hammer. I guess one of the reasons we got here
today was because of a gentleman named Galileo a long time ago who made a rather significant
discovery about falling objects in gravity fields. And we thought that where would a better place to
confirm his findings than on the moon. So we thought we'd try it here for you and the feather
happens to be appropriately a falcon feather or a falcon and I'll drop the two of them here and
hopefully they'll hit the ground at the same time. How about that? So, Mr. Galileo was correct in his
findings.
, Shortly after Galileo's death, mathematician and astronomer Johannes Kepler observed that
planets trace ellipses through the solar system as they orbit the sun.
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Kepler famously described the motion of the planets mathematically, laying the groundwork for the
second last piece of the gravity puzzle which was solved by Christian Huygens, who in the 1660's
described the law of centrifugal force. Together with the help of Edmund Halley, Christopher Wren
and Robert Hooke, Isaac Newton had all the clues he needed to piece together the mathematical
description of gravity. In 1687 Newton's book Philosophiae Naturalis Principia Mathematica which
translate to the Mathematical Principles of Natural Philosophy. Newton laid the mathematical
foundations to explain all of gravitationally related phenomenon including apples falling from trees
and planets in orbit around stars.
Gravity is an attractive force between two objects that have mass. Any object that we talk about in
this course with the exception of light has mass. The earth has mass, I have mass, and you have
mass. There's therefore a gravitational attraction between the earth and me, the earth and you, but
also between you and I at any given time.
The mathematical description of the force of gravity needs to take into account the mass of both
objects, and also the distance between them. In order to get useful information out of any equation,
we also need a universal gravitational constant to tell us how strong the force will be given the
masses and the distances.
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