Chapter 8 Universal Gravitation

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Chapter 8 Universal Gravitation
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Universal Gravitation

• A popular legend is that Newton
• discovered gravity when an apple landed
• on his head. The truth is he expanded on
• the ideas of Galileo and discovered that
• gravity is universal. What Newton
• discovered is that every mass in the
• universe attracts every other mass in the
• universe regardless of whether an
• atmosphere is present or not. Newtons
• laws are what make the idea of gravity
• easy to understand.

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Falling

• Have you ever been on an amusement
• park ride that dropped suddenly? What did
• you feel? You may have described the
• feeling as being weightless. This is what
• happens to the astronauts that are orbiting
• the Earth. Their ride, the shuttle, is
• dropping out from under them faster than
• the gravity of the Earth can pull them
• down. Due to Newtons 1st law an object
• would travel in a straight line unless a
• force acts upon it.

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Falling

• What Newton was able to prove was that
• all objects that orbit are falling. Newton
• showed that the amount an object fell on
• Earth was proportional to the amount that
• an orbiting body fell. Newton figured out
• how far something fell in a second on
• Earth. He then hypothesized that if the
• moon were falling it should fall
• proportionally to something falling on
• Earth. He was correct and was able to
• prove it using a mathematic model that
• held true for all objects.

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Newtons Law of Universal Gravitation

• Again Newton did not discover gravity,
• what he did discover was that gravity is
• universal. Newton showed that the only
• thing that affected gravity were mass and
• distance. The equation
• FG(m1m2/d2)
• F force mmass ddistance
• Guniversal gravitational constant which is
• 6.67x10-11 Nm2/kg2. What G tells us is that
• gravity is a weak force, only when mass is
• very large can we sense gravity.

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Gravity and Distance

• As the distance between object increase
• gravity decreases dramatically. This
• phenomenon is called the inverse square
• law. This law shows for gravity the greater
• the distance from the center of Earth the
• less the object will weigh or the less
• gravity acts on an object, however the
• force of gravity never goes to zero. This
• shows that elevation does affect your
• weight however to get a significant change
• you must move great distances.

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Gravity and Astronauts

• Now does our argument that astronauts,
• and anything that orbits something else,
• are falling hold true? Explain.
• If the astronauts, and all orbiting objects,
• were stationary with respect to the orbiting
• object they would fall straight toward the
• center of the planet. The fact that the
• astronauts are traveling at an incredible
• speed keeps them from falling into the
• Earth.

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Gravity and the Universe

• Why is the Earth round? It and all of the
• other celestial bodies are round because
• of gravity. Since everything attracts
• everything else all matter arranges itself in
• such a way that to maximize attraction.
• Now if everything pulls on everything else
• then the planets should pull on each other
• and they do. Planets will wobble in their
• orbits when planets get close to each
• other. Neptune was discovered in 1845 in
• this way. Uranus had a perturbation and
• astronomers found Pluto.