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Lecture 9

AS101 Lecture Notes - Lecture 9: Net Force, Scientific Method, Weightlessness

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Patrick Mc Graw

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Astronomy Lecture 9
Why doesn’t the Moon fall down?
- What goes up, always comes down
- What goes up must come down unless the only force acting on it is the gravity, or unless the net
force is acting down.
Revisiting action-reaction
- A change in velocity is a change in acceleration
- Since the moon is in a curved orbit there should be force there
- The force of gravity is playing the same role the string is playing
- What way would the moon go, if the earth disappeared? It would move up and down
- It comes in the law of inertia
The Universal theory of gravity
- Everything that has mass, is being felt by everything else
- Gravity is force that is always attracted to an object
- Its dark, because we can see it
- The gravitational force between two objects depends on the product of their masses
- Earth’s mass is much greater than a building’s mass
- The gravitational force between two objects also depends on the inverse-square of their
- Force is going to decrease, the further your object gets away from you
Orbital Motion
- The orbiting bodies are failing toward the center of the Earth
- Body in orbit is always falling but is always missing due to horizontal velocity
- There is a net force toward the center of the earth , but it always keeps going on in the circle
- Weight is the acceleration due to the cause of the gravity
Newton’s Cannon
- Once the object orbits around the earth, it is going to stay there
- If launch it at 11km per second, it will launch so fast that it will never come back to the earth
Why do the planets orbit the Sun?
- The sun and the earth both move
- The sun moves, but not as much as the earth
- Always closer to the more massive body, and sometimes within it
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