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Lecture

# October 18.docx

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Department
Astronomy
Course
AS101
Professor
Shohini Ghose
Semester
Fall

Description
October 18, 2011 The universal law of gravitation: 1. Every mass attracts every other mass. 2. Attraction is directly proportional to the product of their masses. (The more massive, the more attractive). 3. Attraction is inversely proportional to the square of the distance between their centers (Ex: increase distance by 3, decrease force by 9). Fg=G m 1 2 2 d Newton’s Theory of Gravity:  According to the law of gravitation, force of gravity between the Earth and an object of mass m o F=Gm m earth d2  According to Newton’s second law, this force can also be written as o F=ma  Equating the two, we see that the acceleration does not depend on the objects mass o g= Gm earth d 2 Mass vs. Weight:  In physics mass and weight are related but are NOT the same thing.  Mass is the amount of matter in an object  Mass in an intrinsic property of an object  Mass is the same no matter what forces are acting on an object  Weight is the force of gravity exerted on an object  Weight is directly proportional to the mass  Example: doubling your mass doubles your weight  Weight varies from planet to planet since the acceleration due to gravity varies Tides: Gravity in Action:  Tides are caused by small differences in gravitational forces o As the earth and moon orbit around each other, they attract each other gravitationally o Because the side of the earth towards the moon is closer, the moon pulls on it more
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