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
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
According to Newton’s second law, this force can also be written as
Equating the two, we see that the acceleration does not depend on the
o g= Gm earth
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
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
o Because the side of the earth towards the moon is closer, the moon
pulls on it more