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Chapter 12

PHY 101 Chapter Notes - Chapter 12: Hohmann Transfer Orbit, Roche Limit, Gravitational Lens

Course Code
PHY 101
Dr.sharon Zane

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12-1 Newton’s Law of Universal Gravitation
gravity- the weakest fundamental force of nature; it is
responsible for the motion of the sun, moon, earth and
Newton’s Law of Universal Gravitation: F= G(m1m2)/r;
the force of gravity between any two point objects is mass m1
and m2 is attractive and of magnitude F given by the above
r is the distance between the masses, and G is the
universal gravitation constant; G= 6.67 * 10-11
each mass experiences a force of the same magnitude but in
opposite directions, the force between two objects forms an
action reaction pair
according to Newton’s law, all objects in the universe attract
all other objects in the universe by way of gravitational
In general, gravitational forces are only significant when large
masses are involved
The force of gravity decreases with distance; F varies as 1/r2,
it has an inverse square dependence on the distance
Gravity never completely vanishes; the force of gravity
between two masses also depends on the product of the
Superposition: when a given mass is acted on by
gravitational interactions with a number of other masses and
the net force acting on it is simply the vector sum of each of
the forces individually
12-2 Gravitational Attraction of Spherical Bodies
Newton’s law applies to point objects and using it to find the
force of gravity for objects of a finite size can be difficult
Uniform Sphere
The net force exerted by the sphere on mass m is the
same as if all the mass of the sphere were
concentrated at its center; F= G(mM/r2)
The force the earth exerts on a mass m is F= m(GMe/Re2)
where Me and Re are the mass and radius of the earth
m(GMe/Re2) = mg; g= GMe/Re2; this equation can be used
to find the acceleration due to gravity on other objects
in the solar system by using the mass and radius of that
r is the distance from the center of one object to the
center of the other
Weighting the Earth
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