
Hello, A couple of my students were wondering
about the following questions, and rather than
give a partially correct answer I was hoping you
could give me the complete answer. Thank you so
much. 1. Why is there only some gravity on
earth. Yes, I know that 1/6 of the earth's
gravity is found on the moon, but why? 2. If
there was no gravity on earth would there be
hurricanes and tornadoes?

Question Date: 20040910   Answer 1:
The value of the gravitational acceleration on the
surface of a planet of radius R and mass M is
g=GM/R^2 . So the gravity on the moon is 1/6th
that of Earth because the moon is far less massive
than the Earth and has a different radius(R) as
well. The mass of the moon is 1/82 nd of the
Earths and the radius of the Earth is 6300 km
whereas for the moon its only 1738 km. There
are motions in the atmosphere and oceans because
of the rotation of the Earth and because of the
gravity. . If there was "no Gravity" the earth
would not be here! The Earth accreted from a bunch
(trillions upon trillions) of rocks. What forced
these rocks to accrete? GRAVITY!!!!!!!!!! So
with no gravity the earths atmosphere would drift
off to space within a few years!!! no atmosphere,
no terrible IVAN's !!!!!!!
  Answer 2:
Every object in the Universe, planets, stars etc
and also smaller objects such as moons exert a
gravitational pull. This means that other objects
will be drawn towards them. The strength of the
attraction is directly related to the size of the
planet or moon. Therefore as the moon is a
fraction of the size of the earth it's
gravitational pull is 1/6th as strong as that of
the earth. Gravity is a basic property of every
object in the universe and cannot be 'shared'
between objects as your question suggests. We
expect the moon to have less gravity because it is
smaller, the force felt by people walking on the
moon is independent to any force felt by people
walking on the earth. If there was no gravity
on earth there would be no atmosphere (i.e. no air
to breath) so there would be wind at all. There
would also be no life on earth!!Weather patterns
such as hurricanes and tornadoes are caused by
complex interactions between rising and sinking
air (convection) as a result of the suns heat. You
could look into this in more detail on the web
probably. They are not affected by gravity
however, gravity is a constant force.
  Answer 3:
The gravitational acceleration that we feel on the
surface of the Earth is (approximately) governed
by the following equation: a = GM/R^2
where: G = the Gravitational constant (which
presumably is the same everywhere in the
universe) M = mass of the Earth (in kilograms,
for example) R = radius of the Earth (in
meters, for example) For the Earth, if you plug
in the appropriate numbers, you get acceleration
equals 9.8 meters per second squared. Since the
Moon or another planet or the Sun have different
values for mass and radius, you would get a
different answer for the gravitational
acceleration on the surface of that object. For
example, the Moon's radius is 0.27 of the Earth's
and the Moon's mass is 0.012 of the Earth's mass.
So if you take 0.012/(0.27)^2 you get .16 which
is about 1/6. If there was no gravity on Earth
there wouldn't be hurricanes or tornadoes because
there wouldn't be an atmosphere! Gravity is
necessary to keep the atmosphere from literally
floating away.
  Answer 4:
The acceleration of gravity on the surface of the
earth (or any spherical planet) depends both on
the planet's total mass and its density. To a
good approximation the acceleration a =
G*M/(R*R) where G is the gravitation constant, M
is the mass and R is the radius. So, onthe
Moon, the mass is much less, but the radius is
also much lower,giving the Moon a surface
acceleration of about 1/6 that of the earth.Since
the two bodies are so close (238,000 miles)lunar
gravity does affect the earth, mainly this is
visible in the tides. (Earth's gravity keeps the
moon in orbit around the Earth). Without gravity,
there could not be hurricanes and tornadoes since
both rely on energy gained from falling air masses.
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