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Water On The Earth
How much water is there on Earth?
There's
a whole lot of water on Earth!
Something
like 326,000,000,000,000,000,000 gallons (326 million trillion gallons)
of the stuff (roughly 1,260,000,000,000,000,000,000 liters) can be found on our
planet.
This
water is in a constant cycle -- it evaporates from the ocean,
travels through the air, rains down on the land and then flows back to the
ocean.
The
oceans are huge. About 70 percent of the planet is covered in ocean, and the
average depth of the ocean is several thousand feet (about 1,000 meters).
Ninety-eight
percent of the water on the planet is in the oceans, and therefore is unusable
for drinking because of the salt.
About
2 percent of the planet's water is fresh, but 1.6 percent of the planet's water
is locked up in the polar ice caps and glaciers.
Another
0.36 percent is found underground in aquifers and wells.
Only
about 0.036 percent of the planet's total water supply is found in lakes and
rivers.
That's
still thousands of trillions of gallons, but it's a very small amount compared
to all the water available.
The
rest of the water on the planet is either floating in the air as clouds and
water vapor, or is locked up in plants and animals (your body is 65 percent
water, so if you weigh 100 pounds, 65 pounds of you is water!).
There's
also all the soda pop, milk and orange juice you see at the store and in your
refrigerator …
There's
probably several billion gallons of water sitting on a shelf at any one time!
It
would be more proper to ask, "What is the mass of planet
Earth?"
The
quick answer to that is approximately 6,000,000, 000,000,000,000,000,000 (6 x
1024) kilograms.
The interesting sub-question is, "How did anyone figure that out?"
It's
not like the planet steps onto the scale each morning before it takes a shower.
The
measurement of the planet's weight is derived from the gravitational
attraction that the Earth has for objects near it.
It
turns out that any two masses have a gravitational attraction for one another.
If
you put two bowling balls near each other, they will attract one another
gravitationally.
The
attraction is extremely slight, but if your instruments are sensitive enough
you can measure the gravitational attraction that two bowling balls have on one
another.
From
that measurement, you could determine the mass of the two objects.
The
same is true for two golf balls, but the attraction is even slighter because
the amount of gravitational force depends on mass of the objects.
Newton
showed that, for spherical objects, you can make the simplifying
assumption that all of the object's mass is concentrated at the center of the
sphere.
The
following equation expresses the gravitational attraction that two spherical
objects have on one another:
F = G(M1*M2/R2)
- F is the force of attraction between them.
- G is a constant that is 6.67259 x 10-11 m3/kg
s2.
- M1 and M2 are the two masses that are
attracting each other.
- R is
the distance separating the two objects.
Assume
that Earth is one of the masses (M1) and a 1-kg sphere is the other (M2).
The
force between them is 9.8 kg*m/s2 -- we can calculate this
force by dropping the 1-kg sphere and measuring the acceleration that the Earth's
gravitational field applies to it (9.8 m/s2).
The
radius of the Earth is 6,400,000 meters (6,999,125 yards).
If
you plug all of these values in and solve for M1, you find that the mass of the
Earth is 6,000,000,000, 000,000,000,000,000 kilograms (6 x 1024kilograms,
or 1.3 x 1025 pounds).
It
is "more proper" to ask about mass rather than weight because weight
is a force that requires a gravitational field to determine.
You
can take a bowling ball and weigh it on the Earth and on the moon.
The
weight on the moon will be one-sixth that on the Earth, but the amount of mass
is the same in both places.
To weigh the
Earth, we would need to know in which object's gravitational field we want to
calculate the weight.
The mass of
the Earth, on the other hand, is a constant.
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