محمد مصطفى
02-14-2009, 09:55 PM
السلام عليكم ورحمة الله وبركاته
http://www.balagh.com/esteraha/images/2121.jpg
How do scientists measure the
weight of a planet?
A N S W E R E D B Y :
Gregory A. Lyzenga, Professor of Physics,
Harvey Mudd College, Claremont, California
The weight (or the mass) of a planet is determined by
its gravitational effect on other bodies. Newton’s Law
of Gravitation states that every bit of matter in the universe
attracts every other with a gravitational force that is
proportional to its mass. For objects of the size we
encounter in everyday life, this force is so minuscule that
we don’t notice it. However, for objects the size of planets
or stars, it is of great importance.
In order to use gravity to find the mass of a planet, we
must somehow measure the strength of its “tug” on
another object. If the planet in question has a moon (a
natural satellite), then nature has already done the work
for us. By observing the time it takes for the satellite to
orbit its primary planet, we can utilize Newton’s equations
to infer what the mass of the planet must be.
For planets without observable natural satellites, we
must be more clever. Although Mercury and Venus, for
example, do not have moons, they do exert a small pull
on each other and on the other planets in the solar system.
As a result, the planets follow paths that are subtly different
than they would be without this disturbing effect.
Although the mathematics is a bit more difficult, and the
uncertainties are greater, astronomers can use these small
deviations to determine how massive the moonless planets
are.
What about those objects such as asteroids, whose
masses are so small that they do not measurably disturb
the orbits of the other planets? Until recent years, the
masses of such objects were simply estimates, based upon
the apparent diameters and assumptions about the possible
mineral makeup of those bodies.
Now, however, several asteroids have been (or soon
will be) visited by spacecraft. Just like a natural moon, a
spacecraft flying by an asteroid has its path bent by an
amount controlled by the mass of the asteroid. This
“bending” is measured by careful tracking and Doppler
radio measurement from the earth.
انتهى الموضوع
المصدر/ مجموعة كل الفيزيائين العرب على الفيس بوك
http://www.facebook.com/topic.php?topic=7262&post=38403&uid=83814205342#/group.php?gid=18913391805
تيحاتى:)
http://www.balagh.com/esteraha/images/2121.jpg
How do scientists measure the
weight of a planet?
A N S W E R E D B Y :
Gregory A. Lyzenga, Professor of Physics,
Harvey Mudd College, Claremont, California
The weight (or the mass) of a planet is determined by
its gravitational effect on other bodies. Newton’s Law
of Gravitation states that every bit of matter in the universe
attracts every other with a gravitational force that is
proportional to its mass. For objects of the size we
encounter in everyday life, this force is so minuscule that
we don’t notice it. However, for objects the size of planets
or stars, it is of great importance.
In order to use gravity to find the mass of a planet, we
must somehow measure the strength of its “tug” on
another object. If the planet in question has a moon (a
natural satellite), then nature has already done the work
for us. By observing the time it takes for the satellite to
orbit its primary planet, we can utilize Newton’s equations
to infer what the mass of the planet must be.
For planets without observable natural satellites, we
must be more clever. Although Mercury and Venus, for
example, do not have moons, they do exert a small pull
on each other and on the other planets in the solar system.
As a result, the planets follow paths that are subtly different
than they would be without this disturbing effect.
Although the mathematics is a bit more difficult, and the
uncertainties are greater, astronomers can use these small
deviations to determine how massive the moonless planets
are.
What about those objects such as asteroids, whose
masses are so small that they do not measurably disturb
the orbits of the other planets? Until recent years, the
masses of such objects were simply estimates, based upon
the apparent diameters and assumptions about the possible
mineral makeup of those bodies.
Now, however, several asteroids have been (or soon
will be) visited by spacecraft. Just like a natural moon, a
spacecraft flying by an asteroid has its path bent by an
amount controlled by the mass of the asteroid. This
“bending” is measured by careful tracking and Doppler
radio measurement from the earth.
انتهى الموضوع
المصدر/ مجموعة كل الفيزيائين العرب على الفيس بوك
http://www.facebook.com/topic.php?topic=7262&post=38403&uid=83814205342#/group.php?gid=18913391805
تيحاتى:)