Shane -
No, if everybody on the earth were to jump all at once, the weight
of the earth (really, its 'mass') would not change. The earth's mass is
determined based on how hard its gravity pulls on things - for
instance, the force of Earth's gravity on the moon or on you. (More
mass means more gravity.)
Because of this, the earth's mass does include the mass of all of
the people, plants, and animals that live here. This is because all
objects have gravity, no matter how small they are (although very light
objects don't produce very much gravity at all). So the gravity that
you feel pulling you down really comes from the earth plus all of the
people on it. Even if everyone were to jump up into the air, they would
still produce gravity that pulls on everyone and everything else. There
would be a little change in the force on things right near the Earth's
surface, because a little part of the mass would be briefly in new
locations, but the total mass as seen by, say, the Moon wouldn't
change.
If everybody were to leave the earth and go to say, Neptune, then
the mass of the earth would be a little bit less than it is now, since
other objects would feel a little bit less gravity from it. But the
difference would be really tiny, since even the earth's entire
population weighs hardly anything compared to the rest of the planet.
-Tamara
(published on 10/22/2007)
