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#1
06-20-2004, 09:20 PM
 saluki_fan Guest Join Date: Feb 2004 Location: Carbondale, IL Posts: 381
Will the Earth's gravitational pull ever change?

Will the Earth's gravitational pull ever change from 9.8 m/s^2? Just what exactly causes the gravitational rate to be this and why? I also have always wondered how exactly the moon was responsible for Earth's waves also, could somebody answer that as well? Thanks
#2
06-20-2004, 09:26 PM
 DrFidelius Charter Member Join Date: Mar 1999 Location: Miskatonic University Posts: 11,939
One of our physics wonks will be by shortly, but the quick answer is that gravitational force is proportional to the mass of the object. There is right now just exactly enough stuff in the Earth to produce an accelleration of 32 ft/sec/sec at the surface. If the mass of the Earth ever changes, measured gravity will change as well.

I have never heard of the Moon being responsible for waves, as waves are almost all created from the force of wind on the surface of the seas. Perhaps you are thinking of tides?
#3
06-20-2004, 10:15 PM
 saluki_fan Guest Join Date: Feb 2004 Location: Carbondale, IL Posts: 381
Quote:
 Originally Posted by DrFidelius One of our physics wonks will be by shortly, but the quick answer is that gravitational force is proportional to the mass of the object. There is right now just exactly enough stuff in the Earth to produce an accelleration of 32 ft/sec/sec at the surface. If the mass of the Earth ever changes, measured gravity will change as well. I have never heard of the Moon being responsible for waves, as waves are almost all created from the force of wind on the surface of the seas. Perhaps you are thinking of tides?
Thanks for correcting me on that, I meant tides. Physics really isn't my strong suit.
#4
06-20-2004, 10:28 PM
 David Simmons Charter Member Join Date: Nov 2001 Posts: 12,684
Quote:
 Originally Posted by saluki_fan Will the Earth's gravitational pull ever change from 9.8 m/s^2? Just what exactly causes the gravitational rate to be this and why? I also have always wondered how exactly the moon was responsible for Earth's waves also, could somebody answer that as well? Thanks
The force of gravity between two objects is equal to a constant multiplied by the product of the two masses and divided by the distance squared between their centers of mass. The constant is the Newton constant of universal gravitation which was first measured by Henry Cavendish in the late 1700's. The currently accepted value is 6.67259*10-11 m3/(kg*sec) (with an uncertainty in the last two places that doesn't concern us).

If by waves you mean the tides it's this way. A small piece of the earth on the side nearest the moon is attracted to the moon by a larger force than a small piece of the earth on the side furtherest from the moon because the gravitation force is inversely proportional to the square of the distance. In fact all of the small pieces on the near side are so attracted more than all the small pieces on the far side.

This differential force exerts a strain on the material of the earth and pulls it into a sort of oval shape. This oval shape is always pointed (almost) at the moon and so there is a bulge on both sides of the earth nearly lined up with the moon. As the earth rotates the bulges stay lined up with the moon and so any spot on the earth passes through both of them in one rotations. So there are two high and two low tides per day. Naturally the solid body of the earth is more resistant to being deformed than is the water of the oceans so we don't notice the earth deformation but the ocean tides are very noticeable.
#5
06-20-2004, 10:39 PM
 MikeS Charter Member Join Date: Oct 2001 Location: New London, CT Posts: 3,749
Heck, the Earth's mass does increase by roughly 108 kg/day due to infalling spade dust, micrometeorites, etc. So assuming the Earth remains a constant radius in the process, that means that g is changing by about :::pulls out calculator::: on the order of 10-14 m/s2 every year. To put this in perspective, the extra weight on you due to the mass gain of the Earth over one year is about equal to the gravitational attraction between you and a mote of dust (mass 0.1 milligrams) floating in the next room.
#6
06-20-2004, 11:59 PM
 saluki_fan Guest Join Date: Feb 2004 Location: Carbondale, IL Posts: 381
This may be getting off the subject here, but on a surface with less gravitational pull, say the moon, would a person be able to live longer. Would the heart muscles need to work as hard to pump blood to the rest of the body, thus increasing the life expectancy of the person?
#7
06-21-2004, 03:04 AM
 Bob55 Guest Join Date: Sep 2002 Location: Orlando, FL Posts: 1,179
Quote:
 Originally Posted by saluki_fan Will the Earth's gravitational pull ever change from 9.8 m/s^2? Just what exactly causes the gravitational rate to be this and why? I also have always wondered how exactly the moon was responsible for Earth's waves also, could somebody answer that as well? Thanks
I think that the earth's gravitational pull actually changes on the surface of the arth as well - the further you are from the center of the earth the less pull there will be. So someone in the apartment above you may be experiencing acceleration of 9.79(probably followed by hundreds of 9s) m/s^2
#8
06-21-2004, 04:20 PM
 sciguy Guest Join Date: Jan 2003 Location: Denver, CO Posts: 827
The Earth's surface gravitational force (which determines the acceleration) depends on three things:
1. Gravitational constant
2. Mass of the earth
Changing any one of these would change the acceleration. Some research suggests that the Gravitational constant may vary with time (although we'd be talking periods of millions or billions of years). Earth's mass doesn't change much (although it does accumulate some from space impacts) and it's radius isn't likely to change significantly enough to matter.

There's also some variations that make miniscule differences (way, way down the list of decimal places): deposits of high-density materials can increase gravity in a particular area , being in a plane above the survace will decrease gravity, going down in a mine would change it (probably a decrease because the matter now above you is no longer pulling you down, and that outweighs the increase from moving closer to the Earth's center).

Tides happen because the Moon attracts the water more than the Earth on the side nearer the moon, and attracts the earth more than the water on the oppositie side of the moon. This page gives a pretty simplistic breakdown without going through all the math.
#9
06-21-2004, 05:04 PM
 Chronos Charter Member Moderator Join Date: Jan 2000 Location: The Land of Cleves Posts: 72,712
Quote:
 going down in a mine would change it (probably a decrease because the matter now above you is no longer pulling you down, and that outweighs the increase from moving closer to the Earth's center).
This would be correct, if the Earth had a uniform density. Gravity in a mine would scale directly with how far you are from the center of the Earth. However, the density of the Earth is not constant, being higher at the core than near the surface, and it's my understanding that this is just enough that the gravitational pull stays just about constant until you reach the core. The core is approximately uniform density, though, so there it will go down uniformly until you reach the center.

The biggest difference between points on the Earth comes from the Earth's rotation. On the Equator, you're a little further from the center of the Earth (since the Earth is bulged), and you also have a little bit of centrifugal force pulling against gravity, both of which cause you to be a little bit lighter at the equator than at the poles.

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