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Author Topic: How do we know how much the Earth weighs?  (Read 7668 times)

Offline george

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How do we know how much the Earth weighs?
« on: 01/01/2007 16:19:06 »
Scientists seem to have a pretty good idea as to the mass of the Earth, but how did they arrive at this figure?


 

Offline ukmicky

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Re: How do we know how much the Earth weighs?
« Reply #1 on: 01/01/2007 16:27:49 »
The earth as a whole doesn't really weigh anything as its not sitting on anything, in other words if you were floating in space you would be weightless. The earth has mass and its mass is calculated by its gravitational influence on nearby objects in space.
 

Offline Aware

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Re: How do we know how much the Earth weighs?
« Reply #2 on: 01/01/2007 16:39:10 »
George,
What an excellent question.  I'm glad that Michael knew the answer because I had no clue.
   
 

Offline daveshorts

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Re: How do we know how much the Earth weighs?
« Reply #3 on: 01/01/2007 21:20:26 »
Yep because the force 2 massive objects exert on one another object is:


F = G m1 m2
    ---------
       r2

where r is the distance between them
and G is the gravitational constant


because
F = ma
the acceleration m1 causes in m2

a = G m1
    -----
     r2



So if you know G and how the moon or a satellite is accelerating you can work out the earth's mass...

The trick is working out G....

this is really quite difficult as it is really tiny (about 0.0000000000138Nkg-2m2 ) so means measuring absolutely tiny forces produced by quite large masses.

The experiment that can be used to work this out was first done by Henry Cavendish in 1797.

He did it by hanging a heavy lead dumbell on a thin wire,

from the side:


Then moving two 350lb lead weights near to it on either side

From the top:


and he could just about measure the deflection of the dumbell caused by the lead weights...

It is still the hardest of all the fundamental constants to measure, because gravity is so weak.
 

Offline neilep

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Re: How do we know how much the Earth weighs?
« Reply #4 on: 01/01/2007 21:34:46 »
Wouldn't it be easier for me to get on some scales first then pick up the world and weigh it with me then deduct my weight ?
 

Offline Karen W.

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Re: How do we know how much the Earth weighs?
« Reply #5 on: 01/01/2007 21:43:39 »
YES....Indeedy that sounds like a plan to me.. right up my alley!! LOL LOL
 

Offline JimBob

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Re: How do we know how much the Earth weighs?
« Reply #6 on: 01/01/2007 22:29:21 »
Wouldn't it be easier for me to get on some scales first then pick up the world and weigh it with me then deduct my weight ?

Why not put you in space, Neil and use you as the other large body to measure force (F) against?
 

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Re: How do we know how much the Earth weighs?
« Reply #6 on: 01/01/2007 22:29:21 »

 

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