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Author Topic: Do different materials have different gravitational effects?  (Read 2681 times)

Offline bizerl

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For instance, if 1 kg of material A weighs 9.8 N on Earth, is there any kind of material where 1 kg will weigh more or less than 9.8 N if measured in exactly the same place (ie - ignoring any locational gravity effects)?


 

Offline CliffordK

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Generally NO.
1 kg of anything is 1kg of it everywhere.

Generally one calculates gravity based on the distance to the center of mass.  Overlapping bodies, however, have somewhat odd properties, for example your weight will stay more or less the same if one descended through the crust of the earth, but then would start going down as one approached the center of the earth.

http://www.thenakedscientists.com/forum/index.php?topic=45321.0

Now, the one thing that is often ignored is that part of mass is the displacement of fluid. 

So, say you have a 1kg piece of steel with a volume of about 128 cm3.
If you place that 1kg in water, then it will displace about 128 cm3 of water, or about 128 grams of water, and it will weigh about 872 grams.

Now, put it back in air, and it is displacing about 128 cm3 of air.  Air has a density of about 1.29 grams per liter.
So, 128 cm3 of air has a mass of about 0.165 grams.

So, your 1kg of steel will have a buoyancy of about 0.165 grams in air. 
If it was put in a vacuum, it should weigh about 0.165 grams heavier (Total 1000.165 grams).
And, the weight would change in an atmosphere of different gasses, or liquids.

Now, if you compare your steel with something with a lower density such as a 1 kg block of wood that might have a density of about 0.7 g/cc, and have a volume of about 1.4 liters, and displace 1.8 grams of air. 
So, in a vacuum, your 1kg block of wood should weigh about 1.8 grams heavier than 1 kg, (total 1001.8 grams) and also would weigh more than the 1 kg of steel.
 

Offline RD

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Hence the idea of Helium-filled bubble-wrap to save on postage costs (which are by weight).  ;)
« Last Edit: 17/09/2012 06:56:08 by RD »
 

Offline CliffordK

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Hence the idea of Helium-filled bubble-wrap to save on postage costs (which are by weight).  ;)
So, how much does it cost to mail 100 grams of helium?
 

Offline RD

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Hence the idea of Helium-filled bubble-wrap to save on postage costs (which are by weight).  ;)
So, how much does it cost to mail 100 grams of helium?

 

Offline CliffordK

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So, from that clip, it took 70 pence to ship just the air in the box.

I am curious, however, what the post office would say if I tried to ship a lighter than air box to TNS.
 

Offline RD

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I am curious, however, what the post office would say if I tried to ship a lighter than air box to TNS.

The post office should pay you as it would mean less fuel consumption by the vehicles carrying it  ;)
 

Offline syhprum

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I actualy saw this done on TV where the actors took a parcel with negative weight to the shippers and tried to post it but it was a long time ago and I forget the result
 

Offline wolfekeeper

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Well, Cavorite has a negative gravity coefficient, but I've been unable to locate a supply.
 

Offline bizerl

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I am curious, however, what the post office would say if I tried to ship a lighter than air box to TNS.

The post office should pay you as it would mean less fuel consumption by the vehicles carrying it  ;)

I wonder at what point the energy required to keep a lighter-than-air parcel down would become more than trying to lift a regular parcel up...
 

Offline imatfaal

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Well, Cavorite has a negative gravity coefficient, but I've been unable to locate a supply.
  :-)  I know a shop in Diagon Alley that stocks it
 

Offline evan_au

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A 1kg magnet at the North Magnetic Pole would change weight by a small amount depending on whether it was North Up or North Down...
 

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