MadSci Network: Physics

Re: Using a vacuum to lift things

Date: Wed Oct 25 22:57:50 2000
Posted By: Richard Bersin, Other (pls. specify below), Senior Technical Staff Member, Emergent Technologies
Area of science: Physics
ID: 971450293.Ph

Dear Jean:
You are asking questions about air, and vacuum, and lifting things.  Let me 
answer you last questions first.  1 liter of air at 300 degrees K (27 
degrees C)and 1 standard atmosphere of pressure weighs 1.161 grams. 
(Reference Chemical Rubber Handbook)at the surface of the earth.  One cubic 
foot of air contains 27 liters of air (30cmX30cmX30cm=27,000 cubic 
centimeters or 27 liters)which then weighs 31.347 grams.  1,000 grams is 
equal to a weight of 2.2 pounds.  Therefore 31.347 grams is a weight 
equvalent to .031347 kilograms; and 0.031347)X(2.2)=0.0689634 pounds. There 
are 16 ounces in a pound; and therefore 1 cubic foot of air weighs 
16X(0.0689634) ounces, or 1.10 ounce.  There are 1600 ounces in 100 pounds, 
and therefore you need a volume of 1,454 cubic feet to have a displacement 
of air equal to 100 pounds. 

Now if you visit another planet, that has air exactly like the earth in 
terms of gravity, air pressure, etc. let us discuss your experiment.  First 
you take your spherical ball which is dropped on the planet in a collapsed 
state.  Then you inflate the ball and pump out all of the air.  You then 
have your ball, with it's own weight, but no air inside.  Therefore its 
total weight is just that of the ball material itself.  Let us call this 
weight Wb.  If the ball is under perfect vacuum then this is its weight.  
If the volume of air which equals the volume of the ball weighs more then 
Wb, then the ball will rise into the air because it displaces more weight 
of air than its own weight.  The reason the ball rises is because to total 
weight of the ball with no air is lighter than the weight of the displaced 
air.  Whether the ball will rise then really depends on the materials 
which the ball is made of.  If it is a thin plastic material that is very 
light but still rigid to remain a ball and not collapse, then the ball 
rises.  If the ball material is too heavy then the ball will not rise even 
if it is under vacuum inside.  So whether you can lift the ball really will 
depend upon the weight of the ball, Wb, and the vacuum inside of it.  It is 
the combined weights and the total volume of the ball which determines 
whether it will rise.

A vacuum by itself cannot life anything.  The lifting comes from the fact 
that you have outside air at a given pressure, pressing on the ball which 
has a certain weight;  and if the ball displaces nore weight of air than 
its own weight it will rise.  Having a vacuum in the ball is important only 
in that it makes the ball weigh less than if there is air inside the ball.
The force to raise the ball comes from the air on the outside; not from the 
vacuum inside. This is very important to understand-a vacuum can be used to 
move things only if there is something outside the vacuum exerting a force 
to push on it.  Then a net force can be exerted and the object will move.

I hope this answers your questions.

R. Bersin....

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