Expert: Paul Soderman - 2/20/2009

Question
QUESTION: How much weight will one cubical foot of vacuum lift?

ANSWER: Rich
A perfect vacuum can lift a weight equal to atmospheric pressure times the cross sectional area of the object in the horizontal plane over which the atmospheric pressure is acting.  The volume of the vacuum does not matter.

So, imagine a smooth pipe mounted vertically with a sealed top and a vacuum above a frictionless piston with weight W filling the pipe cross section.  At sea level, the maximum weight W that could be held would be

W = pA = 2116.8 lb/ft^2 x A

where A is the cross sectional area of the piston. If A was one square foot, the vacuum could hold 2116.8 pounds.  Of course it is actually the air pressure below the piston that is holding it up.
Paul



---------- FOLLOW-UP ----------

QUESTION: But what if that pipe was a container - say like a rigid balloon with no air pressure inside?  How much could that zero pressure balloon pull if it were let go?

Answer
Rich
This is a buoyancy question.  The lift force from a balloon is equal to the weight of air displaced by the balloon less the weight of the balloon.  At sea level on standard day the density (specific weight) of air is 0.0765 lb/ft^3.  So the lift force would be

L = 0.0765 x volume - balloon weight

where balloon volume is in cubic feet and weight is in pounds.  Note that it doesn't matter to buoyancy what is in the balloon.  If the balloon were filled with air the total weight (air in balloon plus balloon skin) would be greater than the buoyancy force and the balloon would sink.  The vacuum only makes the total weight less.  Course it would be hard to keep such a balloon from collapsing without a very strong and heavy skin.  Helium is a better choice because it is 1/7th as heavy as air and can hold the skin taut and doesn't burn like hydrogen.
Paul