MadSci Network: Physics

Re: Tension in the string of a helium balloon, and volume of the balloon

Date: Mon Jan 11 01:34:53 1999
Posted By: Werner Sieber, Research Scientist, Pigment Division, Ciba Specialty Chemicals Corp.
Area of science: Physics
ID: 915653432.Ph

Dear Anonymous,
The VOLUME of the balloon might be calculated by approximating its shape 
e.g. by a half-sphere + a cone, or, since it certainly has cylindrical 
symmetry, dividing its shape into "slices" which are considered as flat 
cylinders of varying diameter according to the silhouette of the balloon.
In order to lift a mass, the buoyant force has to be at least slightly 
higher in magnitude than the weight (m times g) of the mass. Given that 
force equals counter-force, the excess buoyant force accelerates the mass 
upwards with acceleration a: the total force is thus m times (g+a). This is 
probably what you mean by "tension" in the string. Strictly speaking, 
tension is force per unit cross-section (of the string). I let you work out 
yourself what happens if the buoyant force becomes less than (m times g).
Good flight
W. Sieber

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