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Conservation of Energy and Buoyancy
Name: Vinay
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
Question:
How can the sum of potential energy and kinetic energy
be constant when a helium balloon moves up in air as both potential
energy and kinetic energy increases?
Replies:
The potential energy is going _down_ as the balloon rises. This is
counter-intuitive because the standard way of describing potential energy
is based on the assumption that gravity is much greater than the buoyancy
(buoyancy is ignored). But that is not a good assumption with the helium
balloon. Potential energy is actually stored in the submerged (in air)
helium balloon -- to help visualize, think of how much work it takes to
submerge a basketball under water. With the basketball, you are pushing
water up (it is displaced by the ball) against gravity, storing potential
energy. It works the same way with the balloon submerged in air. Filling
the balloon displaces air, thereby storing potential energy in the
balloon. When you release the balloon, that potential energy is converted
to kinetic energy (there are other factors as well -- it expands as it
rises, for example, but I will neglect those for purposes of this answer).
Hope this helps,
Burr Zimmerman
The potential energy of the helium balloon increases as it ascends,
but as it ascends it allows heavier air to descend to take its
place. The air's potential energy thus decreases. The decrease in
the air's potential energy is of greater magnitude than the increase
in the helium balloon's potential energy, so overall potential
energy decreases.
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
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Update: June 2012
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