MadSci Network: Physics
Query:

Re: Would a bouyant sphere released from ocean depths be heated by frictio

Date: Mon Oct 18 09:20:32 1999
Posted By: Mike Scannell, Powertrain Product Engineer, Ford Motor Company
Area of science: Physics
ID: 939785954.Ph
Message:

Jim,

I'm certainly inclined to agree with you.  Yes, there will certainly be 
friction on the sphere as it rises through the water, but I don't think 
that the sphere would heat so much as to melt rubber or bake the scientist 
inside (though you're right...I don't think I'd want to try it!).

You can compare the ascent of this craft through the water to a person sky 
diving.  Of course water is much more viscous than air, but alot of the 
mechanics will work in the same way.  A sky diver will feel the friction 
of the air on his body as he falls, but at the same time that air will be 
blowing past him and cooling him the whole way.  He will also reach a 
maximum speed of descent as the force of the air resistance becomes equal 
to the force of gravity on his body.

The same will happen to the sphere.  Friction of the water along the 
sphere will create heat, but not much.  As the water rushes past the 
sphere, it will certainly cool it, and I'm sure this cooling will occur 
much faster than frictional heat can build.  Temperatures at the bottom of 
the ocean are around 34 deg F.  In other words, just above freezing.  
Temperatures near the surface are probably going to be around 60-70 deg 
F.  Water is a great coolant, so I would definitely say that it's probably 
going to be pretty cold inside that sphere, not the opposite.  The sphere 
will also reach a maximum speed when water resistance equals the buoyant 
force.  

(Extra physics info:  The buoyant force will be equal to the weight of the 
water that the sphere is displacing, minus the weight of the sphere.  The 
resisting force of the water will be proportional to the square of the 
speed at which the sphere is moving.  Thus the craft will accelerate due 
to the buoyancy until the increasing resistance is equal to the buoyant 
force, and it reaches a maximum speed)

Hope that helps you.  

- Mike "Mad Scientist" Scannell


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