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# Re: [Phys-l] Definition of upthrust or buoyancy

An object under water obviously is being compressed. And I agre we shouldn't conflate buoyancy with compression and tension. But that is not the problem presented.

The pilings here were stated to be less dense than water. So is the human body (for most people). If I am 10 ft below the water and relax, I will float to the surface. And - yes I do feel compressive forces - my ears tell me that.

However, if I am at the bottom of a 10 ft deep pond and my foot gets caught between some rocks, if I relax start to float upwards, my leg is trapped by the rocks and will be under tension (keeping my torso from floating upward).

A balsa wood piling whose base is in concrete is just like my leg and torso - which makes me think that the piling is under tension longitudinaly.

Another example - tie a rock to one end of a string and a balloon to the other. Throw them in a pond and assume the rock is heavy enough for them to sink. The string is under tension. Also if the balloon remains inflated, it's surface is under tension - not compression.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of John Clement
Sent: Thursday, October 21, 2010 11:20 AM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] Definition of upthrust or buoyancy

I think the wording is just fine. Consider the downward air pressure
on the
top of the piling and the weight of the wood above the point at which
you
are calculating. And don't forget that the vertical stress is not the
same
as the horizontal stress.

Make a free body diagram, just as we expect students to do!

John M. Clement
Houston, TX

Find the average normal stress in a cross section of the piling at
height h/2 above the surface
of the pond.