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(OP)
I have a doubt in buyaoncy force calculation,

We know buyonacy force for solid object  = weight of fluid displaced by body.

will it be different for a closed or sealed hallow object. please clarify me.

and please illustrate me with a numerical example.

Replies continue below

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2
It is not different for a closed, sealed hollow body.  Let's look at 3 different cases:

1.  A solid steel cube, 12" to a side.  It weighs 490lbs, but only displaces 62.4lbs of water.  It's gonna sink - and fast.

2.  A steel cube, 12" to a side with a cubic void inside, 9" to a side.  This one weighs 283lbs, but still displaces only 62.4lbs of water.  It will sink, but not as fast as the solid cube.

3.  A steel cube, 12" to a side with a cubic void inside, 11.5" to a side.  This one weighs 58.7lbs while still having the potential to displace 62.4lbs of water.  However, since it weighs less than 62.4lbs, it will float and it will displace exactly 58.7lbs - just enough water to make it float.  If it was floating square to the surface of the water, this means that 11.3" of it would be under water and about 0.7" would be sticking out.

This is the principal that allows huge, 300,000ton super tankers made out of steel to float.  It also explains how college civil engineering departments all over can compete in concrete canoe races.  It doesn't matter how much your object weighs - if it can be configured to displace the amount of water it weighs, it will float.

To take this a step further, let's take my example #1 above and expand on it.  The solid block of steel will not float.  It's not because it weighs 490lbs, it's because it's shape doesn't displace enough water.  If I took that same block of steel, melted it down and worked it into a thin sheet shaped like a bowl, it would still weigh 490lbs, but if gently set into the water, it will float.

(OP)

I am looking for calculation of properties of closed polygon (irregular) based on given set of coordinates.

Can any suggest me how to calculate section properties like centroid,moment of intertia, torsional constant etc., for a give set of coordinates of a closed polygon?

Thanks
P.Ejoumale

Roarks Formulas for Stress and Strain has info on a number of shapes.  But if it is truely irregular you will have to go back to first principles.  Any good mechanics textbook will give you the information you need.

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