Fluid mechanics involving stability

[Jason03]

Im working on a problem involving a 70 lb empty drum. The diameter and length are given. I am trying to find if it will float stably if placed upright in water.

I did a similar problem to this involving a solid cylinder with no problem. But I am trying to figure what special considerations or differences I need to take into account to get the correct answer.

Im calculating the metacenter and the center of gravity as I did in the last problem but I am not coming up with the correct answer for this problem involving the hollow cylinder.

Just trying to see what parameter I need to take into account.

 

[nvn]

What is the drum diameter and length in meters, and what is the empty drum mass in kg? Does the drum contain any water? If so, what is the depth of water inside the drum? What is the density of water inside and outside the drum, in kg/m^3?

 

[Jason03]

The drum is sealed. and i is 2 in diameter by 4 feet long.

I have included my calculations below...My final calculation for the metacenter is coming out to be 13.85 inches...but the correct answer is 10.55 inches...also the metacenter in my second diagram should be higher (slightly below center of gravity)

page 1
http://img16.imageshack.us/img16/6374/page1mu5.jpg

page2
http://img16.imageshack.us/img16/262/page2ok9.jpg

 

[nvn]

Jason03: Vd on page 2 is incorrect. Check your textbook to find out how to compute Vd, and try again.

 

[Jason03]

I went back and looked and the displaced volume formula and you were correct. I was using the standard volume formula when I needed the displaced volume formula which is ((pi*d^2)/4)(x) where x is the distance the cylinder is submerged in the water. When I went back and did the calculations I still came up with the wrong answer. Than I realized my moment of inertia formula was correct but the answer I had written down was just plane wrong. I guess i just typed it incorrectly into my calculator...Now I have the correct answer of a meta center at 10.2 inches from the base.

So thank you!

 

[nvn]

I saw that moment of inertia mistake earlier today, but when I finally made my post, I was only thinking about Vd, and forgot about the moment of inertia mistake. Also, cg on page 2 is wrong.

Your answer is fairly inaccurate. Generally always maintain four significant digits throughout all your intermediate calculations, then round only the final answer to three significant digits, unless the final answer begins with 1, in which case round the final answer to four significant digits. Try it again and you should get 10.55, if you are doing it right.

 

[uradnky]

i have a quick question about stability..sorry if I am hijacking the thread..

If you are determining wether something will be stable or not, is it only dependant on wether the metacenter is above the center of gravity?

 

[nvn]

uradnky: Yes, that is correct, if it is a floating body.