# Simple challenging fluid mechanics question(buoyancy)

hello fluid knowers!
here is a question i have asked from several students every time a different answer was given to me.

question:
we have a steel block floating in mercury as shown below:
http://www.pikipimp.com/pp/pimped_photo/s/image/41/662/573/1.JPG?ts=1235241467627

now we bring the block out of mercury and cut a piece of block for example EFHG and then again put it in the mercury,the question is how pressure on the lower surface of block named CD changes?
-it is equal in both states
-it is lower in second state
-it is lower in first state
-it depends on how we cut the block

Last edited by a moderator:

russ_watters
Mentor
It is lower in the second state. Basically, you have a block with the same dimensions and a lower weight in the second state than in the first. It floats higher and the pressure on the bottom surface is lower.

you were near the right answer but i have another opinion, i will give it away tonight.

Russ is right. Its a simple statics problem. Less weight means there is less mass to be supported against gravity and since the geometry of CD didn't change the pressure will be lower.

Second way the pressure would be less.

i can confirm that your answers were right,but i think there is an exception,if you cut the piece in this way:
-EF and HG can have any length
-Eh and FG are cut so that their length is equal to AC or BD,i mean if AC and BD are for example 1 [m],EH and FG are cut so that their length is 0.99999 [m]
in this way the pressure would be very very near to first state and we can take them equal.

djeitnstine
Gold Member
They can be approximated to be equal but are not.

Q_Goest
Homework Helper
Gold Member
i can confirm that your answers were right,but i think there is an exception,if you cut the piece in this way:
-EF and HG can have any length
-Eh and FG are cut so that their length is equal to AC or BD,i mean if AC and BD are for example 1 [m],EH and FG are cut so that their length is 0.99999 [m]
in this way the pressure would be very very near to first state and we can take them equal.