I have tried to do an experiment where i tried to check the direction of the net force by water on a body. I have some question regarding some concepts. I took an empty sealed bottle and forcefully inserted it in a tub of water completely immersed at a certain depth. Now, as soon as i release my hand from the bottle, the bottle comes up but most amazingly i see that the bottle kind of rotates in the clockwise direction and aligns parallel to the liquid surface and finally floats on the liquid surface. I assumed that the bottle is cylindrical and i tried to calculate the pressure forces (including the weight of the bottle) on all the surfaces of the bottle. I have resolved all these forces in the X- Z direction and found that the net force acts at an angle to the X axis i.e in the X-Z plane. If thats the case, then the bottle should have accelerated in that direction but what i found was that the bottle rotated in the clockwise direction as if there was a net resultant force which acted perpendicular to the axis of the bottle. I am really curious why did the bottle not move in the direction of the net resultant force and moved in a different direction?
Ans)If you suppose the body to be a perfect cylinder, then all pressure forces and weight of the body should balance out to produce a net translational upward force. Though, in non-ideal conditions, the body is not a perfect cylinder and the force you applied is not perfectly perpendicular to the liquids surface. Therefore Bouyoncy acts slightly off the COM to compensate for your imperfect force and therefore produces torque about COM, causing the body to rotate untill it reaches equilibrium (parallel to the surface).