[surajit001]

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 that's 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?

[ebaines]

If the bottle is perfectly symmetric and if you relase it without any induced angle or rotation it should come directly to the surface ina straight path without any angle or rotation. The fact that you are seeing rotation indicates that either the bottle is not symmetric or you are not releasing it in a perfectly vertical orientation. Also, depending on the shape of the bottle, it may be unstable in vertical motion. If the center of drag of the bottle is behind the center of mass then it should be stable as it rises. But if the center of drag is in front of the center of mass it will be unstable and will try to rotate - similar to what happens if you try to throw a dart backwards (fins first).