Re: [Phys-l] A Crude Attempt at Analysis

[brian whatcott <betwys1@sbcglobal.net>]

Yes indeed! I found that keeping the water out was the issue. With cup 
and container both dry, fine. But successive attempts could fail, if the 
interior and base were not both quite dry before  filling with water.
True, a ring of toothpaste will do the job, but you can also suspend the 
inner cup on this toothpaste, for some period of time too, so its not as 
convincing as it could be.

Brian W

On 11/5/2010 8:15 PM, LaMontagne, Bob wrote:
> The free body diagram approach is still best here. If the cube is solid steel, the force on the bottom is the pressure at the top of the cube times its area plus the weight of the cube of steel. It is in equilibrium and the forces sum to zero,
>
> If the density of the cube is the same as water, the force on the bottom is found the same way -except the result is the same as when just water is present - no cube. It is in equilibrium.
>
> If the density of the cube is less than water, the force on the bottom is now the pressure at the top of the cube times the area plus the weight of the cube, the forces sum to zero. It is in equilibrium and doesn't rise.
>
> If water seeps in below the lighter cube, the force on its bottom is now greater - same as when water was there. It is no longer in equilibrium and it rises to the surface.
>
> Bob at PC