Lets keep it short:
I am confused regarding the domain over which buoyancy force due to a liquid acts.
If I have a hollow sphere and I dip it in water, I observe that it floats partially with certain part dipped in the water.
Now I cut a negligibly small hole on the outer part so that the system is NOT affected. Through this hole I release small masses (having finite volume) or maybe pebbles and seal it off quickly. Of course, due to a larger mass, the sphere will sink (maybe fully till the bottom or maybe not).
The Doubt : Does buoyancy force act on the small weights ?
Essentially, does buoyancy force act on an object inside an object ?
Well, if it doesn't, then the object HAS to sink till the bottom due to weight being more than upthrust at any instant.**
**I may be wrong here because I assumed that the body without additional masses floats partially
Best Answer
If it floats partially submerged the density of the sphere is less than the density of water.
As long as you cut out a hole, the amount of the sphere submerged will decrease since the mass is less. Naturally, the smaller the hole the less the effect.
The buoyancy force depends on the amount of water displaced by the sphere. The amount of water displaced by the sphere depends on its density compared to that of the water. It it is less dense, it will float partially submerged, as in your first instance.
As long as adding the pebbles does not result in the density of the sphere, which is its total mass (empty sphere plus pebbles) divided by the total sphere volume (assumed fixed) being greater than water, it will still float.
It will float partially submerged if its density is less that of water, and float totally submerged if its density exactly equals the density of water. It will sink if its density is greater than water.
The buoyancy force acts on the exterior of the sphere and is based on the overall density of the sphere (total mass divided by total volume). It does not individually act on the small weights. The only effect the small weights may have is on which part of the surface of the sphere is submerged (the part of the surface where the weights accumulate.
I'm having trouble following you here. But like I said, the sphere will not sink as long as its density is not greater than water.
Hope this helps.