Difference between volume displaced fluid and volume of the object

[paulh428]

What is the difference between the volume displaced and the object of the volume, according to Archimedes' Principle? Because I'm trying to find logs floating on water and my book gives an equation for buoyant force. Here it is Fb = mg => rho-fluid * V-displaced * g = rho-object * V-object * g. Hope that equation helps. Essentially, my question leads to: shouldn't the V-displaced and V-object be the same thing?

Hope this makes sense. Please ask if you want me to try and make some more sense. :D

 

[James Leighe]

Only if the object is fully submerged.

If the object floats, it will only displace water equal to the volume of the submerged portion.

 

[HallsofIvy]

If the log has volume v and it floats so that exactly half of it is submerged and half is above the water it would displace v/2 volume of water. And that would mean, of course, that the density of the log is 1/2 the density of water.