1. The problem statement, all variables and given/known data if an object is in equilibrium at the interface of 2 fluids in an open tank, why do both provide upthrust? 2. Relevant equations P=P' +dgh 3. The attempt at a solution The upper fluid is not in contact with the lower part of the object, so why would the fluid provide an upward buoyant force? There is no area at which the fluid could exert a force. Yet almost all the questions I have seen that deal with such a principle equate mg with the buoyant force due to the lower and upper liquids.