Derive an expression for the tension in the cord

[lightfire]

A rock with mass m = 3.10 kg is suspended from the roof of an elevator by a light cord. The rock is totally immersed in a bucket of water that sits on the floor of the elevator, but the rock doesn't touch the bottom or sides of the bucket.

Here is the part that I am stuck on:
Derive an expression for the tension in the cord when the elevator is accelerating upward with an acceleration of magnitude a .

I have concluded that
Tension=m(g-a)-1000V(g-a)

Or force of the body-force of the water displaced but it is incorrect.

 

[Volcano]

For an observer in elevator, rock is in equilibrium. Net force is zero.

m(g+a)-(T+F)=0

mg-ma-T-F=0

T=mg-(ma+F)

mg: weight
ma: inertia force
T: tension
F: the lift of water

 

[Doc Al]

I have concluded that
Tension=m(g-a)-1000V(g-a)

Show how you arrived at this answer. (How does the acceleration of the elevator affect the buoyant force?)