Fluid Statics: Solid & Hollow Spheres in Water

[Dreaming big]

Homework Statement

A solid sphere and a hollow sphere of same radius and same material are dropped from the top of a beaker filled with water. The height of water in the beaker is 'H'. Which sphere will reach the bottom of the beaker earlier and why? (Neglect any viscous force)

 

[cepheid]

So the problem wants you to assume that they both sink. Fine. What determines that they sink, (in terms of force balance)? Also, what determines the magnitude of the force that tries to prevent them from sinking?