A thin spherical shell of mass 4.00 kg and diameter 0.200 m is filled with helium (density = 0.180 kg/m3). It is then released from rest on the bottom of a pool of water that is 4.00 m deep. (a) Neglecting frictional effects show that the shell rises with constant acceleration and determine the value of that acceleration. (b) How long will it take for the top of the shell to reach the water surface?