Question

Ans the attachement​

Answer 1

Let total energy is conserved,

Total mechanical energy at start = Total mechanical energy at 'B'

mgh = 21mvB2 +mgD ...........(1)

Minimum speed required to complete rotation is, vb=rg=Dg/2

Hence equation (1) becomes,

mgh = 21m2Dg + mgD

hence, h=5/4D

Answer 2

See the attachement........