don't know how 2 do it

A pendulum consisting of a bob of mass m connected to a light rod of length L is stood upside-down and given a gentle push to start it moving.
(a)Find the centripetal force of the bob when the rod is (i) horizontal and (ii) at the bottom of the swing.
(i)
依機械能守恒，擺錘的速度
0.5mv2 = mgh
v = √(2gh)
= √(2gL)
所以向心力為
F = mv2/L
= m(√(2gL))/L
= 2mg
(ii)
依機械能守恒，擺錘的速度
0.5mv2 = mgh
v = √(2gh)
= √(2g(2L))
= √(4gL)
所以向心力為
F = mv2/L
= m(√(4gL))/L
= 4mg

(b) What is the net tangential force acting on the bob when the rod is horizontal? what is the angular acceleration at this point?
它的轉動慣量 I = mL2
在水平的位置它所受的切向力只有 mg
所受的力矩為 mgL，即
mgL = Iα
mgL = mL2α
α = g/L