Let the string makes angle θ with the vertical. Weight will be balanced by the vertical component of tension, which is Tcosθ=mg and the horizontal component towards the centre will provide the required centripetal force for circular motion. Therefore,
Tsinθ=mω2(lsinθ), where l=length of the string.
⇒400sinθ=0.5ω2×0.5sinθ
⇒ω=40rads−1