The block on the loop shown slides without friction. At what height from A it starts so that it passes against the track at B with a net upward force equal to its own weight? The radius of the loop is R.

Explanation

Let the velocity at B be v, then
`1/2mv^2=mg(h-2R)impliesv=sqrt(2g(h-2R))`
`N+mg=(mv^2)/(R)`
Put `N=mg`, solve to get `h=3R`