A bead of mass m is tied at one end of a spring of spring constant mg/R and unstretched length R/2 and other end to fixed point O. The smooth semicircular wire frame is fixed in vertical plane. Find the normal reaction between bead and wire just before it reaches the lowest point.
I tried as
By energy conservation
PE of spring(final - initial) + PE due to gravity = change in KE
(1/2)(mg/R)R$^2$ + 2mgR = (1/2)mv$^2$
From this I got cetripetal force as 5mg towards the centre,O.
And gravitational force ,mg at B downwards .
So net force is 4mg that is normal reaction
But the answer given as 6mg