We determine the velocity at `C` so that it reaches `A` along a parbolic trajectory, then applying the law of conservation of energy to the motion `ABC` (as non-conservative force are absent)
`v=` velocity at point `C=(3)/(2)sqrt(gR)`
applying conservation of energy from `A` to `C`
`(1)/(2)mu^(2)=2mgR+(1)/(2)mv^(2)`
`rArr u=(5)/(2)sqrt(gR)`