Acceleration of block while sliding down upper half =gsinϕ retardation of block while sliding down lower half =−(gsinϕ−µgcosϕ) For the block to come to rest at the bottom, acceleration in I half = retardation in II half. gsinϕ=−(gsinϕ−µgcosϕ) ⇒µ=2tanϕ