A stone of mass m is tied to a string and is moved in a vertical circle of radius r making n revolution per minute. The total tension in the string when the stone is at its lowest point is

The tension at the lowest point is given byT1 = mr(v2+gr)        = m(v2r+g)       = m(r2ω2r+g)(v = rω)------(i)Now ω = 2πf where f = frequency of revolution. The frequeny of revolution per sec, n = (f60)ω = 2 πn/60 = (πn30)-----(ii)From equation (i) and (ii),    T1 = m[r2π2n2900r+g]                                                       = m[g+(π2n2r)900]