for ur second question, considering a air-resistance free atmosphere, the bob needs a velocity of 0 at the top most position to complete a circle ( since there is no possiblity of tauting)
lets assume that the circle nearly touches the ground .if the radius of the cirle is l. then the total PE at the top is mg(2r).KE = 0 . TE = 2mgl
when it is at the bottommost postition , the entire energy is kinetic
KE = 2mgl v^2 = 4gl
as for ur first question, since there is a possiblity of tauting the minimum velocity at the topmost position must be sqrt(gl) (:: mv^2 / r = mg). applying energy conservation principle the velocity at the bottim must be sqrt(5gl)
Moderation Team
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