A block of mass m at the end of a string is whirled round in a vertical circle of radius R. The critical speed of the block at the top of its swing below which the string would slacken before the block reaches the top is 

(1) Rg

(2) (Rg)2

(3) R/g

(4) Rg

Concept Videos :-

#25-Friction-as-Centripetal-Force- Solved-Examples-13

Concept Questions :-

Uniform circular motion

(4) At highest point mv2R=mg


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