Minimum speed to maintain a circular trajectory

[xxgaxx]

Homework Statement

a rock of mass 0.500 kg is tied to a string of radius 75 cm and is revolving in a vertical circle at a uniform speed. determine the minimum speed for the rock to maintain a circular trajectory without the string collapsing (not staying taut). (Hint: where would that occur in the trajectory?)

m=0.500 kg
r=.75 m

Homework Equations

mg=(mv^2)/r

The Attempt at a Solution

v=(gr)^(1/2) (square root of g*r)
v=2.7 m/s
i don't know where i use the mass of 0.500 kg

 

[Hootenanny]

Homework Statement

a rock of mass 0.500 kg is tied to a string of radius 75 cm and is revolving in a vertical circle at a uniform speed. determine the minimum speed for the rock to maintain a circular trajectory without the string collapsing (not staying taut). (Hint: where would that occur in the trajectory?)

<snip>

mg=(mv^2)/r

Are you sure that the weight of the rock is the only force acting? Furthermore, are you sure that the weight of the rock always acts radially?