Minimum centripetal acceleration for string to stay taut at top of vertical circle

[Student4Life]

Homework Statement

:[/B]
One swings a rock at the end of a string. We wish for the string to remain taut and for the rock to travel in a circulat path, in a vertical plane. What mathematical condition must the centripetal acceleration of the rock satisfy for the string to remain taut when the rock is at its highest point?

Homework Equations

:[/B]
[F[/c] = m[v][/2] / r ?

The Attempt at a Solution

So far I come up with this equation to define what is going on but I am not sure [F[/c] = m[v][/2] / r ?

 

[kuruman]

Homework Statement

:[/B]
One swings a rock at the end of a string. We wish for the string to remain taut and for the rock to travel in a circulat path, in a vertical plane. What mathematical condition must the centripetal acceleration of the rock satisfy for the string to remain taut when the rock is at its highest point?

Homework Equations

:[/B]
[F[/c] = m[v][/2] / r ?

The Attempt at a Solution

So far I come up with this equation to define what is going on but I am not sure [F[/c] = m[v][/2] / r ?

You need to do better than that. Draw a free body diagram for the rock when it is at the top of the trajectory, interpret it in terms of Newton's 2nd law, then write down the equation.