Problem with tether balls - How high should I release the ball

[raferalston]

hi, I am a grade 12 student and am stuck on this question. could somebody help me solve it or solve it for me.
the question is:
A 1 kg tether ball is suspended by a string .8 m long from a nail a point X, .5 m vertically above a horizontal rob at Y. when the ball is released from Z, it swings down and the string catches on Y. how high above Y must the tether ball be released so that it will swing completely around a circle of a radius .3 m about point Y.

 

[HallsofIvy]

At its lowest point, .8 m below X, the ball must have enough kinetic energy that, after it has "caught" on y and swings to its highest point .3m above y (and so .6 m above its lowest point- .2 m below X, it still has some kinetic energy left to "get over the top".

 

[Doc Al]

At its lowest point, .8 m below X, the ball must have enough kinetic energy that, after it has "caught" on y and swings to its highest point .3m above y (and so .6 m above its lowest point- .2 m below X, it still has some kinetic energy left to "get over the top".

Careful... the amount of KE that must be left at the top is non-zero: there is a mimimum amount needed to keep the string taut. (Hint: releasing the ball 0.3m above Y will not give it enough speed.)