A boy ties a string around a small stone and then whirls the stone so that it moves in a horizontal circle at a constant speed. He discovers that no matter how hard he tries, the string can never be perfectly horizontal.
The mass of the stone is 0.15kg and the length of the string between the sotne and the boy's hand is 0.50m. The period of rotation of the stone is 0.40s. Calculate the tesion in the string, assuming that the string is nearly horizontal.
I guess you meant to explain the first paragraph?
If so, it's because of the effects of gravity.
For 2nd paragraph
This is a very basic question, and the formulae can be all found in your notes
Use mrw2, where w = 2 pi / T