The mass of the ball is m, and the length of the string is L

[rbwang1225]

Homework Statement

A man plays a Yo-Yo ball as shown in the figure. The mass of the ball is m, and the length of the string is L. The ball moves in a circle along a vertical plane. What is the difference of the string tension at the lowest point ($(T_{2})$) and the highest point ($(T_{1})$)

The Attempt at a Solution

($\frac{mv^{2}}r=T_1+mg=T_2-mg$)
$\Rightarrow T_2-T_1=2mg$
But the answer is 6mg, I guess the problem is that v is not equal at the two different positions. In this case, I don't know how to solve it.

 

[grzz]

Your guess is correct.
Try to use conservation of energy to relate the top speed with the bottom speed.