Calculating Tension when you swing a ball around a piece of string

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brycenrg
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Homework Statement


You swing a ball around a piece of string(vertically). M = 0.150kg r= 1.10m
Calculate the Tention at the bottom of the arc assuming velocity is twice as fast.

Homework Equations


When I do the problem I'm not sure how to set up the NetForce = ma
How do which force is negative and which is positive? I set tension at the bottom to - making the equation to be
mg - Ft = m(v^2/r)
but i don't get right answer

The Attempt at a Solution


mg - Ft = m(v^2/r)
ft = -4 N
but the answer is 7.35 N
 
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Discussion

Actually I think i just figured it out. The reason it needs to be positive and gravity has to be negative because at the bottom of the path T is pointing inwards because this is Ar. Is this the correct logic? I appreciate it.