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

[brycenrg]

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

 

[brycenrg]

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.