Hi there!
In this instance, the object's centripetal force is provided by the horizontal component of the tension, so:
Tsinθ = mv²/r
**We use sine because in this situation, the angle is with the vertical**
We can plug in the known values for tension and theta:
60sin(60) = mv²/r
51.96 = mv²/r
The radius is equivalent to the sine of the string in respect to theta:
sin(60) = O/H = r/L
2sin(60) = 1.732 m
Now, solve for the velocity:
51.96 = mv²/r
51.96r / m = v²
51.96(1.732)/.400 = v²
v² = 225
v = 15 m/s
