m = mass of the ball attached to friction less surface = 2 kg
r = radius of the circle in which the ball moves = 0.5 meter
T = tension force in the string = 16 N
F = centripetal force
centripetal force is given as
F = m v²/r
the centripetal force required to move in circle is being provided by the tension force here , hence we can write
T = F
T = m v²/r
Tr = m v²
v² = Tr/m
v = √(Tr/m)
inserting the values in the equation above
v = √(16 (0.5)/2)
v = 2 m/s