Answer:
0.60 N, towards the centre of the circle
Step-by-step explanation:
The tension in the string acts as centripetal force to keep the ball in uniform circular motion. So we can write:
(1)
where
T is the tension
m = 0.015 kg is the mass of the ball
is the angular speed
r = 0.50 m is the radius of the circle
We know that the period of the ball is T = 0.70 s, so we can find the angular speed:

And by substituting into (1), we find the tension in the string:

And in an uniform circular motion, the centripetal force always points towards the centre of the circle, so in this case the tension points towards the centre of the circle.