As shown in the picture to the right, a ball is suspended from the ceiling by a string of negligible mass. When the ball is set into motion such that it moves in a horizontal circle at a constant speed of 1.6m/s, the tension in the string is 7.7N. What is the length of the string?

Answer 1

Does the missing figure show the mass of the ball?

There is centripetal force of magnitude mv²/r (horizontal) and the weight (vertical) of mg to form the resultant tension of the string of 7.7N.
r is the horizontal radius, so must be adjusted for the angle the string makes with the vertical.