A child ties a rock to a string and whirls it around in a horizontal circle.

Rotation radius (perpendicular to the turning axis):

r = 1.18 x cos(30) .. .. r = whatever you get... let's call it r since I don't have a calculator

Tension in string:

Vertical component = mg ... (supporting the weight of the rock)
Horizontal component = mv2/ r ... (providing the centripetal force)

So:

tan(30) = mg \(/\) (m * v2 / r) ...

tan(30) = g*r / v2

v2 = g*r / tan(30) ... .. 9.80 x 1.18 / tan(30) .. .. v2 = some number (m/s)2 ...

Period time = circumference / velocity .. .. (2\({\pi}\) x 1.18) / 4.70 = your answer.

Consider this question:

A child ties a rock to a string and whirls it around in a horizontal circle. Assuming a 1.38m-long string making 20.0degrees angle below the horizontal, find the speed of the rock.

I'm assuming it's very similar to yours!

Here's the solution

Consider the tention force of the atring as T, and mass of the object is m.

It is very helpfull to you if I can draw a figure.. but it is difficult to draw a figure here.

Consider the the object and resolve all froces in radial direction, then, Centrafigual force of mgV^2/r is acting to outer direction and T*Cos20 to inword direction

TCos20 = mV^2/r                     ...1

Consider the vertical forces

TSin20 = mg                     ...2

from 2/1 Tan20 = g*r/V^2

r = 1.38 Cos20

V = 5.91 m/s

Enjoy