Find the length of the pendulum thread in meters, the oscillation period of which is 13 seconds.

The oscillation period of the pendulum is calculated by the formula

T = 2 * pi * √ (l / g)

This expression is equivalent to the expression T / (2 * pi) = √ (l / g) or T ^ 2 / (4 * pi ^ 2) = l / g

Hence l = g * T ^ 2 / (4 * pi ^ 2)

Numerically l = 9.8 [m / s ^ 2] * (13 [s]) ^ 2 / (4 * 3.14 ^ 2) = 9.8 * 13 ^ 2 / (4 * 3.14 ^ 2) [m] = 1656.2 / 39.4 [ m] = 42 [m]

Answer: 42 m