Answer:
The length is 5.2 mm.
Step-by-step explanation:
Given that,
Time period T²= 0.021 s²
Gravity due to acceleration = 9.78 m/s²
We need to calculate the length
Using formula of time period of pendulum


Where, l = length
g = acceleration due to gravity
T = time period
Put the value into the formula



Hence, The length is 5.2 mm.