Paragraph : A scientist was asked to find the height of the roof of a dome shaped hall. He took a spherical ball of radius 20 cm which he suspend with the string from the roof of dome. The height of the ball above the ground was 5 cm when suspended. He noted the time of 20 oscillations to be 100 s.
The length of the string used to suspend the bob of pendulum is :

Options

A. 6.39 m
B. 6.14 m
C. 6 m
D. 6.44 m.

Answer

B

Explanation

`T=2pi sqrt((l)/(g))impliesl=(gT^(2))/(4pi^(2))=(20xx5^(2))/(4xx(3.14)^(2))=6.34` m.
length of string used `=l-` radius of bob `=6.34-.20`
`=6.14` m.
So correct choice is (b).