The length of the pendulum is increased by 90 cm and period is doubled then it's original length will be

Options

A. 30 cm
B. 60 cm
C. 90 cm
D. 45 cm

Answer

A

Explanation

`l_(2)=l_(1)+0.9 andT_(2)=2T_(1)`
`T_(1)propsqrt(l_(1))" "T_(2)propsqrt(l_(2))`
`(T_(2))/(T_(1))=sqrt((l_(2))/(l_(1)))=sqrt((l_(1)+0.9)/(l_(1)))`
`2=sqrt((l_(1)+0.9)/(l_(1)))`
`4l_(1)=l_(1)+0.9`
`3l_(1)=0.9`
`l_(1)=0.3=30cm`