The length of seconds pendulum is 1 m on earth. If mass and diameter of the planet are doubled than that of earth, then length becomes

For a second pendulum, T =2∴ 2=2πlg=2πR2lGMor 2=2πR2GM (∵l=1m)……….(1)At another planet2=2π4R2lpG(2M)=2π2R2lpGM…….(2)From eqs. (l ) and (2), we getR2GM=2R2lpGM ∴ lp=0.5m