Question

what is the time period of oscillation of a second's pendulum on the surface of planet having mass and radius double those of earth ?

Answer 1

osillation of pendulum does not depend upon mass and radius of earth.
 it depends upon gravity of that planet...
 Time period is inversly proportional to gravity
    
let see
 let mass of that planet=2M (let M is the mass of earth)
radius of that planet =2R (let R is the radius of earth)

 we know that
       g= GM/R²

g = g × 2M/4R²
 

g = GM/2 R²
gravity on that planet is g/2

now
we know that time period T= 2π√l/g/2
 T°=√2×(2π√l/g)
         given that 2nd s pendulum  means t=2sec
T° =√2×2
T=2√2
 
 

Answer 2

I have solved as considering the pendilum...now it is given that it is second's pendilum so put T=2 sec. u got ur ans..