The earth has a radius of 6400km. The inner core of 1000 km radius is solid. Outside it, there is a region from 1000 km to a radius of 3500km which is in molten state. Then again from 3500km to 6400 km, the earth is solid. Only longitudingal (P) waves can travel inside a liquid. Assume that the P wave has a speed of `8kms^(-1)` in solide parts and of `5kms^(-1)` in liquid parts of the earth. Calculate the time after which it will be recorded in a seismometer at a diametrically opposite point on earth if wave travels along diameter /

Explanation

As the wave travels along the diameter of earth starting from X, is has to travel the following distances to get recorded in a seismometer at `X^(')` , figure.
(i) `s_(1)=` distance through solid portion` =XB+A A^(')+B^(')X^(')`
`=(2900+2xx1000+2900)km=7800km`
(ii) `s_(2)=` distance through molten portion `=BA+A^(')B^(')`
`=(2500+2500)km=5000km`
Now `upsilon_(1)=8km//s ` and `upsilon_(2)=5km//s`
`:.` Total time taken, `t=t_(1)+t_(2)=(s_(1))/(upsilon_(1))+(s_(2))/(upsilon_(2))=(7800)/(8)+(5000)/(5)`
`t=975+1000=1975sec`