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

Explanation

Here required time is `t =t _(1) + t _(2) +t _(3)`
`= (d _(1))/( v _(1)) + (d_(2))/( v _(2)) + (d_(3))/( v _(3))`
`= (2 ( 1000 -0))/( 8) + (2 (3500 -1000))/(5) + (2 ( 6400 - 3500))/(8)`
`= 2 [125 +500 +362.5]`
`=2 xx 987.5`
`therefore t = 1975 s`