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  • The earth has a radius of 6400 km. The inner core of the 1000 km radius is solid. Outside it, there is a region from 1000 km to a radius of 3500 km which is in a molten state.

The earth has a radius of 6400 km. The inner core of the 1000 km radius is solid. Outside it, there is a region from 1000 km to a radius of 3500 km which is in a 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 km/s in solid parts and of 5 km/s in liquid parts of the earth. An earthquake occurs at someplace 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?

Answers (1)

r1=1000km, r2=3500km, r3=6400km, d1=1000km

 d2=3500-1000=2500km

 d3=6400-3500=2900km  

Solid distance diametrically=2d_{1}+d_{3}=21000+2900=7800km 

Time taken by wave produced by earthquake in solid part=\frac{7800}{8}s

Liquid part along diametrically=2d_{2}=2\times 2500

 Time taken by seismic wave in liquid part=2\times \frac{2500}{5}

 Total time=\frac{7800}{8}+\frac{5000}{5}=1975sec=32minand55sec

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