Ncert Exercises & Solutions

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 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^-1 in solid parts and of 5 km s^-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?

Hint: The speed is defined as the distance travelled per unit time.

Step 1: Use the formula of speed.

Speed of wave in solid= 8 km/s.

Speed of wave in liquid= 5km/s

Required time=

[\frac{1000 - 0}{8} + \frac{3500 - 1000}{5} + \frac{6400 - 3500}{8}] \times 2 [∵ diameter = radius \times 2]

[\frac{1000}{8} + \frac{2500}{5} + \frac{2900}{8}] \times 2 [∵ time = \frac{distance}{speed}]

=[125+500+362.5]

\times

2 =1975

As we are considering at diametrically opposite point, hence there is a multiplication of 2.

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