Book: Physics 7th edition, Author: Giancoli Modified 45 the intensity of an eart

Question

Book: Physics 7th edition, Author: Giancoli

Modified 45 the intensity of an earthquake wave passing through the Earth is measured to be 3.0 * 106 J m2 Modify Equation 11-16a for a HEMISPHERICAL wave, as this earthquake approximately

is. That is, assume it spreads out over the area of a HALF sphere. The book has

the wrong units for intensity. It should be watts per square meter, not Joules per square meter.

A) Use this (and the fact that 54.0km is 5.40E4m) to find the power (watts or J/s) of this

earthquake.

B) Find the intensity 10.0km from the source

as the book says. Remember, intensity is

power/area, and your answer should be smaller than the book’s by a factor of 10-squared because

of the inverse r^2

C) Finally, find the power through the area of a square 5.00m on a side

Explanation / Answer

A) Power P = IA

= 3000000 * 4piR2

= 3000000* 2pi * (5.40E4)2

= 5.5 x 1016 W

B) Here R = 10km = 10,000m

I = P/A

= (5.5 x 1017)/2pi*10,0002

= 8.75 x 107 W/m2

C) P = IA

A = side2 = 52 = 25m2

I is not mentioned. Assuming it to be as given in the original question:

I = 3000000 w/m^2

P = 3000000 w/m^2 *25m2

= 7.5 x 107 W

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