1 The radius of a typical human ear drum is about 04 mm Find

(1) The radius of a typical human ear drum is about 0.4 mm. Find the energy per second

(J/s) received by the ear drum when it listens to a sound that is at the threshold of

hearing (1012 W) and the threshold of pain (1 W).

(2) Two points A and B on the surface of the earth are at the same longitude and

separated 60.0 apart in latitude. Suppose that an earthquake orginates at point A

creates a Pwave that reaches point B by traveling straight through the body of

the earth at a constant speed of 7.80 km/s. The earthquake also radiates a Rayleigh

wave, which travels along the surface of the earth similar to a surface wave on water,

at 4.50 km/s. a) Which of these two seismic waves arrives at point B first? b) What

is the time difference between the arrival of these waves at point B? Consider the

earth a sphere of radius 6,371 km.

Solution

1)use the formulat I= P/A.

At the threshold of hearing:

P=(10^-12 W/m²)(4*pi*(0.0004m)²)

= 2.0X10^-18W

=2.0X10^-18J/s

At the threshold of pain:

(2)

Substitute the values.

t_interior=6371 km / 7.8 km/s

=816.79s

a) So, Interior wave first arrives at point B

b) 1482.59s - 816.79s = 665.8s