Hints Ch.17 7 9 18 22 24 25 30
More Helpers
7.  

Note: V = speed of sound and H / 2 = Vpt, where Vp = plane speed.
9.  Parts (a) and (b) are straight forward... For (c), just take the partial derivative like you did in Q3 hints.  You will get a function of the form:
A sin(15.7x  - 858t).
A = max. speed of the molecule.
18. See class notes. Use equation 17.7 and take anti- logs of both sides of the equation..
22. Intensity =  I = P/area of sphere.  Assume the spheres have radii r1 and r2. Thus I2/I1  = r12/r22.  Take the log base 10 of this and a factor of 2 will be an outside factor ! Use definition of the intensity in dB to get the factor of 20. Review the lab  write-up on B-field plot that you did  for the algebra steps. 
24.  See notes from lecture ! Use the result of problem 22.  In dB, was shown in that problem (22):

You need to determine the two radial r distances for this problem. We know that:
(80 - 60) dB = 20 log(r1/r2). But we have two unknowns, the two radii, so we need two equations. Can you think of another one ? Remember that the two observers are 110 m apart. Close your eyes and try thinking about the second equation before you continue reading this hint… Now, open your eyes. After some thought, you probably figured out that the second mathematical relationship is:
r1 + r2 = 110 m. Now solve the two equations for r1 and r2.
25. See fig. P17.25.

(a) In dB, find:

Note:

You need to find the radial distance r1 using the Pythagorean Theorem.
(b) In dB, find :

Note:


You need to determine the radial distance r2 using the Pythagorean Theorem.
(c) Be careful ! You must add the intensities in part (a) and (b). Find I1 + I2 = I f .
Then calculate in dB:
30. (a) Energy equals power multiplied by time. And power equals intensity times area:

(b) In dB: