# Find the period -- from cross section??

• Feb 19th 2011, 04:39 PM
nehpets
Find the period -- from cross section??
Here is a picture (looking down from above) of striped sand ripples from Sea Rim State Park in E. Texas. The bottom of the picture is south and the top is north. The ripple pattern is 24 inches from south to north. Think about taking a cross section that cuts from the bottom of the picture to the top. The height of the sand along this cross section is somewhat periodic. Imagine the period function p(y) describing the height of the sand as a function of the distance in inches north from the southern edge of the pictured pattern.

WHAT IS THE PERIOD???

http://dl.dropbox.com/u/195969/IMG_6368.jpg

Thanks so much for any help. I think I'm just confused by the wording.
• Feb 19th 2011, 05:13 PM
Ackbeet
How many times do you go from a valley to a valley, from bottom to top?
• Feb 19th 2011, 05:16 PM
nehpets
14, I think.

That means the period is 24 in /14, then, right?
• Feb 19th 2011, 05:22 PM
Ackbeet
Not quite. Imagine a similar sort of function: -cos(x). Suppose you looked at this function from 0 to 4pi. How many times would it go from a valley to a valley?
• Feb 19th 2011, 05:28 PM
nehpets
• Feb 19th 2011, 05:30 PM
Ackbeet
Exactly. So the period of -cos(x) is what?
• Feb 19th 2011, 05:33 PM
Ackbeet
I think your sand oscillates 15 times, by the way, not 14.
• Feb 19th 2011, 05:36 PM
nehpets
4 pi / 2 = 2 pi

so the period of that function is 2 pi

does that mean, then, that (in the original function):

24" / 15 = 1.6"

thanks ackbeet
• Feb 19th 2011, 05:37 PM
Ackbeet
Looks good to me, including the units.

You're very welcome!