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The time delay between transmission and the arrival of the reflected wave of a signal using ultrasound traveling through a piece of fat tissue was 0.13 ms. At what depth did this reflection occur?
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Final Answer

$10 \textrm{ cm}$

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OpenStax College Physics Solution, Chapter 17, Problem 75 (Problems & Exercises) (1:10)

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Video Transcript
This is College Physics Answers with Shaun Dychko.  An ultrasound wave is going to reflect off of some other type of tissue here that's probably an interface between two different media here. And so, it's causing a reflection. And then the reflection is detected here. And so, this sound has to travel through the tissue twice. And so, it covers the tissue thickness twice. So this total distance that it covers is two times D, and it does so on a certain time. And this total distance over time is the speed of the ultrasound. Now, we're given what the speed of the sound is, and we know the time as well, and so we'll solve this for D. We'll multiply both sides by T over two, and we end up with this line here. Switching the sides around as well, so we have D over V T over two. So, the speed of sound in tissue is 1540 meters per second multiplied by 0.13 times ten to the minus three seconds, that's 0.13 milliseconds, written in seconds, and divided by two. And this is 0.1 meters, which is 10 centimeters. So, the tissue is 10 centimeters thick.