Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Quick wave question

  1. Sep 11, 2007 #1
    1. The problem statement, all variables and given/known data

    If the ultrasound pulses are 3.5 microseconds (is [tex]\mu[/tex] the right symbol for "micro"?) long, and travel 100mm in 70 micro seconds, what is the smallest distance one part of a baby can be behind another if the reflections can just be told apart (dont get mixed together in time).

    2. Relevant equations

    v = f x wavelength maybe?

    3. The attempt at a solution

    I couldnt find out how to do it in the textbook

    can someone please explain the theory behind it, and possibly leave the calculation to me?

  2. jcsd
  3. Sep 11, 2007 #2


    User Avatar
    Homework Helper

    I'll give an analogy which I think will help illustrate what's happening... suppose you have 2 trucks each of length L... they are driving side by side at the same speed. At some point the first truck turns around in the opposite direction... the second truck turns around at a distance X beyond the spot where the first truck turned around...

    How long does the distance X have to be, so that no parts of the two trucks are side by side... meaning one truck is completely behind the other on the way back?
    Last edited: Sep 11, 2007
  4. Sep 11, 2007 #3
    umm, 0.5L by any chance?

    cant really work it out mathematically, but thats what it would seem right logically in my head. because as one of the trucks turns around, by the time one of the truck goes distance 0.5L to X the one that has turned around has gone back 0.5L right?
  5. Sep 11, 2007 #4


    User Avatar
    Homework Helper

    Yes, exactly right. Thinking of the pulses like trucks that turn around in the same way... the distance between the two parts of the baby has to be 0.5*length of the pulse. Can you calculate the length of the pulse?

    Find the speed of the pulse first. Then you can get the length.
    Last edited: Sep 11, 2007
  6. Sep 11, 2007 #5
    so speed would be 0.1/0.00007 m/s

    distance = speed x time, so i take it the length of the pulse is 0.005, and the so the final answer will be 0.0025m???

    thnx for the help buddy


    just check the answers and im right :D

  7. Sep 11, 2007 #6


    User Avatar
    Homework Helper

    Cool! Good job! :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook