1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Distance of building related to echo

  1. Aug 16, 2012 #1
    1. The problem statement, all variables and given/known data
    A road lies between two parallel buildings. A motorist travels with speed 36 km/h on the road and hits the horn. He heard the echo 1 second later. The speed of sound is 330 m/s
    a. Find the distance of the building
    b. When will he hear the second echo?

    2. Relevant equations
    2d = v.t


    3. The attempt at a solution
    The first echo will come from the nearest building and the second echo will come from the further building.

    a. the distance of nearest building to motorist = d1
    2 d1 = v.t
    d1 = 115 m


    I don't know how to find the distance of further building to the motorist and is the speed of motorist being used in the calculation?

    Thanks
     
  2. jcsd
  3. Aug 17, 2012 #2

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Try to imagine you are the motorist, travelling on the road in between two parallel buildings. What do you mean on "nearest" and "further" building? Show the path of the sound that hits the ears of the motorist in a figure.

    ehild
     
  4. Aug 22, 2012 #3
    untitled-12.jpg

    Let P be the motorist and there are two parallel buildings on his right and left side. The nearest building is the right one and the further building is the left one. The motorist moves "into the page" with respect to the reader (same as (×) sign of magnetic field)

    The sound will hit the right building first and being reflected to his ear. Sound will also be reflected by the left building and he will hear the second echo.

    But I don't know how to calculate the distance...
     
  5. Aug 22, 2012 #4

    NascentOxygen

    User Avatar

    Staff: Mentor

    I'd say the intention is that the road follows a path exactly midway between the buildings.
     
  6. Aug 22, 2012 #5
    Oh I see, the distance is twice of what I've calculated. So the speed of the motorist is not important?

    Thanks
     
  7. Aug 22, 2012 #6

    NascentOxygen

    User Avatar

    Staff: Mentor

    The motorist will be in a different location each time an echo reaches his ears, so I'd say you do have to take into account his movement. I'm picturing it as the motorist in an open-top sports car reaching up and firing a starter's pistol as he passes between the buildings (or as he drives through a canyon with steep sides). How long before the radiating pressure wave gets back to him in some form?
     
  8. Aug 22, 2012 #7
    I am not sure, maybe 1 second because that is the time needed for him to hear an echo.
     
  9. Aug 24, 2012 #8

    ehild

    User Avatar
    Homework Helper
    Gold Member

    To make easier to imagine, see picture. The motorist drives just in the middle of the road between the pair of long buildings, at distance D from both, with speed V=36 km/h. He hits the horn: the sound travels in every direction and reaching the wall of the buildings, reflects. A narrow beam of sound wave reaches the middle of the road just as the motorist also reaches there, Δt=1 s later as the horn was hit. The motorist travelled S=VΔt distance. How long distance travelled the sound during that time if the speed of sound is 330 m/s? The path of the sound and that of the motorist make an isosceles triangle. What is D then?

    ehild
     

    Attached Files:

    Last edited: Aug 24, 2012
  10. Aug 25, 2012 #9
    The distance travelled by the motorist in interval 1 s = 10 m
    The distance travelled by the sound in interval 1 s = 330 m

    D = √(52 + 1652) = 165.076 m

    a. distance between building = 2D

    b. he will hear the second echo 1 s later ??

    Thanks
     
  11. Aug 25, 2012 #10

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Check it. Is D the hypotenuse?

    ehild
     
  12. Aug 25, 2012 #11
    Oh yes, I see my mistake.

    For answer (b), am I correct? I think the answer will be 1 s because the motorist and sound undergo uniform motion
     
  13. Aug 25, 2012 #12

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Yes, of course, the second echo follows the first one after 1 s.

    ehild
     
  14. Aug 25, 2012 #13
    Thanks for the help :smile:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Distance of building related to echo
  1. The echo (Replies: 9)

Loading...