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: Need some Homework PLEASE

  1. Dec 9, 2007 #1
    Need some Homework help PLEASE

    1. The problem statement, all variables and given/known data(problem one i need help with)

    A light string of mass 10.0 g and length L = 3.00 m has its ends tied to two walls that are separated by the distance D = 1.90 m. Two masses, each of mass m = 1.95 kg, are suspended from the string as in Figure P13.64. If a wave pulse is sent from point A, how long does it take to travel to point B?
    Diagram is attached

    I think you apply newtons second law for equilibrim applied to the block and mass per unit length of the string is =M/L
    Sqaure root of (1.95kg)(9.8m/s^2)/(10g)(1.90m)=62.6m/s


    Problem 2 i need help with.

    A stone is dropped from rest into a well. The sound of the splash is heard exactly 2.20 s later. Find the depth of the well if the air temperature is 12.0°C.

    I have completed the rest of my webassign but i am completely lost on these two problems. I have pages of notes trying to solve the first problem but and totally lost on the second. Thanks so much.

    Attached Files:

    Last edited: Dec 9, 2007
  2. jcsd
  3. Dec 9, 2007 #2
    I think I need to see a diagram for #1 and work on both problems before I know how I can help you.
  4. Dec 9, 2007 #3
    Hey, I'm assuming in your first problem a mass is hung from each side of the string? If so, think about what determines the speed of a pulse or wave in a given medium; that is, is determined by the force acting on the medium, or is it a parameter of the medium? What do the masses on the end of the string do to the string?

    The second problem is best tackled in two parts. It is helpful to write out which equations you have at your disposal (in this case, the kinematic equations for motion in one dimension) and how many unknowns you're working with. If there are two unknowns, you'll need two separate expressions relating these two unknowns. As a hint, the speed of sound at varying temperatures can be found in your textbook.

    Hope this helps.
  5. Dec 10, 2007 #4

    Shooting Star

    User Avatar
    Homework Helper

    You have to find the tension in the string joining the two masses, and then apply the formula for speed of wave.

    If t is the time for the stone to fall, then h = (1/2)gt^2. Now find the time for the sound to travel distance h in time (2.2 - t), and eliminate t to find h.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook