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Homework Help: One dimensional problem

  1. May 21, 2006 #1
    Please say whether these 2 problems are right.
    1) A body of mass 2kg moving on a horizontal surface with an initial velocity of 4 m/sec comes to rest after 2 seconds. If one wants to keep this moving on the same surface with same velocity, what is the force that is needed to be applied?
    I solved it in the following way:
    Acceleration = (0 – 4)/2
    = -2m/sec^2
    Retarding force = mass x retardation
    = 2 x 2
    = 4 Newton
    So if a force equal to the retarding force is applied, the body will move with a constant velocity of 4 m/sec^2. But the book answer is 2 Newton. What is your opinion Sir?

    2) A splash is heard 4.28 seconds after a stone is dropped into a well 78.4 meters deep. What is the velocity of sound?
    I solved it in the following way:
    Time taken by the stone to reach the well = [2H/g]^1/2
    (Here H is depth of well & g = 9.8 m/sec^2)
    = 4 seconds
    Time taken by sound to reach the observer = 4.28 – 4
    = 0.28 seconds
    Velocity of sound = 78.4/0.28
    = 280 m/sec
    But the book answer is 341 m/sec. What is your opinion?I think the book answer is wrong.
  2. jcsd
  3. May 21, 2006 #2
    for the first problem i also get an acceleration of 2 m/s^2
    since sum F = 0, then it should follow that F = m*a = 4 N
    The book must have a typo

    for the second problem it also looks like you solved the problem correctly.
    While speed typically does travel around 340 m/s at standard atmospheric conditions, the answer does come out to 280 m/s with the numbers they have given you. What text is this anyways?
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