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İmpulse from the string

  1. Feb 2, 2017 #1
    1. The problem statement, all variables and given/known data
    A ball of mass 1.00Kg is attached to a loose string fixed to a ceiling.The ball is released from the rest and falls 2.00m,where the string suddenly stops it.Find the impulse on it from the string.

    2. Relevant equations.
    Kinematic equations
    ##\vec J=\vec FΔt=Δ\vec P##

    3. The attempt at a solution
    +z direction is upward.
    ##ΔH=v_0t+\frac 1 2at^2## so this becames

    ##-2m=0-\frac 1 2gt^2##
    ##t=0.638 s##

    and

    ##-2m=v_0t-\frac 1 2gt^2##
    we know ##t## so
    ##-2-1.99=v_0(0.638s)##
    ##v_0=-6.25\frac m s##

    then
    ##\vec J=\vec FΔt=Δ\vec P##
    so
    ##P_f=m(-6.25)##
    ##P_i=m0##
    ##ΔP=-m6.25=-6.25Ns=J##

    answer says ##6.25Ns## so theres no minus sign...
    I didnt understand why...The question says "Find the impulse on it from the string" so the thing I found is some other impulse which created by something else..?
     
  2. jcsd
  3. Feb 2, 2017 #2

    jbriggs444

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    Science Advisor

    The final momentum ##P_f## is what again? Did the problem statement not say that the string stops it?
     
  4. Feb 2, 2017 #3
    Is it wrong ?.I just wrote the velocity of the ball...Sure,spring stops it.I see I guess.
     
  5. Feb 3, 2017 #4

    jbriggs444

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    Science Advisor

    What is the "initial" condition just before the string stops the ball?
    What is the "final" condition just after the string stops the ball?
     
  6. Feb 3, 2017 #5
    Ohh ##P_i## is not ##0## its ##-mv## so ##0-(-m v)=mv## I solved
     
  7. Feb 3, 2017 #6
    Thank you
     
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