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Homework Help: Grade 12 Physics Bomb explosion

  1. Apr 22, 2008 #1
    Question:
    A bomb explodes and 3 pieces are flying in different directions.
    Piece 1 : weighs 2kg and is travelling at the North East direction
    Piece 2 : weighs 1kg and is travelling in the south direction ( exactly downards )
    Piece 3 : weighs 3 kg and is travelling in the west direction ( exactly to the left )
    Find the velocity and direction that Piece 1 is travelling at.

    I know to use the momentum equation twice , one with the horizontal direction and one on the vertical direction.

    Horizontal:
    M1V1(initial)+M2V2(initial)+M3V3(initial) = M1V1(Final)+M2V2(Final)+M3V3(final)
    Since all the initial velocities are 0 ( because bomb was at rest before explosion , the entire left side is equal to zero)

    0 = 2kg ( V1F ) cosx + 1kg ( 30 m/s) cos 90 + 3kg ( 20 m/s ) Cos 180
    = 2kg (V1F) cosx - 60 Kg/m/s
    30 m/s = V1F cosx
    Above , i canceled M2V2(FINAL) because i set that as the reference direction and angle , so it counts as 0. and i did minus 60 kg/m/s because its to the negative direction.

    Vertical :
    M1V1(initial)+M2V2(initial)+M3V3(initial) = M1V1(Final)+M2V2(Final)+M3V3(final)
    Since all the initial velocities are 0 ( because bomb was at rest before explosion , the entire left side is equal to zero)

    0 = 2kg ( V1F ) sinx + 1kg ( 30 m/s) sin 90 + 3kg ( 20 m/s ) sin 180

    = 2kg ( V1F) sinx - 30 Kg/m/s
    15 m/s = V1F sinx

    Then divided the final equations to get

    Sinx V1F 15 m/s
    --------- = -------
    Cosx V1F 30 m/s



    Tanx V1F = .5 m/s

    now i have no idea wat to do to find the direction and speed :( can anyone help please?
     
  2. jcsd
  3. Apr 22, 2008 #2
    You neglected to cancel out V1F in your final equations.
     
  4. Apr 22, 2008 #3
    No , i just rewrote it as Tanx V1f

    Sinx/Cosx = Tanx ( trig identity )
     
  5. Apr 22, 2008 #4
    any ideas?
     
  6. Apr 22, 2008 #5

    alphysicist

    User Avatar
    Homework Helper

    Hi Shin,

    I think you misunderstood Phlogistonian's post.

    You had:

    Sinx V1F 15 m/s
    --------- = -------
    Cosx V1F 30 m/s

    this simplifies to

    tan x = (1/2)

    The V1F is not inside the trig function and so will cancel since it is in both numerator and denominator.
     
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