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Homework Help: Calculating recoil

  1. May 31, 2015 #1


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    1. The problem statement, all variables and given/known data
    A 20kg cannon ball is fired from a 600kg cannon, with an initial velocity of 450 m/s. Calculate the initial velocity of recoil of the cannon.

    2. Relevant equations
    Conservation of momentum formula

    m1u1 + m2u2 = m1v1 + m2v2

    m = mass
    u = initial velocity
    v = final velocity

    3. The attempt at a solution
    m1u1 + m2u2 = m1v1 + m2v2

    let v = recoil velocity

    20 x 0 + 600 x 0 = 20 x 450 + 600 x v
    0 = 9000 + 600v
    -9000 = 600v
    v = -15 m/s

    The negative is from the negative direction so the answer is 15 m/s. Is my logic correct, because I know how momentum is conserved, but is this the right way to think about it? Also how can Newton's law explain this?

  2. jcsd
  3. May 31, 2015 #2
    Yeah your idea is correct to solve this problem. Newton third law says
    "When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body."
    This is the thing happens here.You exert a force and cannon ball fired.Canon ball goes forward but canon goes back for this affect.Newton third law explain this easily.
  4. May 31, 2015 #3


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    Ok thanks a lot
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