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Homework Help: Linear Momentum Bullet

  1. Feb 12, 2017 #1
    1. The problem statement, all variables and given/known data
    A bullet of mass 0.011 kg is fired at a speed of 850 m/s. It embeds itself in a man of mass 90 kg.

    a) Find the momentum of the bullet before it strikes the man.

    b) Assuming it takes 0.1 seconds for the bullet to come to rest after hitting the man, calculate the average force that the bullet exerts on the man.

    2. Relevant equations
    I=Pf - Pi

    3. The attempt at a solution
    a) Po = m*Vo = (0.011 kg)(850 m/s) = 9.35 N/s

    b) I = Pf - Po = (0.011 kg)(0 m/s) - 9.35 N/s = - 9.35 N/s
    F = I/t = -9.35 N/s / 0.1 s = -93.5 N

    Question is why is the force negative. Shouldn't it be positive as the force is directed in the same direction as the bullet's movement and thus a positive direction?
  2. jcsd
  3. Feb 12, 2017 #2


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    Homework Helper
    Gold Member
    2017 Award

    Yes, the force on the man should be positive. But did you calculate the impulse of the force on the man or did you calculate the impulse of the force on the bullet?
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