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Homework Help: Negative or Positive

  1. Dec 24, 2009 #1
    1. The problem statement, all variables and given/known data
    Suppose a 6.00g bullet traveling at 100m/s strikes a bulletproof vest and comes to rest in about 600microseconds. What average force will it impart to the happy wearer?

    2. Relevant equations
    Use the Momentum-Impulse Theorem
    I took the initial direction of the bullet as the positive direction.

    3. The attempt at a solution
    By using the momenum-impulse theorem, where vi = 100m/s and vf = 0m/s, I calculated the force to be -1.00kN. However, the negative sign tell me that this force is against the bullet, which should be the force that the vest apply to stop the bullet, not the force the bullet imparted to the wearer. The force that the bullet will impart on the wearer should be in the positive direction, which is the initial direction of the bullet. Therefore, I think the average force that the bullet imparted to the wearer should be +1.00kN. Nevertheless, the answer in the textbook for this question is -1.00kN, so I don't know whose right, the textbook or me.
  2. jcsd
  3. Dec 24, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    I would say you were right.
    If you take the direction of the bullet to be +ve then the force obviously acts in that direction on the victim.
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