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Homework Help: Thermodynamics problem

  1. May 4, 2009 #1
    1. The problem statement, all variables and given/known data
    A forensic investigator notes a 7.2g lead bullet stopped in a door frame melted completely on impact. Assuming the bullet was shot at RT (20C) what would the investigator calculate as the minimum nozzle velocity of the gun to be? (C_lead = 130J/kg*C), (L_lead = 2.5*10^4J/kg) Lead melts at 327C

    2. Relevant equations
    Q = mc(delta)T
    K = 1/2*m*v^2 (?)

    3. The attempt at a solution
    I know that the objective of this problem is to use the energy to determine the velocity. I found that Q = 287.4J. I'm not sure what to do from here, however. If I multiply 2.5*10^4J/kg by 0.0072kg, I get 180J. Do I subtract this from 287.4J, and use the result to calculate velocity? Is the difference kinetic energy? I'm not sure if I'm on the right track here. Any help is appreciated!
  2. jcsd
  3. May 4, 2009 #2

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    What is the total energy needed to melt the bullet? Remember, it has to be first heated up to the melting point, and the total energy is equal to energy needed to heat it and then melt it. This energy has to come from the kinetic energy.
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