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Homework Help: Projectile fired to melt ice

  1. Aug 18, 2012 #1
    1. The problem statement, all variables and given/known data
    Newt needs to melt some ice, so he fires a test projectile (0.025 kg) at 30.0 degrees C at a speed of 240 m/s into a large block of ice at 0 degrees C, in which it becomes embedded (comes to rest). Approximately what mass of ice melts? (i.e. ignore the heat lost by the projectile in going from 30-0 degrees C).

    2. Relevant equations

    3. The attempt at a solution
    The heat of fusion of water is 3.35x10^5J.
    e=(0.025 * 240^2)/2=720J
    720J/3.35x10^5J=0.002, so 0.002kg of ice melts.
    I get the feeling that's incorrect, especially since I didn't do anything with the temperature (although it mentions that I should ignore the heat lost by the projectile).

    Thanks for any help you can provide.
  2. jcsd
  3. Aug 18, 2012 #2
    Energy release of the projectile is its kinetic energy. So assuming the ice is at 0 C, the amount that melts equals the projectile KE divided by the latent heat of the ice. The rotational kinetic energy of the projectile is omitted. This is what you have done and it is correct. I assume the heat of fusion of ice is 3.35X10^5 J/kg. You left off the kg.
  4. Aug 18, 2012 #3
    Thanks a million!
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