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Calorimetry: Thermal energy going into translational KE

  1. Jan 22, 2012 #1
    1. The problem statement, all variables and given/known data

    Suppose a cup of boiling water (m=250g) instantaneously cools to room temperature (25°C) with the liberated thermal energy going into translational KE. How fast will the cup fly off the table? Assume the water molecules have 18 degrees of freedom.

    2. Relevant equations

    Uwater=9nRT (not sure about this one)

    3. The attempt at a solution

    So what I first did was try to calculate Q:

    Q=-18750 cal

    I then converted calories to joules:

    -18750 cal x 4.186 J = -78487.5 J.

    I am not sure where to go after this. Any help?
  2. jcsd
  3. Jan 22, 2012 #2

    I'm guessing that since the cup is on a horizontal table, the degrees of freedom is 9. Therefore the energy must be halved.

    Q = 39243.75 J

    Plug it into:

    KE = (1/2)mv2

    v2 = (2KE) / m
    = (2*39243.75) / .250
    = 313950

    √v2 = √313950
    v = 560 m/s

    Can anyone confirm that this is correct?
  4. Jan 24, 2012 #3
    Any help?
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