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Homework Help: Thermal properties of matter caluculation

  1. Nov 6, 2007 #1
    A squash ball of mass 46g is struck against a wall so it hits with a speed of 40m/s, and rebounds with a speed of 25m/s.
    Calculate the temperature rise (s.h.c. of rubber is 1600J/kg/K)

    This is fine. I use the equation:

    heat energy = mass x shc x temperature change

    (40 - 25) = 0.046 x 1600 x temperature change

    15/73.6 = temperature change

    temperature change = 0.2K

    Then it asks why is it unecessary to know the mass?

    And I cannot for the life of me think why. Is there another equation I'm supposed to know? Have I made a mistake? Am I overlooking something incredibly obvious?

    Any help/hints would be very much appreciated.

    Thanks,
    Rachael
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 6, 2007 #2

    Astronuc

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    Staff Emeritus
    Science Advisor

    The part of the solution (40-25) is not the heat energy. It's best to write units with the corresponding values.

    The 40 m/s - 25 m/s is simply the change in velocity (which is also the change in specific momentum). The change in energy is the change in kinetic energy and KE = 1/2 mv2. But looking at the righthand side one multiplies the mass * specific heat.

    If we deal with the specific kinetic energy and specific heat, we can eliminate mass from the equation. Thus

    [itex]\Delta[/itex]v2/2 = shc*[itex]\Delta[/itex]T
     
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