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Homework Help: Conservation of Energy and internal energy

  1. Nov 18, 2012 #1
    1. The problem statement, all variables and given/known data
    A 0.75-kg block slides on a rough horizontal table top. Just before it hits a horizontal ideal
    spring its speed is 3.5m/s. It compresses the spring 5.7 cm before coming to rest. If the spring
    constant is 1200N/m, the internal energy of the block and the table top must have:
    A. not changed
    B. decreased by 1.9J
    C. decreased by 2.6J
    D. increased by 1.9J
    E. increased by 2.6J

    2. Relevant equations
    delta E = delta Emec + delta Etherm + delta Eint = 0

    3. The attempt at a solution
    basically I just did conservation of energy which I get that the change in mechanical energy is 1/2kx^2 - 1/2mv^2 = -2.6 J. Which means that the change in internal energy should be positive 2.6 J.
    Why does the answer key say that it's C?
  2. jcsd
  3. Nov 18, 2012 #2


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    2017 Award

    Staff: Mentor

    I think the problem is the unclear definition of "internal energy". 2.6J are lost to heat or other things. If those things do not count as internal energy (?), the energy decreased by 2.6J. Looks strange...

    Edit: Your second post was not very polite.
    Last edited: Nov 18, 2012
  4. Nov 18, 2012 #3
    any ideas?
  5. Nov 18, 2012 #4


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    Science Advisor
    Homework Helper
    Gold Member

    At start we have 4.59J KE. At end, 1.95J of PE, so 2.64J heat. So C only makes sense if it means the total non-heat energy in the system. But the wording clearly excludes the spring, so it must be a mistake.
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