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Homework Help: Momentum and Heat

  1. Dec 3, 2011 #1
    1. The problem statement, all variables and given/known data
    A 30 kg boy on a 5 kg sled takes a running start from the top of a slippery hill, moving at 4.00
    m/s. The hill is 60 m long at an angle of 11.5 degrees above the horizontal. The boy and sled
    reach the bottom of the hill moving at 10.0 m/s. (a) How much mechanical energy has been
    transformed into thermal energy? (b) If the steel runners of the sled have a mass of 1.500 kg and
    half of the heat generated remains in the runners, by how much does the temperature of the steel
    runners rise during the ride downhill? Specific heat for steel = 452 J/(kg·°C). [ANS: 1.94°C]

    2. Relevant equations

    Wnc=delta KE+ delta PE
    delta T = Q/nc

    3. The attempt at a solution

    Wnc= delta KE + delta PE
    = 10920 J

    delta T = Q/nc
    = .5(10920)/(1.5*452)
    = 24.16 C

    the answer as shown is 1.94 C so i dont know where i am going wrong... i am also solving part b because part a is theory.

    Thank you
  2. jcsd
  3. Dec 3, 2011 #2


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    Homework Helper

    You can only do part B correctly if you have part A done correctly.
    That energy figure is way too high.

    For a start .5(35)(10^2) - .5(35)(4^2) is not equal .5(35)(6^2)

    That is like saying (a - b)2 = a2 - b2 which it certainly is not

    Also the change in height is way less than 30m. The slope is only 11.5o. The change in height is [co-incidentally] alarmingly close to 11.5 - I thought I had made a calculator error.
  4. Dec 3, 2011 #3
    you used sine law to find the height correct ???

    and part a was just a theoretical answer of explanation not writing.
  5. Dec 3, 2011 #4


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    Homework Helper

    if you call sin(11.5) = h/60 the sine law, then yes.
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