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Calculate Friction in Spring hanging from ceiling

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  1. Feb 29, 2016 #1
    1. The problem statement, all variables and given/known data

    A massless spring with force constant k = 200N/m hangs from the ceiling. A 2.0 kg weight is attached to the free end of the spring and released. If the weight falls 17 cm before starting back upwards, how much work is done by friction during its decent? (Note: choose ground zero for gravitational potential energy to make this easy!)

    2. Relevant equations

    Hooks Law;
    E=1/2*k*x*x


    3. The attempt at a solution

    http://www.wikispeedia.org/tmp/untitled.png [Broken]
    E= 1/2 * 200 * 0.098 * 0.098 Joules

    What is worries us is that we didnt use the 17cm in our solution.

    Thanks
     
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Feb 29, 2016 #2

    gneill

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    Staff: Mentor

    Hi cellurl2, Welcome to Physics Forums.

    Start by identifying all the places that energy can be stored, come from, or go, in the system.
     
  4. Feb 29, 2016 #3
    (corrected)
    PE lost is m*g*deltah = 2kg*9.8*0.17m= 3.332 joules

    PE gained by spring= 1/2 * k * x* x = 1/2 * 200N/m * 0.17m*0.17m= 2.89 joules

    So? is that it , friction (spring tension/heat??) just 3.332-2.89= 0.442 joules
     
    Last edited: Feb 29, 2016
  5. Feb 29, 2016 #4

    gneill

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    Staff: Mentor

    Check the value that you used for the spring constant. :wink:
    As it turns out, yes, it will be the difference. Energy is lost from the system due to frictional heating.

    This is why I suggested that you start by identifying all the places that energy can reside or got to in the system. Once you've done that, these sorts of problems become a matter of summing things up accordingly for the given scenarios.

    It's important to understand the assumption you've made: You have assumed (correctly) that there will not be any kinetic energy involved at the two positions that you've chosen to compare the energy in the system. In other problems of this type you may be given information about the instantaneous velocity at some position that is not at an extreme of the motion.
     
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