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Law of conservation of energy problem [reupload]

  1. Nov 15, 2016 #1
    1. The problem statement, all variables and given/known data
    THIS PROBLEM CONSISTS OF NOTHING MORE THAN WHAT IS BEING STATED (NO VALUES, NO EQUATIONS, NOTHING) THIS IS A WORD PROBLEM WHERE NO CALCULATIONS ARE TO BE MADE: A mass hangs from a vertical spring and is initially at rest. A person then pulls down on the mass, stretching the spring. Does the total mechanical energy of this system increase, decrease or stay the same? Explain.

    2. Relevant equations
    none

    3. The attempt at a solution
    So what i said was the total mechanical energy of the system is constant, meaning that it stays the same (I believe this is so because one is decreasing the gravitational potential energy of the body whilst increasing the elastic potential energy such that the total mechanical energy is kept constant)

    Is that right? If not, please explain to me where my interpretation of the problem has come to faults with the correct answer
     
    Last edited: Nov 15, 2016
  2. jcsd
  3. Nov 15, 2016 #2

    gneill

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    Staff: Mentor

    Hi Kianlos.

    Is the person who pulls down on the spring considered to be part of the system? Where did the energy come from for him to pull down the spring? It wasn't potential energy already associated with the rock, otherwise the rock would have stretched the spring further.

    If the person lets go of the spring after stretching it, what happens? Would the rock return to its previous rest position and stop there?
     
  4. Nov 15, 2016 #3
    The person who pulls down on the spring is considered not to be part of the system (the system only consists of the spring and the mass)... Okay, if it wasn't potential energy already associated with the rock then can I just go on to say that the total mechanical energy of this system increases instead? I say this because if the object is initially at rest and we are not to factor in gravitational potential energy and are to pull upon the said object bringing about some form of stretch, is it just safe to say that the total mechanical energy of the system increases?
     
  5. Nov 15, 2016 #4

    jbriggs444

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    Science Advisor

    Gravitational potential energy is a factor. What @gneill and @Kianlos are trying to get you to consider is whether the change in gravitational potential energy is the same, greater than or less than the change in potential energy in the spring.
     
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