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Work, Energy, Displacement

  1. Feb 16, 2015 #1
    1. The problem statement, all variables and given/known data

    In the pictures.
    2. Relevant equations
    KE = 1/2mv^2.
    PE = 1/2kx^2
    Work external = Delta KE + Delta PE
    W = F* d


    3. The attempt at a solution
    In the pictures.


    Comments:

    On Problem 3 ai, I am not sure whether I have identified the correct horizontal forces. The hint gave two, but I believe there could also be a force that the wall exerts on the spring.

    I am also not sure which way the force points when a spring is stretched and extended. I believe it is in the direction of the equilibrium unextended position. Please let me know if my answers/reasoning is correct. Thank you.
    2q1c97a.jpg

    2a5akqv.jpg
     
    Last edited: Feb 16, 2015
  2. jcsd
  3. Feb 17, 2015 #2

    haruspex

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    They want you to identify the forces acting on the SB system from outside that system. Is the force between spring and block such a force?
     
  4. Feb 17, 2015 #3

    NascentOxygen

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    Bear in mind that forces always occur in pairs: an action and an equal and opposite reaction. You can't exert a force on anything if it doesn't push back.
     
  5. Feb 17, 2015 #4
    I see, so the force on the spring by the block is equal and opposite to the force by the block on the spring?. So is what I have for 3 a i correct?
     
  6. Feb 17, 2015 #5
    No, correct? The only force that I can see is the wall.
     
  7. Feb 17, 2015 #6

    haruspex

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    That's right, the external forces are from string and wall.
     
  8. Feb 17, 2015 #7
    The wall does zero work while the String does positive work right? Since displacement and force is in the same direction?
     
  9. Feb 17, 2015 #8

    haruspex

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    Yes.
     
  10. Feb 17, 2015 #9
    Ok thanks for the help. Do you see any discrepancies on the back side?
     
  11. Feb 17, 2015 #10

    haruspex

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    Have another think about a iv. This is the work done by external forces on SB.
     
  12. Feb 17, 2015 #11
    It should be positive right? The string does positive work while work does negative work. But why is the work net external not equal to delta potential energy here?
     
  13. Feb 17, 2015 #12
    What situations can we use conservation of energy for net external work?
     
  14. Feb 17, 2015 #13

    haruspex

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    No, I'm suggesting you need to distinguish between work done on a system and and work done by a system. They have opposite signs.
     
  15. Feb 17, 2015 #14
    The system gains potential energy because the spring is stretched. Isn't Wnet external equal to - delta PE? So it would be indeed be negative? That was my reasoning and it makes sense.
     
  16. Feb 17, 2015 #15

    lightgrav

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    system gains PE because spring is stretched (spring PE is always positive, like KE is always positive).
    but no, Wnet,external (positive) is where that Energy comes from that is stored in the spring as PE (positive).
     
  17. Feb 17, 2015 #16

    haruspex

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    Work done on the system equals gain of work by the system. Both positive in this case.
     
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