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Homework Help: Where is the force coming from to compress the spring

  1. Mar 13, 2017 #1
    1. The problem statement, all variables and given/known data

    Given a plank is moved with uniform velocity v. To which a mass is connected by a spring of force constant k. What is work done by the external agent on the maximum compression of the spring
    2. Relevant equations
    Work done by spring=0.5*k*x2
    Work-energy theorem: total work done =change in kinetic energy

    3. The attempt at a solution
    in order to compress a spring there has to be a force.but since it is given plank is moving with uniform velocity.where is the force coming from?
    Since I couldn't figure or understand that I couldn't proceed further.

    Attached Files:

  2. jcsd
  3. Mar 13, 2017 #2
    The screenshot makes it a bit clearer. The "plank" is moved with constant velocity--but what about the mass, m? It seems we have to neglect friction.
  4. Mar 13, 2017 #3
    Then also why the mass moves...?
  5. Mar 13, 2017 #4
    What would happen to the mass if there was no spring, and no friction, and the plank suddenly started moving at velocity v0?
  6. Mar 13, 2017 #5


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    This is the necessary assumption, not made clear in the question.
  7. Mar 13, 2017 #6
    It's not a terribly well written question--nothing about friction, for one thing--but I don't think anything else makes sense.
  8. Mar 14, 2017 #7
    Ok then what is the work done by spring in this situation .is it negative or positive?
  9. Mar 14, 2017 #8


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    Is that the exact wording of the question?

    There is no mention of it starting from rest?
  10. Mar 14, 2017 #9
    Try to visualise what will happen if the whole system is at rest and then someone starts pulling the "plank". As I said, it might help if you start by imagining there's no spring and the mass is on a very slippery surface.

    Also note that the question talks about "compression" of the spring.
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