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Solution discussion

  1. Jul 16, 2005 #1
    I was asked to solve the following problem:

    A box is sliding down a plain inclined 30 degrees above the horizontal.
    towards a spring attached to the start of the plain.
    when the box hit the spring its velocity is V(0).
    the box continue moving for another 0.1m until a complete stop.

    * there is no friction on the plain.
    * the spring is weightless, and was in rest before the impact.
    * Values : Box mass = 0.5Kg , K constant = 400N/m.

    i was asked to find V(0) at the moment of the impact.

    i used the equations :

    1/2 (0.5Kg) (V(0) ^ 2)
    = (0.5 kg) (9.8m/s^2) ((-0.1m)sin30) + 1/2(400N/m)(-0.1m)^2

    My questions:
    1. what type of collision occurred ? Plastic or elastic?
    2. was i right? i got 0 points for this question, and would like to know why...
    3. was there any energy loss in the system?

    Thanks . :tongue:
  2. jcsd
  3. Jul 16, 2005 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Looks OK to me. You are applying conservation of energy. You are assuming no energy loss in the collision of mass with spring. (That's the usual OK assumption--since you are treating the spring as massless.) The "complete stop" is only momentary--it will spring back up.

    Since you are using conservation of energy, you are assuming no mechanical energy loss. I don't know why you got it wrong. Did you check your arithmetic?
  4. Jul 17, 2005 #3
    thanks Doc.
    actually the question was vice versa - it was just easier for me describing the problem like this in this forum.

    In the original question the equations were given, and i was asked to "write" a problem, that should be solved with the equations.
    the problem i invented was described above.

    I was told I'm wrong because "I assumed that the collision was elastic, and that is not sure"
    is the any way the collision is not elastic when the spring is without mass???
    is there any energy loss in this situation?

    thanks again, and looking forward to get some answers that will help me deal with my lecturer... : :confused:
  5. Jul 17, 2005 #4

    Doc Al

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    Staff: Mentor

    I am confused about exactly what you were given and exactly what you were asked to do.

    What equation were you given? If it's the one you provided, then it sure looks like conservation of energy to me. If the objection is to the collision, then get rid of it! :smile: Your invented problem can be stated: A mass attached to a spring on an incline is seen to be moving at speed v. How far does the spring compress before the mass reverses direction?
  6. Jul 17, 2005 #5
    Ok, the equations were written in the test :
    1/2 (0.5Kg) (V(0) ^ 2)
    = (0.5 kg) (9.8m/s^2) ((-0.1m)sin30) + 1/2(400N/m)(-0.1m)^2

    i had to "invent" a problem, that should be solved with the exact same equations.

    the problem that i invented was described above.. unfortunatly, i choosed somehow a problematic solution, which involve a collision with a weightless spring.

    i was told its wrong because the collision might not be elastic, and therefore there will be energy waste. i think that the collision must be elastic.
    Am i right?
  7. Jul 17, 2005 #6
    And , may i add, this was a part of a test... so unfortunatly i cannot change the question. i need to know if i can win this case with the current solution..
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