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Homework Help: Conservation of Mechanical Energy

  1. Jun 29, 2015 #1
    1. The problem statement, all variables and given/known data
    This is my first time posting here guys so correct me if I am wrong. This question was on my test and I wanted to know if I did it right. There is a block of mass m and it is moving at a certain velocity before pressing against a spring which compresses distance x.

    Find Change in Kinetic Energy
    Change in Potential Energy
    Is this a conservative energy. Why?
    Energy lost to Friction
    Coefficent of Friction.

    2. Relevant equations
    KE = 1/2mv^2
    PE of Spring = 1/2kx^2
    3. The attempt at a solution

    I dont know the exact measurements for the variables but I used the kinetic energy formula to find intial KE and the final should be 0 because it turns into Potential correct? I got negative for that.

    I found change in potential by setting intial to 0 and using the equation.

    I then noticed that kinetic and potential weren't equal and potential was smaller so there was no conservation of energy.

    The loss in energy is friction? I then used this to find the coefficent. Is this correct?
  2. jcsd
  3. Jun 29, 2015 #2


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    Welcome to Physics Forums.

    The approach you describe sounds right, although it's difficult to say for sure without seeing exactly what you did.
  4. Jun 29, 2015 #3

    Energy cannot be negative correct? I got negative Kinetic Energy but reported it as positive would this be right?
  5. Jun 29, 2015 #4


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    Energy is a bit like money. If you lose it, you could describe that as a negative change.
  6. Jun 29, 2015 #5
    Then say I get Intial KE as 100J and Final is 0 because it converts to PE then Change in KE is -100?
    Assuming the PE is 80J.

    If I were to find the work done by Friction would it be -100 - 80? = -180J?
    I thought it would be 100 - 80 = 20J.
  7. Jun 29, 2015 #6


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    It really just like bookkeeping. You start with KE = 100J and end with PE = 80J, so:

    1) There is a loss of 20J.
    2) The change in energy is -20J.

    These are equivalent.

    It's then s equally valid to say:

    a) The work done by friction is 20J
    b) The work done by friction is -20J.

    a) assumes you know that friction takes energy out of the system and b) makes this more explicit.
  8. Jun 29, 2015 #7
    Thank You. That makes sense so much more sense now.
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