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Homework Help: Law of conservation of kinetic energy?

  1. Jan 11, 2009 #1
    Is there a difference between the "law of conservation of energy" and "law of conservation of kinetic energy?" If so what is it?

    and what IS the law of conservation of kinetic energy? It doesn't seem to be in my textbook or on the net..
  2. jcsd
  3. Jan 11, 2009 #2


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    That's because it doesn't exist. Kinetic energy is not conserved. Two blobs of equal mass hit at equal velocity and stick together. The resulting blob has no kinetic energy. Poof. Kinetic energy all gone.
  4. Jan 11, 2009 #3
    o dear. sigh. I have a lot more to worry about now
  5. Jan 11, 2009 #4


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    Here is an example of the conservation of energy.

    Say a bomb explodes. Initially, the bomb has chemical energy. When it explodes, the types of energy present are heat,light and sound. But energy neither made or destroyed, so it had to come from the chemical energy.
    So applying the law of conservation of energy, you'd say that
    Chemical energy -> heat+light+sound

    For kinetic energy. If two bodies collide and kinetic energy is conserved, then the kinetic energy before impact =kinetic energy after impact. Kinetic energy is conserved in elastic collisions.
  6. Jan 11, 2009 #5
    ok ok hold on, my question here is though
    I have 2 carts with springs at the back of them and I've pushed them together and realeased
    Then in order to prove that energy has been conserved, I'd need to calculate the elastic potential energy right..?
  7. Jan 11, 2009 #6


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    Yes. Total energy is conserved. The initial elastic potential energy must equal the final kinetic energy.
  8. Jan 11, 2009 #7
    o goodness. Yes I DO have a lot more to worry about.
    Which means I need to find the spring constant.
    What does the K mean? in F=k delta x ? Is there a way I can find x with info only about force? I'm not sure I even have k....
  9. Jan 11, 2009 #8

    The K is the spring constant. In F=kx, F= force, x=displacement, and k=the spring constant
  10. Jan 11, 2009 #9


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    I think you'd better state the full problem you are working on.
  11. Jan 11, 2009 #10
    Hahaha. On the Standardize test last year, they actually have that as the right answer. (because the other answers definitely didn't make sense at all). Heh.

    k is known as the string coefficient. It is a constant that every spring has. Think of it this way...

    IF I were to pull on a slinky..it would be pretty easy to stretch it. So the k value would be small. If I were to pull a car spring (made of steel and all), I wouldn't be able to do it. So the k value would be big.

    It is essentially a value that take into account how tough your spring is..and adjust the force necc to stretch it appropriately.

    Spring constant are usually given. In some situations, you might set up a conservation of energy (PE --> KE or vice versa) to solve for k...then calculate x, F (which ever is not known)

    In reality, you can prob look up the K value online or in some physics handbook if you are doing a lab and need that value
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