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Homework Help: Collisions/conservation of energy[conceptual]

  1. Mar 29, 2010 #1
    as alluded to in the title, this isn't so much a "problem" as a concept for a lab/experiment we performed.
    so, for some reason I was sure that a collision between two hockey pucks of roughly equal masses would result in an elastic collision, but I calculated the KE, and the two values were different, meaning it wasn't elastic. is this, uh, okay? from the way our professor explained the concept of elastic collisions[though not necessarily the lab itself], this type of collision[two similar masses moving at angles and producing a glancing collision] should result in conserved KE. but, maybe I just didn't understand what he was trying to say.
  2. jcsd
  3. Mar 29, 2010 #2


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    In the real world, there is no such thing as a perfectly elastic collision, and therefore, some kinetic energy will always be lost. But the loss of energy between a collision of 2 pucks should not be as huge, say, as the loss would be in the case of 2 marshmallows colliding . How much did you lose? Did you do the calculations correctly?
  4. Mar 29, 2010 #3
    the calculated KE before the collision was around 8.58*10^-3J. KE after was about 5.97*10^3J. so it lost, approximately. 2.5*10^-3J of energy following the collision.

    I may have done the calculations incorrectly, but the more plausible situation is that my derived equations may be incorrect; I'm asking on here to see if these answers would be somewhat plausible, or if I should try to completely change everything.
  5. Apr 5, 2010 #4
    Elastic collision processes are taught in a complicated way by most teachers. There is an utterly simple method - it is geometrical, however. But once we appreciate the result, we can do the algebra and get the required numbers.

    First, forget about real life situations, because they are most complicated - in fact, that is the reason great scientists have resorted to thought experiments (ideal world situations).

    1D collisions (head-on) are the simplest to understand. Solve the problem in center of mass reference frame, first. (just as you do a multiplication or division problem using logarithms). Then , in the next step you can transform the result to the frame of your interest. (just as you do a multiplication or division result using antilogarithms).
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