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Homework Help: Can total KE ever increase for inelastic collisions?

  1. Apr 12, 2017 #1
    1. The problem statement, all variables and given/known data
    Is following statement True or False?
    For inelastic collusions, the total KE of colliding objects just after collision is less than the total KE just before collision.
    The answer is given as False with a mention saying: "This is normally true but in some cases it could increase(why?)".
    2. Relevant equations

    Total KE initial = Total KE final
    The above is true only for elastic collisions.

    3. The attempt at a solution
    For inelastic collisions, some of the KE is always lost and therefore total KE of colliding objects must decrease.

    The only way that total KE could increase is when some of the mass of colliding objects gets converted to KE, but not sure if this really happens.
  2. jcsd
  3. Apr 12, 2017 #2

    TL;DR. Yes, but you need a source of energy that gets released upon collision (e.g. chemical energy in gunpowder, spring getting released). If you're looking at 2 ordinary blocks, you're always going to have KE decrease after an inelastic collision. Where there is an increase in KE in a closed system, there is always a source: Potential energy, chemical energy, mass energy even...
  4. Apr 12, 2017 #3
    But isn't mass a source of energy? Maybe in some collisions involving sub-atomic particles, mass gets converted to energy.
  5. Apr 12, 2017 #4
    Correct. I've personally never studied relativistic collisions that resulted in a decrease of mass and an increase of KE (although an exploding atomic bomb certainly converts mass to KE). In classical physics, especially in the low-energy limit, mass is conserved in collisions, so classical examples of an increase in KE would be if you triggered the release of a compressed spring during the collision.
  6. Apr 12, 2017 #5
    In the case of an expanding spring tied to a block A that is expanding when this block A collides with another block B, how could we say that it's an inelastic collision? Is it that collisions in real life are usually inelastic since some energy always dissipated?
    Last edited: Apr 12, 2017
  7. Apr 12, 2017 #6


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    Because kinetic energy is not conserved. The KE of block A plus the KE of block B is increased as a result of the collision. An "elastic collision" is one in which KE is conserved. An "inelastic collision" is one in which it is not.

    [I assume that you have in mind a spring that is compressed and which is released during the collision so that it imparts its stored potential energy to the blocks]
    That is usually the case. However it is not relevant to classifying a collision in which kinetic energy increases.
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