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Some conceptual collision/energy stuff

  1. Feb 20, 2008 #1
    1) I was thinking about the definitions of KE and PE... can't the values KE and PE only ever be positive...? so

    ME = KE + PE

    Can total mechanical energy ever be negative? I'm thinking it cannot

    2) also lets say you're in a car and you collide with another car... I'm trying to compare the impact of an inelelastic vs. elastic collision.. which would actually cause more damage to you? if the cars stick... or if they rebound?

    I'm really clueless on this
    Last edited: Feb 20, 2008
  2. jcsd
  3. Feb 21, 2008 #2
    1] Mechanical energy is somewhat relative to a reference point. Like, if you consider that a level 1000km above you has potential energy 0, then your P.E would be hugely negative and if you are at say.. rest.. your KE will be '0' and mechanical energy will hence be negative.

    2] It doesn't really matter what kind of collision you are in but what really matters is how your vehicle gives in to the stress. The type of collisions that u mention controll/predict only the final velocities and trajectories of the bodies colliding. For safety data, you need to know the yield strength and similar parameters for your vehicle.
  4. Feb 21, 2008 #3
    This is basic, basic high school physics.. and I was going through the textbook, these were just conceptual questions to think about

    1) It just asked if mechanical energy can be negative

    2) It simply asked if an inelastic or an elastic collision would cause more damage

    you're thinking too much IMO. There is a definitive answer... this is basic high school level physics
  5. Feb 21, 2008 #4
    With an elastic collision we can think of the momemtum being distributed equally, atomically. If an elastic impact yielded .00000001 Newtons per atom, we could not make the same generalization about an inelastic collision.
  6. Feb 22, 2008 #5


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    Yes, it can. For two reasons: one is a bit silly, and comes from the fact that you can define your "0" for potential energy just anywhere: potential energy is defined up to an arbitrary constant. For instance, the potential energy in a uniform gravity field with acceleration g of a mass m is usually written as: V = m x g x h, where h is the "height" (but measured from where ?). But you can just as well write: V = m x g x h + C, where C is an arbitrary constant. You can even see this C as meaning that you took another reference point for your "h".

    So total mechanical energy, being KE + V, is also defined up to an arbitrary constant (the one in C). So you can give it any value, positive or negative, to your liking.

    But another reason is "more serious". By convention, one usually takes potential energy to be 0 at "infinite separation". IF you take this convention, then negative total energy means that you have a "bound state": that not all the particles/constituents of your system can get arbitrarily far away from eachother, but that at least some need to stay "together".
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