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Conservation of Momentum question

  1. Jun 26, 2011 #1
    Little item that is being debated on another forum and I thought I'd attempt to get a "second opinion" here.

    Picture this, a simple projectile
    ( just for the sake of argument lets say an aluminum soda can, full or empty, not a big deal)
    and the projectile is first shot from an air-cannon at 500 ft/s against a target that is designed to give maximum resistance such that the projectile will not penetrate or move the target. The can completely destroys itself against the target and in high speed photos of the event the last bit of the can to impact the target is seen to be traveling very near that original 500 ft/s ...

    Second run at this, the air-cannon is again fired and this time the target is designed to allow penetration by the projectile, but offeres some resistance.
    however upon viewing the video of the event, the projectile is NOT seen slowing down in the slightest.

    Now the debate gets juicy, the loyal opposition claims that the first example can be used to PROVE that the second example should not have slowed down at all, while penetrating said target.

    What I'm looking for is discussion, debate, dialog (or?) on this subject to sort out what is the REAL science involved here.

    I'll tell you this, when I did physics courses in school, I did VERY well
    however I have NO degree of any sort because I'm the character who
    was thrown out of Chenistry for being a FREE RADICAL ....

    anyhow Thanks in advance ... if anybody wants to jump on this ...
  2. jcsd
  3. Jun 26, 2011 #2
    Welcome to Physics Forums.

    My first reaction is that this violates the first situation clearly violate the law of conservation of momentum: before the collision the net momentum of the can+wall system is in the direction of the can’s motion. After the collision there is no momentum. You might rephrase it like this:

    A projectile moves at 500 ft/s until it has a perfectly inelastic collision with a second body that is so much more massive that the projection that the larger mass does not move significantly. The larger mass is so rigid that it does not compress significantly. The projectile deforms in such a way that there is negligible slowing of the trailing edge while the rest of the projectile deforms.

    The problem with this scenario is that if the projectile moves with a constant velocity, the wall can offer no resistance (Newton’s second law). I don’t see a way to rephrase that preserves the given criteria and is consistent with the laws of nature.

    This seems one of those questions that comes down to asking “What does physics predict will happen if physics doesn’t work?”
  4. Jun 26, 2011 #3


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    Homework Helper

    In reality, the can does slow down when it impacts the target. It happens very quickly though.
    Again, if the can is stopped by the target, then it must be slowed down. Maybe it just happens too quickly for the camera to see.
  5. Jun 27, 2011 #4
    so just to wrap it up
    any object that is seen to penetrate a target
    can be expeceted to decelerate in proportion to
    the resistance encountered while penetrating said target.

    For all the high powered physics majors on this forum,
    did I get it right?
  6. Jun 28, 2011 #5
    That is strictly true if you are thinking about this purely theoretically. It is important to note that any experiment to test this (especially one done at high velocity) will likely produced apparently impossible results if measurements are not made precisely enough or frequently enough.
  7. Jun 28, 2011 #6
    Imagine this happening in a vacuum, where there is no air pushed by the barrier when it is struck by the can. The parts of the inter-atomic lattice of the barrier might move, but they would return to their original configuration with no net change?

    Sometimes electrons slip in between the electron clouds of objects. This is called 'quantum tunneling'. The objects do pose a probabilistic resistance to tunneling proportional to how far apart their electron clouds are, but I don't think this 'resistance' affects the velocity of the electrons that make it through.
  8. Jun 28, 2011 #7

    This is the simple projectile that was being discussed, an F-4 Phantom jet colliding with a reinforced concrete wall.

    The analogy to a "an aluminum soda can, full or empty, not a big deal" is a bit misleading.

    DS8.5 expected the jet to exhibit noticable deceleration on impact.
    Last edited by a moderator: Sep 25, 2014
  9. Jun 28, 2011 #8
    Thank U ever so much SAM.H

    For the readers of this forum to get the big picture
    Look up ANY of the video showing the impact of the alleged FLT175 airliner against the wall of the South Tower, Just exactly HOW does anyone who did not flunk out of middle school science, justify THAT?
  10. Jun 28, 2011 #9
    And it does. It looks pretty stopped to me.
    Last edited: Jun 28, 2011
  11. Jun 28, 2011 #10
    Really? For the record there are A LOT of people who did MUCH BETTER in their education at a MUCH HIGHER LEVEL than you did, that "justify" that.
    Last edited: Jun 28, 2011
  12. Jun 28, 2011 #11
    DS8.5 expected the entire plane to decelerate, like it was a spear striking a cement wall (i.e. the tail of the plane to stop when the front hit the wall).
  13. Jun 28, 2011 #12


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    Staff: Mentor

    9/11 conspiracy theories are on the forbidden topics of the PF. You have come to the wrong place to try to push your theory. Thread locked.
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