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Sweet Dreams Are Made Of This

  1. Apr 3, 2004 #1
    How about we bet our lives on this one? I’ll even let you be the one who decides who will win the bet knowing full well that you wont be able to decide rightfully.
    Here it comes:

    Lets consider body A and body B in evacuated space giving no resistance to their motion form unchangeable viewpoint. A is at rest. B is traveling with constant speed towards A. I bet I’ll bring Newton’s mechanics into contradiction with its self, viewing A and B before, at the time of and after the impact! What do you bet on?

    Before the impact neither A nor B is subjected to force because of Newton 1.

    At the time of the impact:
    A changes its speed from 0 to constant value. This change in time it takes exerts force on A. So A exits the impact with constant speed yet subjected to force.
    B changes its speed from constant value to 0. This change in time it takes exerts force on B. So B exits the impact at state of rest yet subjected to force.
    It is because of Newton 2 and definition of force as change of impulse in time.

    After the impact Newton 1 is impossible!
  2. jcsd
  3. Apr 3, 2004 #2

    matt grime

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    Erm, eh? Many assumptions made, non-stated (equal massm perfectly inelastic collison rtc) but what are you claiming is violated? A and B have because of the impulse changed from being in a state of rest to moving and vice versa. what's wrong there? you're also getting speed and velocity confused - a body moving in a circle with constant speed is under the influence of some force.
  4. Apr 3, 2004 #3


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    I'm trying to picture what you are talking about. Are the two bodies identical in mass? Are both of them nonrotating as seen in an inertial frame? Are you making any claims as to how incompressible they are, or how elastic they are? Are they uncharged, so there are no electromagnetic forces to consider? Do you ignore gravitational attraction between them?
  5. Apr 3, 2004 #4
    If I have understood you correctly, your thought experiment agrees with the law of conservation of momentum, all three of Newton's laws of motion, and classical physics completely.

    What is it about laws of nature that you find to be so objectionable?
  6. Apr 3, 2004 #5

    Doc Al

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    OK, fair enough. You have two bodies in space, neither one experiencing a force. They move with constant velocity. You choose to view things from an inertial reference frame in which body A is at rest.
    Right! (By stipulation: you are ignoring gravity and other long-range influences. No problem.)
    Here's what happens during the collision. Body A and body B exert equal and opposite forces on each other, per Newton's third law. During the time of interaction, they accelerate per Newton's 2nd law. Only under special conditions will the bodies exchange speeds as in your example. But in all collisions, momentum will be conserved.

    After the collision, A and B no longer exert forces on each other. So they proceed once more with constant velocities, per Newton's 1st law.

    But at least you haven't mentioned Archimedes! :biggrin:
  7. Apr 3, 2004 #6
    Deda, if you are going to go around makeing posts about how the laws of physics are all wrong, at least do it with some common sense. I suggest you read the books/papers by the deaceased physicist's, Pierre Teilhard de Chardin and Michael Talbot. Perhaps after you read their books/papers, you will be able to post with some intelligence.
  8. Apr 4, 2004 #7
    the conservation laws are not the problem.
    The problem is that Newton 2 don't support Newton 1!
  9. Apr 4, 2004 #8
    Can I have my headline on CNN and DW-TV?
  10. Apr 4, 2004 #9
    I like it when you call me big P.A.P.A.

    Doc Al you might be disappointed but here comes Archimedes:

    In order to have this kind of process, body A and body B must have equal and opposite masses on different sides of their common center or same masses on same side. That’s why:
    They have equal and opposite forces on opposite sides of the center;
    They have equal and opposite distances on opposite side of the center;
    The center is the mid point between them.
    The center is the only available viewpoint that respects their balance.
    The center doesn’t provide them with feedback, neither attractive nor repulsive so their distances don’t alter their forces.
    Their constant forces are putting them in displacement with constant rate.
    When they impact they change their forces respectively to how elastic the collision is and carry the motion in direction suggested by their new forces.
    Unlike Newton says the main point in my physics is that it is the force making the displacement in direction of the actual force. Small change but enough to alter the physics you defend entirely.

    As for BookWorm:

    Unlike you I don’t read books so I can parrot them. I’m sure I have well functional mind and I’m using it most efficiently. Assume you read those books then how would you oppose me. And there is one more thing. We are not here to attack our personalities but we are here to fight for what’s right in physics.
  11. Apr 5, 2004 #10
    I'm an old man, and I'm very tired; but I wish you the best of luck in your battles.

  12. Apr 5, 2004 #11

    matt grime

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    That would be a "well functional mind" that cannot construct a sentence, nor do physics, at least not correctly.
  13. Apr 5, 2004 #12
    Go easy on him guys, I think English isn't his first language.

    Deda: Your posts are very confusing to us, it would benefit all parties if you put more effort into clearly explaining your thoughts. What is all this stuff about constant forces and changing forces? Last I checked, we are only talking about the collision between two objects. Force only exists at the moments during which the two objects are in contact with each other and are exchanging energy (when body A accelerates and body B decelerates). Before and after that short pulse, there is nothing but momentum which is a potential energy NOT a force.
  14. Apr 6, 2004 #13
    You could as well suggest PF to be in Macedonian for a while. But I guess it wont help you either to understand me well.

    The velocity of B must alter gradually from C <> 0 to 0 in order B to start its rest. The force on B must change from 0 to 0 to meet Newton 1. It's only the force on B that we can blame for the change of its velocity because of Newton 2 in this form:
    [tex]F \Delta t = m \Delta V[/tex]
    What is the force F acting on B like to bring V gradually form C <> 0 to 0?
  15. Apr 6, 2004 #14
  16. Apr 7, 2004 #15
    As if there was an equal sign between the laws of nature and Newton's mirage!
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