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Laws of motion question

  1. Dec 6, 2005 #1

    I have a question about a basic principle of physics: the laws of motion.

    If two identical buses were traveling towards each other at a speed of 40 mph, assuming a lot of things are constant, their resulting motion in either direction would be 0, correct?

    Is this principle the same for every state of matter that collides with an equal but opposite force, like liquids, gases, or plasmas?

    Or do different states of matter cause more varied outcomes than two solid objects running into each other?

    For instance, if two identical water waves were to collide, would the net result be 0 as quickly and as definitively as with two solids?

    In that vein, would two really hot pressure waves traveling towards each other head on, would this outcome be any different than two solid objects, like buses, colliding?

    Is there a larger amount of "slippage" or "fluidity" of the molecules in different states of matter like liquids or gases than in solids, which causes them to behave differently (in the short term) than two solids colliding?

    One more example, if you could play the exact same song out of two speakers facing each other, if they collided midway, would the resulting motion of the two sound waves then be stopped?

  2. jcsd
  3. Dec 7, 2005 #2


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    I don't know what you mean by "their resulting motion" being 0 in the case of the two buses. One can define the "center of mass" of the two buses and that would be stationary from the perspective of someone observing from the (fixed) highway.

    With regard to sound waves, you can have "cancellation" under some circumstances but generally two sound waves or two water waves propagating in opposite directions will essentially be unimpeded by the other. (Amplitude and phase matter and can change the picture radically but your run of the mill waves have little effect on each other.)

    That is not to say the wave patterns are the same when you have two waves instead of just one.
  4. Dec 7, 2005 #3
    How can two waves of the same type that collide other move forward as if nothing had happened? It seems like they'd both slow down at least.
  5. Dec 7, 2005 #4


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    They don't collide. They pass through each other. The presence of one wave does not alter the propagation characteristics of the other wave, until the amplitude of the waves gets large enough to introduce nonlinearities. Take a look at the math behind transverse waves like water surface waves or waves on a string, and I think you'll see what we mean. You get superposition of the two waves, as already mentioned, and under the proper conditions that can cause *local* cancellation of the amplitude. So for your example of two speakers driving sounds out, yes, at the mid-point, if the two speakers are wired out of phase, you will get a cancellation of the sounds. But only at that exact midpoint. It is not because of any collision, it is a simple arithmetic addition and subtraction of sound pressure waves.
  6. Dec 7, 2005 #5
    I will look into those examples. Let me pose this question then:

    If two equal explosions went off towards each other at exactly the same time, would the pressure waves cancel each other when they collided?

    Thanks a lot.
  7. Dec 7, 2005 #6


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    No, they will not cancel each other out. Compression occurs at the shock front and colliding shock waves would (more or less) double the compression. Similarly, as the shock fronts proceed they are followed by rarefaction whose effect will also be augmented. Beyond that, they will each continue to propagate onward and outward from their respective sources.
  8. Dec 9, 2005 #7
    Thanks ya'll !
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