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Are collisions relative?

  1. May 31, 2008 #1
    Consider two objects, a and b, traveling toward each other at, say, 100 m/s.
    There are three observers, OA, OB, and OC. With respect to OA, a is stationary. With respect to OB, b is stationary. From OC's point of view, both objects are approaching each other at 50 m/s.
    From OA's perspective, b collides with a at 100 m/s. From OB's perspective, the opposite occurs. From OC's perspective, both objects collide at 50 m/s each.

    It seems to me that the result of each of these scenarios would be different. OA would see a go backwards, OB would see b do so, and OC would see both cease motion.
    Only one of these events would actually occur.
    Does that not imply that one of the observers had a "correct" viewpoint?

    Additionally, it seems to me that a collision between two cars traveling at 50 m/s toward each other will have different results than one car traveling toward a stationary one at 100 m/s. Is that correct?
    Last edited: May 31, 2008
  2. jcsd
  3. Jun 1, 2008 #2

    Doc Al

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    No. The speed and direction that something moves clearly depends on who is doing the observing. But it has no physical consequence--you can easily transform measurements from one frame to another. (This is the principle of relativity in classical mechanics.)
    No. Let's keep it simple. Say the two cars are sliding towards each other on a frictionless surface (ice, for example) so that no other forces are involved. Whether one car is at rest and the other is moving at 100 m/s, or both move at 50 m/s is simply a matter of reference frame. Pick the reference frame that is most useful--but any will do.
  4. Jun 1, 2008 #3
    I suppose I'm just visualizing it poorly. Thanks, though.
  5. Jun 1, 2008 #4


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    The strength of the impact is directly related to the velocity DIFFERENCE of the two objects directly before impact.

    Thus, in two distinct inertial frames, although they assign different velocities to the two object, they'll calculate the same velocity difference, yielding the same prediction as to what happens in the collision.
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