The Traffic Accident Reconstruction Origin -ARnews-

Re: Snowmobile speed response to archive thread

Gene K. Baxter ([email protected])
Tue, 3 Nov 1998 04:31:17 -0500 (EST)

Brian. Thanks for your response. I believe it has pointed out that our lack of communication in this thread has been related more to "semantics" than to "physics." It also provides a good lesson to all of us by demonstrating how the imprecise use of engineering terminology can cause great confusion between experts, let alone the poor lawyers and jury. First, I incorrectly used the term "rotational inertia" as a synonym for "angular momentum." Then, you incorrectly used the term "rotational inertia" as a synonym for the "moment of inertia." In simple terms, the "moment of inertia," your symbol "I," is just: I = mk^2 (mass x radius of gyration squared), so the "moment of inertia" depends on the shape and mass distribution of the object, as you pointed out. Therefore, you and Bruno are correct in that the "moment of inertia" of the track system does not change significantly as the snowmobile leaves the ramp. In addition, you are correct in that the "angular momentum" is equal to
the "moment of inertia" x the "angular velocity." And, I believe you will also agree that there must be a conservation of angular momentum for the snowmobile system as it leaves the ramp because there are no significant external forces that act on an airborne snowmobile. Therefore, as the "angular momentum" of the track system increases in one direction (say, the clockwise direction) due to the sudden lack of load on the track, then the angular momentum of the snowmobile body will increase in the opposite direction, counterclockwise. Therefore, the snowmobile will rotate such that the rear-end moves toward the ground, giving the appearance that the rear-end has dropped.
Gene K. Baxter
[email protected]

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