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I Motorcycle physics question

  1. May 7, 2017 #1
    In a closed system, I know the rider wouldn't be able to change the combined center of gravity of the system by moving his weight around. If he pushes the bike to lean to the right, he would go to the left by the amount that would keep the combined cog the same.

    But a bike with tires and a road under it is not a closed system, the tires wouldn't let the lower part of the bike left or right when tryingo to initiate lean. Does this mean it is possible to push with your bodies inertia to the bike, and the combined cog will be changed? I thing if you do so above the cog of the bike, you could, but I am not sure.
  2. jcsd
  3. May 7, 2017 #2


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    Staff: Mentor

    It is a bit like a swing set where you can start swinging without touching the ground - just as inverted pendulum, with the support below the bike instead of above the swing.
  4. May 7, 2017 #3
    Why does it work if it wouldn't work in a closed system? Is it because tires stop the lower part of the bike move laterally?
  5. May 8, 2017 #4


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    Right. Friction (and the non-zero contact width) can lead to a net force and torque on the system.
  6. May 9, 2017 #5


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    In the case of a swing, the supporting chains or ropes are displaced forwards or backwards by the person leaning forwards or backwards, pushing forward or pulling backwards on the supporting chains or ropes, resulting in a backwards or forwards component of tension in the supporting chains or ropes, and this initiates movement.

    The bikes effective angular inertia, which is affected by the gyroscopic reactions in the wheels, resists a change in lean angle due to the rider leaning to one side or the other. The result is some of that force is applied to the pavement, coexistent with the pavement applying a force onto the bike in the same direction as the rider leans.

    The dominant effect is related to steering. On a typical bike, the point of contact of the front tire is "behind" the pivot axis, so if the rider leans to the left, the front tier initially steers to the right, an indirect form of counter steering.
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