How Does Friction Affect a Spinning Bike Tire Dropping to the Ground?

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    Bike Momentum Tire
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Discussion Overview

The discussion revolves around the effects of friction on a spinning bike tire when it is dropped to the ground. Participants explore the implications of different friction coefficients and their impact on the tire's motion, including acceleration and final velocity, without accounting for drag.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant poses a question regarding the acceleration and final velocity of a spinning bike tire when dropped, specifying the need for mass, moment of inertia, and angular momentum.
  • Another participant emphasizes that the outcome depends on the coefficients of static and kinetic friction between the tire and the ground, as well as the coefficient of restitution if the tire bounces.
  • A further contribution outlines various scenarios based on friction levels: with no friction, the tire would maintain its angular velocity; with low friction, it would roll and slide, dissipating energy; and with high friction, it would roll without sliding, conserving energy after landing.
  • The discussion also notes that total angular momentum is conserved immediately before and after the tire touches the ground, and highlights the instability of the system due to small angle variations from the vertical.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as multiple competing views regarding the effects of friction and the resulting motion of the tire are presented.

Contextual Notes

The discussion includes assumptions about the coefficients of friction and the conditions of the drop, which may affect the outcomes. The complexity of the problem is acknowledged, with participants noting that simplifying assumptions could lead to different conclusions.

Rockazella
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You have a bike tire with X mass and Y moment of inertia.
You hold the tire on a vertical plane just slightly above the ground. You then spin it to give it Z amount of angular momentum.

If you were to let the tire drop to the ground, what would its acceleration and final velocity be? (not taking into account drag)
 
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Originally posted by Rockazella
You have a bike tire with X mass and Y moment of inertia.
You hold the tire on a vertical plane just slightly above the ground. You then spin it to give it Z amount of angular momentum.

If you were to let the tire drop to the ground, what would its acceleration and final velocity be? (not taking into account drag)

It would depend on the coefficient of friction (both static and kinetic) between the tire and the ground. If the wheel bounces, you'd need the coefficient of restitution as well.

It's not a simple problem (unless you make a bunch of simplifying assumptions)
 
The answer depends on the friction coefficients between the tire and ground. For example if there was no friction at all, the tire would keep on rotating around its axis with constant angular velocity (conservation of angular momentum relative to the center of the wheel)and at the most could bounce up and down. With "low" friction the tire would start rolling and sliding, energy would be dissipated continuously until pure rolling sets in. With "high" friction the tire would start rolling without sliding and no energy would be dissipated after landing.
In the general case (with friction and considering an almost zero vertical landing speed just to disregard bouncing) what is conserved (immediately before and after touching the ground) is the total angular momentum with respect to landing point since all the interaction forces are bounded there and hence produce a null external momentum with respect to this pole.
The system is also pretty unstable (a small angle with the vertical direction will create a totally different evolution).

Hope this helps!
 

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