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Q & A: Towing a car with a flat tire (tyre)

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Most recent answer: 10/22/2012
Q:
I have studied that friction is not dependent on surface area;only on coefficient of friction and normal reaction.but why it is hard to pull a flat tyre than a fully inflated tyre
- monty (age 25)
mp
A:
Nice question Monty.
First of all there are two (at least) kinds of friction, sliding and rolling.  The sliding kind is ideally thought to be of sort you are referring to, the frictional force is proportional to the total normal force and the coefficient of sliding friction.   In the case where the tire is flat and the wheels are locked and not able to roll, then according to the (approximate) rule there would be no difference.  I'm not sure how accurate that is in this case since the flattened tire might find additional obstructions to hang up on.     In the case where the wheels are not locked but can roll, then you have a different regime.  The rolling friction loss usually comes from the small deflections of the material.   In the case of a flattened tire there is much more deflection.  Energy is lost in these deflections and increases the force needed to keep it rolling.

LeeH


(published on 10/22/2012)

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