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xissburg

About rolling wheels....

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I cant understand it! :P for example the wheel of a car. The motor applies torque on the wheel. The wheel apply a force with magnitude F = torque * wheelRadius on the ground. Supose that this force is not greater than the maximum static friction force at there. So the ground applies the same force in opposite direction on the wheel. This force is applied at the contact point of the wheel with the ground so the wheel will get this force on its center of mass and also a torque. Since this force is equal to the force the wheel applied on the ground the torque now will be 0... !!! ... and the wheel will not roll :O ... What really happens a there?!?

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Okay, basically you're right but you're forgetting the First Law. Firstly, if the vehicle is already moving and just keeps on going, what you describe is correct – the frictional 'drag' from the ground counterbalances the torque applied by the engine, thus there is no net torque on the wheel and so it keeps rotating at the same rate.

When accelerating or decelerating, obviously, the torque's don't balance so the angular speed of the wheel changes.

Edit: Also, for the purposes of most games you would ignore the 'real' physics behind a wheel, applying the forwards-motion forces directly to the vehicle and calculating the wheel's rotation from its speed.

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