Why rolling friction is less than sliding friction..?

Asked by  | 11th Sep, 2012, 10:47: PM

Expert Answer:

rolling resistance is often expressed as a coefficient times the normal force. This coefficient of rolling resistance is generally much smaller than the coefficient of sliding friction.

Any moving wheeled object will gradually slow down due to rolling resistance including that of the bearings, but a train with steel wheels running on steel rails  will roll farther than a bus  of the same mass with rubber tires running on tarmac. Factors that contribute to rolling resistance are the (amount of) deformation of the object, the deformation of the surface, and movement below the surface. Additional contributing factors include wheel diameter, forward speed load on wheel, surface adhesion, sliding, and relative micro-sliding between the surfaces of contact. It depends very much on the material of the wheel or tire and the sort of ground. For example, rubber will give a bigger rolling resistance than steel on some surfaces (polished steel) and a lower rolling resistance on other surfaces (pavement/tarmac). Also, sand on the ground will give more rolling resistance than concrete.

  Hard wheel rolling on and deforming a soft surface, resulting in the R ,reaction force from the surface having a component that opposes the motion. (W is some vertical load on the axle, F is some towing force applied to the axle, r is the wheel radius, and both friction with the ground and friction at the axle are assumed to be negligible and so are not shown. The wheel is rolling to the left at constant speed.) Note that R is the resultant force from non-uniform pressure at the wheel-roadbed contact surface. This pressure is greater towards the front of the wheel due to hysteresis.

Answered by  | 12th Sep, 2012, 12:17: PM

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