Rolling friction
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Rolling friction is the friction that occurs when an object (e.g a wheel or tire) rolls. It is much smaller than sliding friction except for special cases like ice skating. It is caused by the deformation of the wheel or tire or the deformation of the ground. 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 friction than steel. Also, sand on the ground will give more rolling friction than concrete. A vehicle rolling will gradually slow down due to rolling friction - but a train running on a steel rail will roll much further than a car or truck on rubber tires.
It is worth noting that for all vehicles that travel upon wheels (such as cars and bicycles), the sum of rolling friction and static friction is what causes the vehicle to slow when the brakes are applied. The actual force applied in braking (for example, clamps applied to disk brakes) is internal, and by Newton's First Law cannot cause a change in the vehicle's motion. Therefore the slowing is caused by contact between the road and the car's tires; the static friction force between road and tire is the "equal and oppsite reaction" specified in Newton's Third Law. Rolling friction can be compared to sliding friction, as when the brakes "lock up", they slide upon the driving surface and do not sufficiently slow the car.
Several factors affect the magnitude of rolling friction a tire generates:
- Material - Tires with higher sulfur content tend to have a lower rolling friction. This is one strategy that most hybrid car vendors use to improve fuel efficiency.
- Extent of inflation - Partially inflated tires tend to suffer higher rolling friction. This may lead to overheating and may have played part in the infamous Ford Explorer rollover accidents.
- Hard rail steels last longer but may also have lower static friction - they may also suffer fatigue cracking because the cracked area is not worn away by the passing trains.