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Rolling friction is a resistive force acting to slow the motion of a sphere, cylinder or wheel that is traveling in a straight line over a level surface. This force is largely the result of the material properties of the object and the surface with which it is in contact. Deformations or irregularities in the object or surface are important factors that affect the magnitude of rolling friction, as is the area of contact between the two.
There are tiny irregularities on the surface of most things no matter how smooth they might appear. When an object at rest is in contact with a surface, these irregularities catch on each other, resisting motion. This is static friction.
The amount of force applied to an object to initiate motion must be sufficient to overcome static friction. To remain in motion, an object in motion must overcome resistance caused by the irregularities. This resistance is kinetic friction.
When the materials composing an object and surface are both relatively rigid, there is little deformity in the shape of the object or elasticity of the surface. In this case, the primary factors determining rolling friction are the static or kinetic friction and molecular friction. Molecular friction is not a product of surface irregularities but of chemical bonding and electrical interactions.
Less-rigid materials might allow the object or surface to become deformed in shape or elastic because of their interaction. An object such as a tire or a basketball can become deformed as it rolls. The change in shape of a soft object moving over a hard surface increases the object's area of contact with the surface. The kinetic friction is increased, and the magnitude of the rolling friction also rises.
If the surface over which an object travels is soft or elastic, the object will sink into the surface and must force its way through. This is known as the plowing effect and is the major component of rolling friction in this circumstance. The force of resistance is not evenly distributed over the area of contact with the object. The deformation of the surface is greatest in the direction of the object's motion. This, in effect, increases static friction and thus rolling friction.
If both the object and surface are elastic, the interaction between the two is relative and can be manipulated for a desired outcome. Deforming an object's shape might make the elasticity of the surface more manageable. It is often easier to drive a car through soft dirt if the car's tires are less than fully inflated, for example.
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