Rolling friction is a force that slows the motion of an object on a surface due to material properties and surface irregularities. Static friction resists movement, while kinetic friction resists motion once initiated. Rolling friction is affected by molecular friction, surface deformity, and contact area. Soft or springy surfaces increase rolling friction due to the plowing effect. Elasticity can be manipulated for desired results.
Rolling friction is a resistive force that acts to slow the motion of a ball, cylinder, or wheel traveling in a straight line on a flat surface. This force is largely a result of the material properties of the object and the surface it is in contact with. Deformations or irregularities in the object or surface are important factors affecting the amount of rolling friction, as well as the contact area between the two.
There are small irregularities on the surface of most things, no matter how smooth they may appear. When an object at rest is in contact with a surface, these irregularities interlock with each other, resisting movement. This is static friction.
The amount of force applied to an object to initiate motion must be sufficient to overcome static friction. To stay in motion, a moving object must overcome the resistance caused by irregularities. This resistance is kinetic friction.
When the materials that make up an object and a surface are both relatively stiff, there is little deformity in the object’s shape or surface elasticity. In this case, the primary factors determining rolling friction are static or kinetic friction and molecular friction. Molecular friction is not a product of surface irregularities but of chemical bonds and electrical interactions.
Less rigid materials might allow the object or surface to warp in shape, or springy due to their interaction. An object like a tire or a basketball can deform as it rolls. The change in shape of a soft object moving on a hard surface increases the contact area of the object with the surface. Dynamic friction increases and the magnitude of rolling friction also increases.
If the surface an object travels on is soft or springy, the object will sink into the surface and have to work its way through. This is known as the plowing effect and is the major component of rolling friction in this circumstance. The drag force is not evenly distributed over the contact area with the object. Surface deformation is greatest in the direction of movement of the object. This, in effect, increases the static friction and therefore the rolling friction.
If both the object and the surface are elastic, the interaction between the two is relative and can be manipulated for a desired result. Deforming an object’s shape may make the surface’s elasticity more manageable. It’s often easier to drive a car through soft dirt if, for example, the car’s tires aren’t fully inflated.
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