<P> Plastic deformation in solids is an irreversible change in the internal molecular structure of an object . This change may be due to either (or both) an applied force or a change in temperature . The change of an object's shape is called strain . The force causing it is called stress . </P> <P> Elastic deformation in solids is reversible change in the internal molecular structure of an object . Stress does not necessarily cause permanent change . As deformation occurs, internal forces oppose the applied force . If the applied stress is not too large these opposing forces may completely resist the applied force, allowing the object to assume a new equilibrium state and to return to its original shape when the force is removed . This is known as elastic deformation or elasticity . </P> <P> As a consequence of light pressure, Einstein in 1909 predicted the existence of "radiation friction" which would oppose the movement of matter . He wrote, "radiation will exert pressure on both sides of the plate . The forces of pressure exerted on the two sides are equal if the plate is at rest . However, if it is in motion, more radiation will be reflected on the surface that is ahead during the motion (front surface) than on the back surface . The backwardacting force of pressure exerted on the front surface is thus larger than the force of pressure acting on the back . Hence, as the resultant of the two forces, there remains a force that counteracts the motion of the plate and that increases with the velocity of the plate . We will call this resultant' radiation friction' in brief ." </P> <P> Rolling resistance is the force that resists the rolling of a wheel or other circular object along a surface caused by deformations in the object or surface . Generally the force of rolling resistance is less than that associated with kinetic friction . Typical values for the coefficient of rolling resistance are 0.001 . One of the most common examples of rolling resistance is the movement of motor vehicle tires on a road, a process which generates heat and sound as by - products . </P>

Four field applications of friction on horizontal surface