Mechanical forces acting upon the rocks cause physical weathering. Temperature fluctuations cause expansion and contraction of rock surface resulting in the formation of cracks apd fissures. During cold weather, the water present in rock crevices gets frozen and the formation of ice results in its expansion. The force of expansion causes breaking up of rock. Broken rock fragments roll down the slopes and break further into smaller pieces. Hails, rakfall and fast flowing streams are important agents of physical weathering. You might have seen rounded stone pieces of varying sizes on the bed of river where fast flowing river reaches the plains from its place of origin. Wind is another agent of physical weathering particularly when it carries sand particles which causes abrasion of rock surface, due to friction. In the hill forests, it is commonly seen that tree roots often penetrate through the rock crevices and in course of time, with the radial growth of roots, the rocks get disintegrated.