Friction is caused primarily by two key factors: the roughness of surfaces and intermolecular forces between them. The tiny bumps and valleys on surfaces, called asperities, physically prevent smooth sliding. Additionally, electromagnetic forces, such as van der Waals forces between molecules at the surface, play a significant role. When surfaces slide against each other, these forces resist the motion, generating friction. Friction also results from energy lost due to vibrations or phonons created as atoms on surfaces move past each other. Overall, friction arises from a combination of physical surface roughness and molecular-level electromagnetic interactions.
