# Friction it on a level surface. For most

Friction is caused by one surface moving over another. It is a force that can resist the relative motion of an object.  For instance, if you push a book across a desk, you are exerting a force that will cause the book to slide across the desk. As the book slides, it slows down and eventually stops moving. This force that opposes the motion of an object, in this case, the book, is referred to as friction.  Friction between surfaces as they move in contact with each other can cause kinetic energy to be converted to thermal energy (heat). The amount of frictional force created depends on the materials involved, and every combination is different. The friction formula is dependant on two values, the coefficient of friction and the normal force on an object.  The coefficient of friction is used to describe the relationship between the forces as two surfaces interact. The coefficient of friction is assigned the Greek letter “mu” (?), and is unitless. The coefficient of static friction is the minimum force required to get an object to slide on a surface, divided by the forces pressing them together. A coefficient of 1 would mean that it takes just as much force to lift an object as it does to slide it on a level surface. For most everyday objects and materials, the coefficient has a value between 0 and 1. Additionally, it is imperative to note that this coefficient can exist either as static friction (?s) or kinetic friction (?k). As for the coefficient of static friction (?s), it refers to the frictional force between two surfaces or objects when none of them are in motion. On the other hand, the coefficient of kinetic friction (?k) relates to the frictional force when one or both of the objects are in relative motion with each other. As for the normal force on an object, usually represented with Fn or just n, is referred to as the force that is directly perpendicular to the two surfaces in contact. For instance, if a book is resting upon a surface, then the surface is exerting an upward force on the book in order to support the weight of the book. This is in accordance to Newton’s Third Law which states: “For every action, there is an equal and opposite reaction.” In this case, the normal force on the book is the reaction force.  Therefore, frictional force is the coefficient of friction (?) multiplied by the normal force (n) exerted on an object. The S.I unit for frictional force is Newton (N).