Learn about friction and it's application in our daily life Table Of Contents 1. Defininition of Friction 2. Calculations on friction 3. Application on Friction 4. Laws of Friction Friction is a force which opposes relative motion between two surfaces in contact with each other. Coefficient of static friction, µ is defined as the ratio of the limiting frictional force (F) to the normal reaction (R). Coefficient of static friction = µ = F/R = Frictional force/Normal Reaction The normal reaction, R is equal to the weight, W of the block i.e. R = W = mg, where m is the mass of the block in kg and g is the acceleration due to gravity.