CONCEPT: Frictional force: It is a type of contact force. Contact forces: It arises when two bodies are in contact with some other object: solid or liquid When two bodies are in contact mutual force arise due to newton's third law of motion. The component of contact force normal to the surface is called normal reaction and the component parallel to the surface is called friction. Static friction: It is the force of friction that balances the external force applied on the body and keeps the body at rest. It opposes the impending motion of the body. [ alt="F1 Jitendra K 26.5.21 Pallavi D2" src="//storage.googleapis.com/tb-img/production/21/05/F1_Jitendra%20K_26.5.21_Pallavi_D2.png" style="width: 212px; height: 86px;"> If fs is the static frictional force μs is the coefficient of static friction and N is the normal reaction force, the law of static friction may be written as: ⇒ fs ≤ μsN The maximum value of static friction: ⇒ (fs)max = μsN EXPLANATION: Static friction does not exist by itself. When there is no applied force there is no static friction. It comes to play as soon as the external force is applied. The component of contact force normal to the surface is called normal reaction and the component parallel to the surface is called friction. Static friction opposes the impending motion. Impending motion means the motion that would take place ( but does not actually takes place) under the applied force if friction were absent. Hence option 4) is correct.