C- Normal contact force.
- The normal force is the support force exerted upon an object that is in contact with another stable object.
- For example; if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book.
- On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person.
- We use “N” or “R” as symbol.