When a force is applied to an object resting on a surface, a retarding force is that which opposes the force being applied to the object. A retarding force is usually that of friction, which must be overcome when moving an object resting on a surface. If the object is being moved upward along an inclined plane, gravity represents an additional force that must be overcome.
The retarding force represented by friction can take on various forms. Dry friction refers to the retarding force existing between two solid surfaces in contact with each other. Skin friction, also known as drag, represents the force offering resistance to a fluid moving across the surface area of a body.