The vector size which is a measure of mechanical impact on a body from other bodies or fields as a result of which the body gets acceleration or changes the form and the sizes (is deformed). In each timepoint force is characterized by the size, the direction in space and a point of application.
The size of friction force of sliding is calculated on a formula, where by - sliding friction coefficient (in many cases instead of use k). At the movement on a horizontal surface force of normal pressure is, as a rule, equal to body weight and can coincide with gravity. At the movement on the inclined plane it is necessary to decompose gravity on components parallel to the inclined plane and perpendicular to it. The perpendicular component of gravity provides force of normal pressure, and, therefore, and sliding friction force.
The physical quantity which is a measure of inertial properties of a body is called as the inert mass of this body. In this sense weight acts as property of bodies not to give in to change of speed both in size, and in the direction.