Dictionary.net

The power which produces or tends to produce motion, or an alternation in the direction of motion. Mechanical forces are of two sorts; one of a body at rest, being the same as pressure or tension; the other of a body in motion, being the same as impetus or momentum. The degree of resistance to any motion may be measured by the active force required to overcome that resistance, and hence writers on mechanics make use of the terms resisting forces and retarding forces. When two forces act on a body in the same line of direction, the resulting force, or resultant as it is called, will be the sum of both forces. If they act in opposite directions, the body will remain at rest if the forces be equal; or, if the forces be unequal, it will move with a force equivalent to their difference in the direction of the greater. If the lines of direction make an angle with each other, the resultant will be a mean force in an intermediate direction.