What Kind of Force Causes a Change in Motion?

Unbalanced forces that have a net force in a certain direction are what lead to changes in motion. When multiple forces act on a body, the resultant force, calculated as the vectorial sum of the individual forces, determines whether the body moves or not.

When this resultant force is equal to zero, the individual forces that contribute to this resultant force are balanced, and no motion results. If the resultant force equals any quantity other than zero, the forces that contribute to the resultant are unbalanced, and the body moves. Bodies being acted upon by balanced forces are said to be in equilibrium.

To measure the total forces acting on a body and determine resulting movements, free-body diagrams are constructed depicting the magnitude and direction of each force. Reaction forces resulting from the contact of the body with another body must be taken into account. A simple free-body diagram of an upright person shows the person's weight as a downward force and the upward reaction from the floor as an equal and opposite force. The person is in equilibrium because these two forces balance out. The reaction force from quicksand does not balance out with the weight of a person sinking in it, causing net downward motion.