The normal force on an object is not always equal in magnitude to the weight of the object. Some problems contain multiple force vectors acting in different directions on an object.

Always analyze the direction in which an object accelerates so that you can determine whether F net = m a or F net = 0 If an object is accelerating vertically, the normal force is less than or greater than the weight of the object.

object — accelerating vertically, the normal force is less than or greater than the weight of the object. drag force — force that always opposes the motion of an object in a fluid; unlike simple friction, the drag force is proportional to some function of the velocity of the object in that fluid. terminal velocity — constant velocity achieved by a falling object, which occurs when the weight of the object is balanced by the upward drag force. static friction — force that opposes the motion of two systems that are in contact and are not moving relative to each other.