Newton's First Law of Motion

Newton's first law states that an object remains at rest, or continues to move at constant velocity, unless it is acted on by a resultant force.If the forces on an object are balanced, ΣF = 0, the object is in equilibrium. It may be stationary, or it may be moving at a constant speed in a straight line. Balanced forces do not mean the object is at rest.An object's tendency to resist any change in its motion is called inertia. The greater the mass of an object, the greater its inertia and the larger the resultant force needed to change its velocity.A resultant force is needed to change either the speed or the direction of motion. A force perpendicular to the motion changes the direction without changing the speed, as in circular motion.For example, a car moving at a steady speed along a straight road has a driving force exactly equal to the total resistive force. If the driving force is removed, the resistive forces become a resultant force and the car slows down.