According to Newton's first law, an object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force. It is the natural tendency of objects to keep on doing what they're doing.
All objects resist changes in their state of motion.
In the absence of an unbalanced force, an oject in motion will maintain this state of motion. This is often called the law of inertia . The law of inertia is most commonly experienced when riding in cars and trucks. In fact, the tendency of moving objects to continue in motion is a common cause of a variety of transportation accidents - of both small and large magnitudes. Consider for instance the unfortunate collision of a car with a wall.