According to Newton’s First Law, when the body is at rest or keep moving in a straight line at a constant speed, unless it is acted upon by force. In fact, in classical Newtonian mechanics, there is no important distinction between rest and uniform motion in a straight line. They may be regarded as the same state of motion seen by different observers. One moving at the same velocity as the particle and the other moving at constant velocity with respect to the particle. This postulate is known as the law of inertia.