s2-1 Newton's Laws Force Inertial(惯性) Reference frame 1 Newton’ first law( The law of inertia惯性) A body at rest remains (te 持) rest and a body in motion continues to move at constant(但定) velocity unless acted upon by an external force(外力)§2-1 Newton’s Laws Force Inertial（惯性）Reference Frame 1.Newton’ First Law （The Law of Inertia惯性） A body at rest remains（保 持 ） rest and a body in motion continues to move at constant（恒定） velocity unless acted upon by an external force（外力）