Collins Concise English Dictionary © HarperCollins Publishers::
Newton's laws of motion pl n - three laws of mechanics describing the motion of a body. The first law states that a body remains at rest or in uniform motion in a straight line unless acted upon by a force. The second law states that a body's rate of change of momentum is proportional to the force causing it. The third law states that when a force acts on a body due to another body, then an equal and opposite force acts simultaneously on that body