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Normally, when we say an object is at rest, we mean that it is at rest with respect to the surface of the earth; when we say a car is moving at a speed of 40 mi/hr, we imply that the motion is taking place at this speed relative to the road. A boat sailing on the river moves with respect to the river's banks, but it also moves with respect to the flowing water in the river. The lift on the wings of an airplane is generated by the motion of the airplane through the air, but it is quite important to know the plane's motion with respect to the ground. When we speak of the motion of a car or a train, we normally mean the motion with respect to the ground, but when we speak of the rated speed of an airplane, we refer to its motion with respect to the air. To avoid confusion in the discussion of motion, it is important to refer the motion to a frame of reference, usually thought of as fixed on the earth or fixed relative to the stars, in which the motion is measured. For many problems it is convenient to use moving frames of reference; it is then necessary to specify the nature of the motion of the frame. The frame of reference generally takes the form of a set of coordinate axes in which the motion is pictured.