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A measurements must be made relative to a specified point. Velocities are normally made with respect to earth unless otherwise specified. However, the velocity if specified can be made with respect to an object or point that is moving compared to the surface of earth. 
Direction can be specified as N, E, S, W, up, and down or draw coordinate axes. Up or down can be positive as long as it is specified. Left or right can be positive as long as it is specified. The Z axis is used for three dimensions. The decision is arbitrary; however, in some problems picking certain reference frames can make the problem easier. Once a reference frame is picked for a problem, it must be used for all parts of the problem. Remember, the acceleration of gravity is always toward the center of Earth. The velocity and accelerations can be in opposite directions with opposite signs. |
