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The length of a vector, or the absolute value of a scalar quantity. The magnitude is always a positive scalar value.

For instance, the velocity may be positive or negative in a one-dimensional problem, or it will be a vector in a two- or three-dimensional situation, but the speed is the magnitude of the velocity, and is always a positive scalar number.

in the case of uniform circular motion, the magnitude of the position vector (when measured from the center of rotation) is constant, even though the vector itself is rotating. Similarly, the magnitudes of the velocity and the acceleration vectors are also constant, but the vectors themselves are rotating constantly.