Mass in special relativity (including the kinetic energy of the gas) only when it is measured in the center of momentum frame. Just as is the case for single particles, the calculated "rest mass" of such a container of gas does not change when it is in motion, although its "relativistic mass" does change. The container may even be subjected to a force which gives it an overall velocity, or else (equivalently) it may be viewed from an inertial frame in which it has an overall velocity (that is, technically, a frame in which its center of mass has a velocity). In this case,
Velocity Moments image, formula_4 is the intensity of the pixel at the point formula_5 in the image, and formula_6 is the basis function. Cartesian velocity moments are based on these Cartesian moments. A Cartesian velocity moment formula_7 is defined by where formula_2 and formula_3 are again the dimensions of the image, formula_11 is the number of images in the sequence, and formula_12 is the intensity of the pixel at the point formula_5 in image formula_14. formula_15 is taken from Central moments, added so the equation is translation invariant, defined as where formula_17 is the formula_18 coordinate of the centre of mass for