Drag equation by experiment. Of particular importance is the formula_8 dependence on flow velocity, meaning that fluid drag increases with the square of flow velocity. When flow velocity is doubled, for example, not only does the fluid strike with twice the flow velocity, but twice the mass of fluid strikes per second. Therefore, the change of momentum per second is multiplied by four. Force is equivalent to the change of momentum divided by time. This is in contrast with solid-on-solid friction, which generally has very little flow velocity dependence. The drag force can also be specified as, formula_9 where, "P" is pressure
Velocity Moments image formula_14, and similarly for formula_20. formula_21 introduces velocity into the equation as where formula_23 is the formula_18 coordinate of the centre of mass for the previous image, formula_25, and again similarly for formula_20. After the Cartesian velocity moment is calculated, it can be normalised by where formula_28 is the average area of the object, in pixels, and formula_29 is the number of images. Now the value is not affected by the number of images in the sequence or the size of the object. As Cartesian moments are non-orthogonal, so are Cartesian velocity moments, so different moments can be closely