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
Angular velocity two-dimensional case above, one may define the angular velocity vector as: where "θ" is the angle between r and v. In terms of the cross product, this is: From this, one can recover the tangential velocity as: If a point rotates with angular velocity formula_28 in a coordinate frame formula_29 which itself rotates with an angular velocity formula_30 with respect to an external frame formula_31, we can define formula_32 to be the composite angular velocity vector of the point with respect to formula_31. This operation coincides with usual addition of vectors, and it gives angular velocity the algebraic structure of