Normal velocity
= Normal velocity
{title2=$v_n=v\cdot n$}
The <normal velocity> is the scalar component of a <velocity> in a specified unit-normal direction. Its sign depends on the orientation of the normal. At a material interface, no penetration equates the fluid and interface <normal velocities>; it does not require their tangential components to agree. <Surface acoustic impedance> uses a fixed inward normal-velocity convention to determine the sign of energy absorbed by a boundary.