For 10 points, name this vector quantity equal to the displacement over the time, whose magnitude is speed.
In fluids, the curl of this quantity's field gives vorticity. The Lorentz factor is equivalent to one minus this quantity squared over the speed of light squared all raised to the one half power. Kinetic energy is one-half times mass times this quantity squared, and momentum is equivalent to the mass multiplied by this quantity. Its time derivative is acceleration. For 10 points, name this vector quantity equal to the displacement over the time, whose magnitude is speed.
In fluids, the curl of this quantity's field gives vorticity.
{velocity} [do not accept “speed”]