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,
Escape velocity a body that falls under the force of gravitational attraction of mass "M", from infinity, starting with zero velocity, will strike the massive object with a velocity equal to its escape velocity given by the same formula. When given an initial speed formula_2 greater than the escape speed formula_3 the object will asymptotically approach the "hyperbolic excess speed" formula_4 satisfying the equation: In these equations atmospheric friction (air drag) is not taken into account. A rocket moving out of a gravity well does not actually need to attain escape velocity to escape, but could achieve the same result (escape) at