In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero. It is the speed needed to "break free" from the gravitational attraction of a massive body, without further propulsion.
For a spherically symmetric body, the escape velocity at a given distance is calculated by the formula
In physics, in particular in special relativity and general relativity, a four-velocity is a four-vector (vector in four-dimensional spacetime) that replaces velocity (a three-dimensional vector).
Events are described in time and space, together forming four-dimensional spacetime. The history of an object traces a curve in spacetime, called its world line, which may be parametrized by the proper time of the object. The four-velocity is the rate of change of four-position with respect to the proper time along the curve. The velocity, in contrast, is the rate of change of the position in (three-dimensional) space of the object, as seen by an inertial observer, with respect to the observer's time.
A physical quantity (or "physical magnitude") is a physical property of a phenomenon, body, or substance, that can be quantified by measurement.