Velocity quantity, momentum, is a vector and defined by In special relativity, the dimensionless Lorentz factor appears frequently, and is given by where γ is the Lorentz factor and "c" is the speed of light. Escape velocity is the minimum speed a ballistic object needs to escape from a massive body such as Earth. It represents the kinetic energy that, when added to the object's gravitational potential energy, (which is always negative) is equal to zero. The general formula for the escape velocity of an object at a distance "r" from the center of a planet with mass "M" is where
Propellant mass fraction important role in the rocket equation: Where formula_10 is the ratio of final mass to initial mass (i.e., one minus the mass fraction), formula_11 is the change in the vehicle's velocity as a result of the fuel burn and formula_12 is the effective exhaust velocity (see below). The term effective exhaust velocity is defined as: where "I" is the fuel's specific impulse in seconds and "g" is the "standard acceleration of gravity" (note that this is not the local acceleration of gravity). To make a powered landing from orbit on a celestial body without an atmosphere requires the same mass