Bypass ratio the velocity of a very large volume and consequently mass of air produces a very large change in momentum and thrust: thrust is the engine's mass flow (the amount of air flowing through the engine) multiplied by the difference between the inlet and exhaust velocities in—a linear relationship—but the kinetic energy of the exhaust is the mass flow multiplied by one-half the square of the difference in velocities. A low disc loading (thrust per disc area) increases the aircraft's energy efficiency, and this reduces the fuel use. The Rolls–Royce Conway turbofan engine, developed in the early 1950s, was an early
Mass–energy equivalence the velocity is small, the relativistic mass and the rest mass are almost exactly the same. Also Einstein (following Hendrik Lorentz and Max Abraham) used velocity- and direction-dependent mass concepts () in his 1905 electrodynamics paper and in another paper in 1906. However, in his first paper on (1905), he treated as what would now be called the "rest mass". Some claim that (in later years) he did not like the idea of "relativistic mass". When modern physicists say "mass", they are usually talking about rest mass, since if they meant "relativistic mass", they would just say "energy". Considerable debate