Thrust is a
reaction force described quantitatively by
Newton's
second and third laws. When a system expels or
accelerates mass in one direction, the accelerated mass will cause a force of equal
magnitude but opposite direction on that system. The force applied on a surface in a direction perpendicular or
normal to the surface is called thrust. Force, and thus thrust, is measured in the
International System of Units (SI) as the
newton (symbol: N), and represents the amount needed to accelerate 1 kilogram of mass at the rate of 1 metre per second squared.