The forces of flight are the four forces of lift, drag, thrust, and weight, which act on an aircraft in flight. An aircraft is in a state of equilibrium when the thrust and drag are equal and opposite. It will continue to move forward at the same uniform speed. If thrust or drag becomes greater than the opposite force, the aircraft loses its state of equilibrium. If thrust is greater than drag, the aircraft will accelerate. If drag is greater than thrust, the aircraft will lose speed and eventually descend. When lift and weight are equal and opposite, the airplane is in a state of equilibrium. If lift is greater than weight, the aircraft will climb. If weight is greater than lift, the airplane will descend.