follow-up signal. A signal in an autopilot system that nulls out the input signal to the servo when the correct amount of control surface deflection has been reached.
footcandle (illumination). The amount of illumination on a surface when all points are one foot from a uniform light source of one candela, the amount of light produced by one international candle.
foot-pound (measurement of torque). The amount of torque produced when one pound of force is applied one foot from the point of rotation.
foot-pound (measurement of work). The basic measure of work in the English gravitational system. One foot-pound is the amount of work done by one pound of force when it causes a movement of one foot in the direction of its application. One foot-pound is equal to 1.355818 joules.
foot-poundal (measurement of work). The basic measure of work in the English absolute system. One foot-poundal is the amount of work done when one poundal of force causes a movement of one foot in the direction of its application. One foot-poundal is equal to 0.0421 joule. The difference between a foot-pound and a foot-poundal is that foot-pounds use a measure of weight, and foot-poundals use a measure of mass.

force. Energy brought to bear on an object that causes or tries to cause it to change its direction or speed of motion.