The pound-force (lbf) is the standard unit of force in the U.S. Customary system. It is defined as the force required to accelerate a one-pound mass (lbm) at the standard acceleration due to gravity. Unlike the Newton, which is based on fundamental physical principles, the pound-force is tied to Earth’s gravitational field, making it a gravitational unit of force. In engineering applications, force values are often given in pounds-force, with larger forces expressed in kilopounds-force (kip), where 1 kip = 1000 lbf. When working with U.S. Customary units, careful attention is needed to distinguish between mass (lbm) and force (lbf), especially in dynamic calculations where acceleration differs from standard gravity.