Foot-pound (energy)

The foot-pound force (symbol: ft⋅lbf, [1] ft⋅lbf,[2] or ft⋅lb [3]) is a unit of work or energy in the engineering and gravitational systems in United States customary and imperial units of measure. It is the energy transferred upon applying a force of one pound-force (lbf) through a linear displacement of one foot. The corresponding SI unit is the joule, though in terms of energy, one joule is not equal to one foot-pound.

Not to be confused with Pound-foot (torque) or Foot-poundal.
Foot-pound
Unit systemEnglish engineering units and British gravitational system
Unit ofEnergy
Symbolft⋅lbf, ft⋅lb
Conversions
1 ft⋅lbf in ...... is equal to ...
   SI units   1.355818 J
   CGS units   13,558,180 erg

Usage

The foot-pound is often used to specify the muzzle energy of a bullet in small arms ballistics, particularly in the United States.

The term foot-pound is also used as a unit of torque (see pound-foot (torque)). In the United States this is often used to specify, for example, the tightness of a fastener (such as screws and nuts) or the output of an engine. Although they are dimensionally equivalent, energy (a scalar) and torque (a Euclidean vector) are distinct physical quantities. Both energy and torque can be expressed as a product of a force vector with a displacement vector (hence pounds and feet); energy is the scalar product of the two, and torque is the vector product.

Although calling the torque unit "pound-foot" has been academically suggested, both are still commonly called "foot-pound" in colloquial usage. To avoid confusion, it is not uncommon for people to specify each as "foot-pound of energy" or "foot-pound of torque" respectively.

Conversion factors

Energy

1 foot pound-force is equivalent to:

Power

1 foot pound-force per second is equivalent to:

Related conversions:

  • 1 watt ≈ 44.25372896 ft⋅lbf/min = 0.737562149333 ft⋅lbf/s
  • 1 horsepower (mechanical) = 33,000 ft⋅lbf/min = 550 ft⋅lbf/s

See also

References
  1. IEEE Std 260.1™-2004, IEEE Standard Letter Symbols for Units of Measurement (SI Units, Customary Inch-Pound Units, and Certain Other Units)
  2. Fletcher, Leroy S.; Shoup, Terry E. (1978), Introduction to Engineering, Prentice-Hall, ISBN 978-0135018583, LCCN 77024142.: 257 
  3. Budynas, Richard G.; Nisbett, J. Keith (2014-01-27). Mechanical Engineering Design. McGraw Hill Education. ISBN 978-0073529288.
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