Convert between the pound-force foot or pound-force inch and other major units of torque.
Convert between the foot pound-force and other major units of energy.
The foot pound and pound foot are a fertile source of confusion: confusing mass with force, and confusing torque with energy, because both have the dimensions energy and length.
In everyday speech in the United States, the foot pound is a unit of torque used, for example, in expressing how strongly to tighten a nut or bolt. Essentially, a manufacturer who specifies that a certain nut be tightened to 1 foot pound of torque is calling for turning the nut by applying a force of 1 pound to the wrench, at a right angle to the wrench, at 1 foot from the axis of the bolt. The inch pound and inch ounce are similarly conceived.
In the rest of the world, this unit of torque is usually called a pound foot. In SI the unit of torque is the newton meter, with the name of the unit of force first and of distance second. The pound foot follows that pattern. More correctly, since we are really not dealing in pounds, a unit of mass, but in pound-force's, engineers call the auto mechanics' foot pound a pound-force foot.
Suppose instead of being applied at right angles to a lever arm, the pound-force is applied in a straight line over a distance of one foot. Metrologists call this a foot pound-force, and it is a unit of energy, not torque. Similarly, we distinguish between two different newton meters, one the unit of torque, where the force is applied at right angles as described above, and the other the unit of energy better known under its special name, the joule.
To summarize (the pink background indicating common but questionable usages):
|foot pound||ft·lb||US (in trades)||torque|
|pound foot||lb·ft||UK, Australia, India, China,etc.||torque|
|pound-force foot||lbf·ft||correct form||torque|
* The two parts of symbols like these should be separated by a middot, but since middots are sometimes not easily available, hyphens and periods are also seen.
Google's N-gram Viewer shows how the extent of the use of these terms changed during the 20th century.
Convert between foot poundals and other major units of energy.
Of course, the downward force exerted by a mass of 1 pound varies from place to place; it is less on Mt. Everest than it is on the beach, and even less on the Moon. The pound-force was defined to eliminate that uncertainty as well as to create a unit that is, unambiguously, a unit of force.
Another related unit was also created to be unambiguously a unit of force, the poundal, the force need to accelerate 1 pound at a certain rate. From this unit of force we get another obsolete unit of energy, the foot poundal. Thankfully, we know of no one who has used the poundal foot as a unit of torque.
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Last revised: 6 August 2011.