Changes

One inch was originally the length of three [[barley]] corns, which form a distance approximating the breadth of the bridge of an adult human's nose. Today the inch is defined in terms of the [[centimeter]] with 2.54 centimeters per inch.  An inch is also one-twelfth of a foot.

One inch was originally the length of three [[barley]] corns, which form a distance approximating the breadth of the bridge of an adult human's nose. Today the inch is defined in terms of the [[centimeter]] with 2.54 centimeters per inch.  An inch is also one-twelfth of a foot.

=== Foot (length) ===

=== Foot ===

The US customary foot, originally the length of an adult human foot, has a length of twelve inches.

The US customary foot, originally the length of an adult human foot, has a length of twelve inches.

=== Yard (length) ===

=== Yard ===

The yard, originally the length measured from the tip of the nose to the tip of the third finger with the arm fully outstretched, is defined as thirty-six inches, or three feet.

The yard, originally the length measured from the tip of the nose to the tip of the third finger with the arm fully outstretched, is defined as thirty-six inches, or three feet.

The US Customary system uses [[Avoirdupois]] weight. Effective July 1, 1959, the US Customary avoirdupois pound has been set equal to the international pound, and this in turn is based on a prototype pound in the British Exchequer.<ref name=pounddef>[http://www.sizes.com/units/pound_avoirdupois.htm Pound avoirdupois] at Sizes.com</ref> All other units of weight derive from this prototype. To be specific:

The US Customary system uses [[Avoirdupois]] weight. Effective July 1, 1959, the US Customary avoirdupois pound has been set equal to the international pound, and this in turn is based on a prototype pound in the British Exchequer.<ref name=pounddef>[http://www.sizes.com/units/pound_avoirdupois.htm Pound avoirdupois] at Sizes.com</ref> All other units of weight derive from this prototype. To be specific:

=== Pound (force) ===

=== Pound ===

One [[Avoirdupois Pound|pound]] (lb) is either the mass of the international pound (or 0.45359237 [[kilogram]]), or more properly the force that gravity exerts on this mass on the ground on [[earth]].

One [[Avoirdupois Pound|pound]] (lb) is either the mass of the international pound (or 0.45359237 [[kilogram]]), or more properly the force that gravity exerts on this mass on the ground on [[earth]].

=== Ounce (force) ===

=== Ounce ===

One ounce is one sixteenth of a pound.

One ounce is one sixteenth of a pound.

In recognition that weight, being a force, is not an acceptable substitute for the measure of mass, physicists have added a mass unit to the US Customary system: the ''slug''.

In recognition that weight, being a force, is not an acceptable substitute for the measure of mass, physicists have added a mass unit to the US Customary system: the ''slug''.

=== Slug (mass) ===

=== Slug ===

One slug is that mass which, when subjected to a force equivalent to a one-pound weight, will accelerate by one foot per second per second. Thus a pound is that force required to accelerate one slug of matter one foot per second per second. A slug would weigh thirty-two pounds on the ground, because the acceleration due to [[gravity]] at ground level on [[earth]] is thirty-two feet per second per second.

One slug is that mass which, when subjected to a force equivalent to a one-pound weight, will accelerate by one foot per second per second. Thus a pound is that force required to accelerate one slug of matter one foot per second per second. A slug would weigh thirty-two pounds on the ground, because the acceleration due to [[gravity]] at ground level on [[earth]] is thirty-two feet per second per second.