B Why Is an Object's Weight Equal to the Force of Gravity It Experiences?

[Herman Trivilino]

Nevertheless, the informal use of "pound" as a unit of force, instead of the more precise term "pound-force", is rather common.

It's nearly ubiquitous in introductory physics textbooks. Even NASA uses it. Nevertheless, there is no officially-sanctioned definition of the pound-force.

For example:

ACDelco digital torque wrench (amazon.com)



which also has "kg" instead of "kgf".

The kilogram-force was officially dropped by the SI as of, IIRC, 1960. Prior to that a so-called standard value of $g$ equal to 9.806 65 N/kg was used to define the kilogram-force. It was simply an invention designed to allow the measurement of mass using units of force. There is no place on Earth where $g$ has a value of 9.806 65 N/kg for any significant length of time.

Unfortunately, no such standard value was ever adopted for the definition of the pound-force, although the above value appears to be used by NASA for their definition. So they end up defining the pound-force as the product of 0.453 592 37 kg and 9.806 65 N/kg, resulting in an unjustifiably large number of significant digits.

 

[jbriggs444]

There is no place on Earth where $g$ has a value of 9.806 65 N/kg for any significant length of time.

According to the Wiki article the figure was the result of a measurement (made in 1888 in Paris) and a correction (specified in the previous year by the CIPM) to obtain a corresponding estimate applicable to 45 degrees latitude).

Wisely, no one bothered to correct the figure to account for more accurate gravitational measurements obtained in the years since.