Force & Work: Understanding the Calculus Behind it All

[opticaltempest]

I am reading over an explanation of Work in my calculus textbook (Early Trancendentals 5e - Stewart). The following example has me confused. Why are pounds treated as a force and not a mass?

http://img71.imageshack.us/img71/8987/forceandwork6ok.jpg

 

[andrewchang]

because lbs are a unit of force. the slug is the unit of mass for the english system

 

[Orefa]

The pound has actually been officially defined as a unit of mass, even though I believe it was used as a unit of force a long time ago. Nowadays the term pound-weight is used to refer to the gravitational force exerted by a pound, and if you notice the main question states "20-lb weight" to specify this, even though it neglects to be so specific in the rest of the text.