Why Is "P" Used in Momentum Formula (p=mv)?

[Mrs. Callahan]

While teaching momentum, we use the formula p=mv.

Every year it comes up as to why the letter p is used in the formula and not other letters or symbols. I have conducted on-line searches and I stumble across explanations that mention Newton and his Principia along with other possible suggestions stated here:

http://2000clicks.com/mathhelp/PhysicsMomentum.aspx

Is there a reliable source out there that can difinitively explain the p in the formula p=mv?

 

[Vanadium 50]

We get similar questions a lot, such as "why m for slope". I think these questions tend to obscure the important thing in the equations: they express relations between quantities, and are not formulas (i.e. recipes) to be memorized and incanted later as needed. No, I don't know why p, but why not p?