If you look at Slide 103.  The notation has me a little confused.  In the diagram of the cartoon woman walking, the arrow points in the opposite the direction of the person's motion but its notation is Fgp.  But I think it should be Fpg.  The person's foot is pushing on the earth backwards.  WIll you check the notation. Perhaps I misunderstand.

Peter, I've seen the nomenclature working in the opposite direction in different textbooks. Either is correct, it just has to be consistent.

On this slide, the first letter in the subscript is what is being pushed on, and the second is what is doing the pushing.

So the person is pushing the ground to the left, Fqp, and the ground is pushing on the person to the right, Fpg.

John