1. When we walk we push backwards against the ground with our feet and the opposing force pushes us forwards, is this opposing force friction? Or is it the Earth pushing us in response to us pushing it (with our foot) re Newton’s 3rd Law of Motion? Is there one pair of interaction forces occuring here or are there two pairs? 2 . Re the above question; If I push a door, the door pushes me; but there is no friction involved, yes? Is there just one pair of interaction forces involved here? 3. If I push a book across a table the book pushes me (but there is also the opposing frictional force between the bottom of the book and the table, yes?). Are there two pairs of interaction forces at work here?