At the risk of weighing in on a topic that I don't often think about...
Is it easier to consider the comparable problem of a walker walking on a
non-motorized treadmill? In that case, the force exerted by the foot moves
the tread in the direction of the force. Force and motion are in the same
direction. The static friction maintains the contact between the foot and
the tread. The walker doesn't move - but does push the tread behind
him/her.
Then consider this from the point of view of an ant glued to the tread. The
ant sees the walker approaching, and then receding.
Larry Woolf; General Atomics; 3550 General Atomics Court, San Diego, CA
92121; Phone:858-455-4475; FAX:858-455-4268; http://www.sci-ed-ga.org