CoachA contact force is exerted at the points where two bodies touch. A force at a distance needs no contact: weight — the gravitational pull of Chapter 4 — is the only one used in this chapter.
Examples
Example 6.5 (Inventory on a towed sled)
A sled towed across the snow: weight (distance), normal reaction of the snow, tension of the rope, friction opposing the sliding. Nothing else touches the sled: the inventory is complete — weight always, then one contact force per touching object.