= Solution
At position (1), the <AFM probe> is far from the surface, with approximately zero <force>. At (2), attractive interactions bend the <AFM cantilever> toward the surface; when the attractive <force> gradient exceeds the <cantilever spring constant>, the equilibrium becomes unstable and <jump-to-contact> occurs. Position (3) is on the mechanical-contact branch: changes in imposed height largely bend the <AFM cantilever> and, for a compliant specimen, indent the sample. This branch can include compressive contact and adhesive contact under tension; contact does not itself imply a positive <force> at every point. At (4), during withdrawal the tip remains attached and pulls the <AFM cantilever> downward until the <pull-off force> is reached and it detaches. \b[Approach and retraction follow different paths because <adhesion> creates a hysteretic contact.] The numbered points identify stages on the source curve, not four extra question parts.
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