Machines rarely want the motion a motor gives them. A crank and slider turns rotation into back and forth, a cam's shape becomes a follower's path, and four bars turn a crank into a rocking arm, if their lengths allow it.
Cranks and sliders, cams and followers, and four-bar linkages.
A crank and slider turns rotation into back-and-forth motion: one turn is one stroke out and back, and the stroke is twice the crank's radius, whatever the rod's length. Students find the stroke by turning, work out a pump's flow, and size a crank to a brief.
A cam is a shaped wheel; a follower rests on it and rises and falls as it turns. The cam's shape decides the follower's path, which a displacement graph shows. Students read graphs from four cams, match cams to jobs, and choose a cam for a toy.
Four bars joined at pivots turn a crank into a rocking arm, if their lengths allow it. Students find a crank that jams, learn the rule that predicts whether it can turn all the way (shortest + longest ≤ the other two), and design a windscreen wiper with a big enough swing.
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