The jerky form of motion described as ``stick‐slip,'' which is sometimes observed in the sliding of surfaces, has been studied by means of the stick‐slip apparatus built in this laboratory. A comparative study of motion during the slip has been made for a number of combinations of unlubricated metals. The variation of friction with velocity has been determined for several typical cases. Most slip traces are symmetrical about their point of inflection indicating that the kinetic friction remains approximately constant. The static friction is greater, and a rapid drop to kinetic friction usually occurs. The kinetic friction is least at the end of slip, so that the friction‐velocity relation is not reversible. There is evidence that the friction does not return immediately to its higher static value when the sliding surfaces come to rest. These results are considered in terms of several theories of friction.
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