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Mobility analysis confirms the 5-bar mechanism has one degree of freedom (1 DOF). As seen in the figure above, the mechanism is comprised of five links, and has full joints at O2, A, B, C, and at the sliding contact between the slider (link 5) and the ground. O4 is a half joint.
M = 3L - 2J1 - J2 - 3 = 3(5) - 2(5) - 1 - 3 = 1 DOF