http://kmoddl.library.cornell.edu/model.php?m=476&movie=show
This mechanism consists of a rotating circle that has a knob near the edge of the disk. The knob is inserted into the slot of a long arm that swings back and forth. The arm is fixed at the point where it is attached to the broom shaped piece at the bottom, point A.
As the disk rotates, the arm is forced to swing back and forth. Although the disk is rotating at a constant speed, the arm swings more quickly to the left than to the right. If the knob were to start at point B, and make a complete clockwise rotation, the arm would swing slowly from the left to the right, and quickly from right to left, as the knob approaches the 6 o clock position again.
distance = rate x time
Since the angular velocity of the disk remains constant, we know that the distance and time are proportional.
The distance between B and A is smaller than the distance between C and A. With a smaller length between the knob and point A as the knob approaches the bottom of the circle, the arm is forced to take a shorter amount of time to swing around. When the knob goes through the top half of the circle, the distance between the knob and point A is at a maximum, and therefore more time is taken for the arm to swing around.
The mechanism is called "reciprocating" because the movement of the disk and the arm drive each other. I found this mechanism appealing because it is not consisted of one driving mechanism and one output mechanism. The two parts of the mechanism reciprocally propel each other.
Your analysis of this reciprocating mechanism clearly explained to me how it works in motion. I like the thought you gave to how distance affects the rate of motion for the arm, and also how you proposed and explained the alternate direction of movement to illustrate the entire mechanism's movement in the second paragraph. Great job!
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