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Have you ever seen an off-balanced washing machine wobble wildly? This critter jitterbugs similarly—and for similar reasons—in a playful demonstration of eccentric motion.
Use the paper clips attached to the motor to connect the battery to the motor. You can put the paper clip ends either between the brass fasteners and the battery terminals or, better yet, between the two prongs on the fasteners themselves, which will help hold them in place as Skippy vibrates. Bend the fasteners as necessary to establish a good contact with the battery terminals.
When the circuit is complete, the motor should turn and Skippy should begin to wiggle, jiggle, and dance (see animation below).
As you may have guessed, the secret to Skippy’s jumpiness is the off-center hole you made in the bottle cap. For an object to spin smoothly, the axis of rotation (in this case, the motor axle) must coincide with the object’s center of mass. If these two things don’t coincide, the result is what’s called eccentric motion—and a wobble that can cause the entire system to “walk” as the motor spins. You may have experienced an unbalanced washing machine wobble as it spins with an uneven distribution of mass.
Skippy lends itself to creative further investigations, races, and design challenges.
Can you make Skippy stand still, go in a straight line, or go in a circle? What happens if you change the length of Skippy’s legs, or make a Skippy with only three legs, two legs, or one leg? Does reversing the direction of the motor’s spin affect performance?
You can also try changing the location of the hole in the bottle cap, or attach additional mass by drilling a hole in the outer wall of the cap and attaching a small screw. (See the “optional” items in the Materials section.) How does this affect Skippy’s motion?
For a personalized and artistic touch, decorate your Skippy. Replace its legs with markers and see what patterns it traces.
Skippy has been compared to a bug or a robot. Look up the definition of these two things and decide for yourself: Does Skippy qualify as either? Why or why not?
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Attribution: Exploratorium Teacher Institute