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Science Snack
Make a jumpy mechanical pet that takes itself for a walk.
Make a jumpy mechanical pet that takes itself for a walk.

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.

Tools and Materials
  • Six jumbo paper clips
  • Needle-nose pliers
  • Cardboard, approximately 4 x 6 inches (10 x 15 cm) and 1/8 inch (0.3 cm) thick (the exact size and thickness are not critical; foam board or stiff matboard can also be used)
  • Pushpin
  • Hot-glue gun and glue sticks
  • Clothespin (the kind with a spring)
  • AA battery
  • Small DC hobby motor that will run on one AA battery
  • Utility knife
  • Two brass fasteners (roundhead paper fasteners), 1 inch (2.5 cm) long
  • Plastic cap from a soda or water bottle
  • Electric drill and drill bits: 5/64-inch (0.2 cm) or AWG #48 or #49 [American Wire Gauge sizes will hold the cap on the motor shaft a little more tightly, but a 5/64-inch (0.2-cm) bit will also work, and is more commonly available]
  • Optional: small sheet-metal screw (such as an 8, 10, or 12 x 1/2-inch [1 cm] Phillips pan-head screw); 1/8-inch (0.3 cm) drill bit; Phillips screwdriver
  1. Straighten and then re-bend one of the paper clips as shown in the photo below, using needle-nose pliers as necessary.
  2. Repeat Step 1 three more times, so you have four paper clips that look like those in the photo below.
  3. Make a final vertical bend in each paper clip so that two look like the leftmost paper clip, and the other two look like the rightmost one in the photo below. 
  4. Use the pushpin to poke holes through the cardboard in the positions shown by the dots in the photo below (click to enlarge). Make your holes about 1/2 inch (1 cm) away from each corner.
  5. Using the photo below, put all four paper clips in place, being careful to achieve the arrangement shown. To do this, just push the long leg of a paper clip down through one of the holes, making sure that the bent portion lies flat against the top surface of the cardboard. (You can re-bend a little, if necessary, to get it to lie flat.) Once all four paper clips are in place, generously apply hot glue to hold them to the cardboard.
  6. Use hot glue to attach the clothespin to the cardboard, as shown below. When the glue is dry, hold the jaws of the clothespin open and insert the battery. The clothespin should hold the battery firmly in place (see photo below).
  7. Use the sharp point of the knife blade to make two small slits in the cardboard, one just below and parallel to each end of the battery. Insert a brass fastener up from under the cardboard, going through each slit (click to enlarge the photo below).
  8. Turn the device over and liberally apply hot glue to the top of each fastener to hold it in place (see photo below). After the glue has dried, make sure the flat sides of the brass fasteners are firmly in contact with the ends of the battery. Bend the fasteners, if necessary, to keep them in place.
  9. Using hot glue, attach the motor to the end of the cardboard opposite the clothespin. Place the motor so its case is even with the edge of the cardboard, and its shaft extends out past the edge. Turn the motor so the two contacts at the rear are at roughly equal height (see photo below).
  10. Drill a 5/64-inch (0.2-cm) hole in the top of the bottle cap, about halfway between the center and the edge. Push the bottle cap onto the motor shaft (see photo below). If the cap slips around on the shaft, remove it and put a small blob of hot glue in the hole before putting the cap back on. It should fit tightly on the motor shaft.
  11. Straighten the two remaining paper clips. Then use the needle-nose pliers to make a small hook at one end of each paper clip. Hook the clips through the holes in the two motor contacts—one on each contact—and use pliers to tighten them (see photo below).
To Do and Notice

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).

What’s Going On?

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.

Going Further

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?