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How to Build a Joule Mobile

A joule is a measure of energy used internationally to describe how much force is expended to move or lift an object. Building a joule mobile, or an object that can be propelled with no outside force, means that your mobile will move only as a natural reaction to the movement of one of its components. A component movement can include unwinding, dropping a weight or a release of air. Making a joule mobile can be done with common, household materials and can be done with or without establishing a distance of travel ahead of time.

Things You'll Need

Hand drill
Four metal jar lids
2 thin wooden skewers, 8-inch,
6-inch wooden skewer
2 thin metal tubes, 8-inch long
6-inch by 10-inch plywood
Empty thread spool
2 pieces of balsa wood, 3-inch by 13-inch
2 small rubber washers
Wood epoxy
2 wood blocks, 2-inch square
Heavy twine
2-pound weight
Handsaw

Instructions

- 1
  Assemble the body of the mobile. Drill a hole through the center of your jar lids. Epoxy one metal tube across the front of your balsa wood, and the other one across the back. Insert one end of the wooden skewer through a jar lid and epoxy in to place; now thread the other end of the skewer through the front metal "axle," and epoxy the other lid on to the skewer. Repeat for the back of the mobile.
- 2
  Construct your pulley system. Insert your empty thread spool on to the center of a 6-inch wooden skewer and slide one small rubber washer on either side. The washers should fit snuggly on to the skewer to prevent the spool from sliding horizontally. Using epoxy, attach the wooden dowel horizontally across your two long pieces of balsa wood and let dry.
- 3
  Attach the pulley to your mobile. Cut a small 1/4-inch deep slot in to each piece of your wood blocks using a small handsaw. Attach one on either side of your mobile at the midway point of the longer sides using epoxy. Let dry for at least one hour. Apply epoxy to the open ends of your pulley system and insert each end in to your slots. Let it dry.
- 4
  Secure string to the axis. Run a length of string from the bottom of your mobile to the top of the thread spool, add 4 inches and cut. Measure from the thread spool to either side of your front axle, add 4 inches and cut two pieces of string. Tie these two pieces to the end of your first piece so that they form a "V" shape. Knot each of the two "V" ends around the left and right axis, leaving plenty of room for the axis to turn.
- 5
  Tie the weight in to place. Drape the single end of string over the thread spool, and loosely wind the two "V" ends of string around the axis until the single end is up to the thread spool. Tie your 2-pound weight on to the end of the string. When you let the weight fall, it should pull the string, causing the wheels to turn, moving the mobile.

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