Center of Gravity Title

Here's how to make a cool sculpture that seems to defy gravity!

Materials

1.  Polymer clay such as "Sculpey" brand, that can be hardened by heating in an oven.  A pound of clay is approximately enough for 100 students (get extra).

2.  Wooden dowels, 3/32" dia. x 2 5/8" long (At Michael's Craft Supplies, or in bags of 250 for under $2 here).  Minimum three dowels per student.

3.  Stiff paper, such as that from a manila file folder.  1/2" x 3/4" piece per each student.  Or cut strips of paper 3/4" wide, and have students cut their own 1/2" wide slices.

4.  Hot glue guns and glue sticks.

5.  Tool for cutting the dowels after assembly.  Scissors are not the best tool because kid-grade scissors are too dull and the handles are usually too small to get good leverage against the dowel.  Best is to use "diagonal cutters" or one can use "fingernail" clippers with big jaws.  One set of cutters per 5 students should be sufficient.

6.  Strip of masking tape, 1/2" wide by 2" long, per student.

7.  Latex or vinyl gloves to protect hands from sticky hot glue.

8.  (Optional) Sandpaper sticks for smoothing the dowel ends.



Cut the heavy paper

Step 1.  Cut a paper rectangle (if not pre-cut).

Use the stiff paper from a manila folder.  Print out these figures to help you build the sculpture.


Fold the paper

Step 2.  Fold the paper rectangle, then flatten it out.

Use this picture to see where to fold it.

Put tape on a dowel

Step 3.  Put a Piece of Tape On a Dowel.


Add glue

Step 4.  Position the Rectangle Onto the Pattern. 

Add a dab of glue on the lower part of the rectangle as shown in the picture.


Add the Middle Dowel

Step 5.  Add the Middle Dowel

Put the middle dowel down first and tape it.

Add the other dowels

Step 6.  Add the Other Dowels. 

Lay down the other two dowels into the glue and let the glue cool.


Add glue

Step 7.  Add some hot glue to the top of the dowels and fold over the paper.

Be careful, the glue is hot and will stick to your finger!  Use gloves!

Add glue

Prepare a cube of clay
Step 8.  Cut out two pieces of clay about the size of a 1/2" cube.
 
Try to make both pieces the same size.

Add balls to dowels

Step 9.  Make Two Clay Balls.  Add Them to the Dowels.
 
Roll each blob of clay between your palms to make a smooth ball.  Don't press too hard and the ball will become smooth and round.  Poke them into the ends of the dowels.

Find the Center of Gravity

Step 10.  Find The Center Of Gravity
 
Lay your sculpture onto the pattern.  Make a little dot with a pencil on the middle dowel as shown.

Cut at the center of gravity

Step 11.  Cut The Dowel A Little Above The Center Of Gravity.
 
Use the cutters to cut the dowel just above (closer to the folded paper) the center of gravity.

Test the Balance

Step 12.  Test The Balance.
 
Try to balance the sculpture on your finger.  If it falls off, you need to cut off a little more of the middle dowel.  Cut tiny pieces each time until the sculpture will balance.

Sand dowel flat
Step 13.  Sand the End Flat.
 
If you sand the end of the center dowel, you will be able to balance the dowel on the end of a sharpened dowel.  When your sculpture is all balanced and adjusted, you can bake the clay in an oven at 275 degree F for 10 minutes.  Place your sculpture on a sheet of aluminum foil when in the oven.


Balance

An Amazing Video

Here is a video that shows how balance is improved by using a long pole like the weighted arms of our sculpture.  In the video, the wirewalker, Phillipe Petit, is not as stable as our sculpture;  he must still work to stay balanced.  But balance is easier with the long pole.  Notice how, when he is lying on the wire, he suspends one leg below the wire to help lower his center of gravity;  still not below the wire, but at least lower than otherwise.


(Just click on the picture to start the video. )

This file is very large, over 125 MB, but should stream as an .mp4 video with a QuickTime plug-in on your browser;  let us know if you have problems.  Thanks to PBS for this video;  please don't sue me for ripping and posting this most beautiful video so that other students can learn from this fantastic story.  Note:  This video contains material about the World Trade Center in the mid-1970's.  Images of the World Trade Center may be distressing to some individuals, so please consider this before viewing.


Raising the Center of Gravity
Instructor's Notes

The principle of static balance is based on the idea of a "potential well".  This is like a marble rolling back and forth in the bottom of a teacup;  the marble eventually settles to the bottom of the teacup because the "potential energy" of the marble increases for any position of the marble other than the bottom of the cup.  This is the same as saying that it takes energy to move the marble from the bottom of the cup.

In our sculpture, the "center of gravity" of the sculpture lies halfway between the two balls (if they are the same size and if they are much more massive than the sticks that hold everything together).  In actuality, the "center of gravity" lies a little above the halfway point between the clay balls (because the sticks also have mass).  When the middle dowel is cut off to a point just above the center of gravity, and the sculpture is placed on a fingertip, the center of gravity rises just a little (and moves to one side) as the sculpture tilts.

Raising the center of gravity takes some energy, just like pushing a marble up the inside wall of a teacup.  This means that the sculpture will be stable and slowly come to rest without falling over after it is "kicked".  Students will find that the sculpture becomes more and more stable as the center dowel is shortened.  This is because the center of gravity rises further for the same amount of tilt.  This is another way of saying that the potential well becomes "steeper".  More "kinetic energy", energy of motion, would be needed to escape over the top of the potential well.


Here are some pictures of an exhibit at the Museum of Science in Exposition Park near Downtown Los Angeles.  It is a bicycle suspended on a highwire that museum visitors can ride.  But it is a very special bicycle in that it has an additional heavy mass suspended below the bicycle, and attached very rigidly to the bicycle via a long pole.  Without the suspended mass, the center of gravity of the rider and bicycle would be above the highwire and would fall as the bicycle tilted (meaning it would not be stable).  But the heavy mass suspended a long distance below the bicycle shifts the center of gravity below the highwire (the pivot point).  Therefore, just as in our sculpture, the center of gravity rises as the bicycle tilts.  And then it is stable.







In the case of the wirewalker in the video, the falling motion of an unbalanced wirewalker is slowed by the "rotational inertia", the resistance of the long pole to being spun about its center.  The weight of the pole alone is not the critical element;  it is the fact that the weight of the pole is spread out over a long distance that gives the pole "rotational inertia".  This allows the wirewalker sufficient time to readjust his body to lie directly above the wire.