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Activity

Balancing sculpture

Summary: 
Make a toothpick or grape balance on your finger or other object, by adding weight under it, which lowers the centre of mass to make it more stable.
Science content (2016 curriculum): 
Physics: Motion and Forces, Newton’s Laws, Gravity (K, 2, 6)
Science topic (2005 curriculum connection): 
Physical Science: Force and Motion (grade 1)
Lessons activity is in: 
Materials: 
  • for a clay and stick sculpture: toothpick, skewers, modelling clay (firm play dough also works, soft play dough does not)
  • for a fruit sculpture: grape/apple etc, forks or skewers and clay
Procedure: 

There are many adaptions of this concept.

Clay and stick sculpture
I have found this simple one quick to assemble and sturdy enough to stay together.

Push the toothpick into a small piece of the clay, then push the skewers into the same piece of clay on each side of the toothpick (see first photo).
Add additional blobs of clay at the ends of the skewers.
Stand the toothpick on your finger, the table, or other surface.
If pushed to one side, it will right itself.

Encourage students to experiment with various configurations of the materials and see how their new models balance, or not.
(e.g. what happens if you remove one skewer, what happens if you change the position of the skewers, does it work without the clay at the end of the skewers?...)

Fruit sculpture
Quick to set up.
Try and balance the small end of a grape on your finger. It's pretty much impossible.
Add some mass below the grape, using forks, or other heavy objects, stuck into the grape at an angle.
If the forks are heavy enough an apple will work too.
A banana with a curve can be balanced on your finger if it curves downwards, as the curve puts some mass below the balance point, making it more stable.

Explanation in terms of force
The sculpture balances because most of the mass is low down underneath the balance point. When gravity pulls on these masses, the force pulling down on the balance point effectively comes from underneath it.
If the sculpture is tipped to one side, gravity pulls the weights back down to the lowest position.

Explanation in terms of centre of gravity/mass:
The exact position of the effective force on the toy is in midair underneath the balance point, and is called the centre of gravity/centre of mass. As long as the centre of gravity is below the balance point, the sculpture is stable. If the sculpture is tipped in any direction, the centre of gravity is raised. This results in gravity pulling it back towards an upright position, and returning it to its upright position.

Applications of this concept:
Engineers try to design a sports car's centre of gravity as low as possible to make the car handle better.
When high jumpers perform a "Fosbury Flop", they bend their body in such a way that it is possible for the jumper to clear the bar while his or her centre of mass does not.
When balancing on a beam, we stretch out our arms and move them around, to keep our centre of mass over our feet.

Notes: 

If the clay gets warm and squishier, the skewers do not stay in place. Try these other materials to hold the skewers:
foam ball (though ends up getting many holes in it as different configurations are tried)
foam or old flip-flop (also accumulates holes, but should be secure): https://www.youtube.com/watch?v=L5vnpg8NhBE

Another model: attach washers to ends of a stiff wire. wrap it around a popsicle stick. balance stick on finger. NSF "Teaching Physics with Toys" p. 45

Grades tested: 
Gr 1
Gr 2
Gr 3
Gr 4
Gr 5
Gr 6
Gr 7
Teacher: 
Ingrid
Romy Cooper
Teaching site: 
Eton Arrowsmith Camp
General Gordon Elementary
General Gordon Elementary Science Club
ingridscience afterschool
Activity originally developed and delivered: 

Latest version at Gordon Elementary Science Club. Fruit at ingridscience afterschool.