© 2019 by Pernille Dahl & Cole Robbins. 

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Balancing Act & Spill the Beans

Parts needed:

Balancing Acts

  • 2 Models

  • 1 1x2 Baseboard

  • 1 Tack

  • 2 Bamboo skewers

  • 2 Nuts, 3/8” or larger

  • 8” Dowel, ¼”

  • Popsicle sticks

  • 1 penny

Spill the Beans

  • 1 Baseboard

  • 7 Pennies

  • 3 Salsa cups

  • 2 Popsicle sticks

  • 1 Bamboo skewer

  • Cloth

  • Beans

Extra Tools: 
  • Drill

  • Drill platform

  • 15/64 bit for drill

  • Hole punches

  • Duct tape

  • Markers

  • Colored paper

  • File folders or stiff paper

Project Description:

These projects are about optical illusions as well as center of mass.  Since the nuts are small and the figures are large it looks like they should fall over.  But the nut is heavy and the figures are light, so it makes sense that the nuts will swing to the bottom.  If you made the figures from thick wood, they would probably fall over because they would be heavier than the nuts. 

 

In the Spill the Beans project, the salsa cups are hanging from two points:  where the skewer enters the bottom cup, and where it exits.  The pennies are heavy, so the center of mass is near them and swings underneath the two support points.  Then, as you add beans to the top cup, the center of gravity of the two cups moves up bit-by-bit, bean-by-bean, until it rises above the points of support.  At that point the system must turn so that the center of mass moves back under the support.  But when it moves, the beans all fall out and it goes back to the original position. 

 

Count how many beans it takes as you play.  When it tips over, it will always be the same number of beans, within two or three, no matter how carefully they are placed.  That’s because the mass of that number of beans raises the center of mass above the support points. 

 

Your body has a center of mass too. While standing, your feet always stay directly under your center of mass.   If you try to stand on your hands, you may notice that your hands are not as big as your feet, and you will have to work harder to keep them under your center of mass.  To walk on high heels can be hard because your points of support on the ground are so very small.  It is easy for your center of gravity to move outside those points of support, at which time you’ll stumble or fall.  Similarly, if you want a table to be very stable, you’ll put the legs far apart.  Tall, slender tables with legs close together fall over easily.

 

If you try to hang from your hands, it will be easy.  Gravity just pulls your center of mass directly under the pole you are hanging from.  In physics this is called “stable equilibrium,” and occurs whenever you hang something.  If you move the exact same system upside down, there will still be a point of equilibrium – when the mass is balanced above the point of support – but it will not be stable unless there is a large enough area of support.  Instead, it will tend to fall over until it reaches stable equilibrium again. 

 

If your Dancer on the Table is not working, you can bend the weight around so that the center of gravity is below the point of support.  A seesaw works because it is balanced on the center pivot.  If one kid is a lot heavier, it will not be balanced, so that kid will always be on the ground.  But if she moves toward the center, there will be a point where she is balanced with the other kid around the central pivot.  Both mass and distance from the pivot are important when determining balance and equilibrium.  If you carry 40 pounds of water in two 20 pound buckets, you can put one on each side of you, so that your center of mass is right above your feet.  It is hard to get your feet under one bucket of 40 pounds, unless you put it on your head. 

Concepts:

  1. Everything has a point called the center of gravity, or center of mass.  You can think of all the mass of the object concentrated there.

  2. An object standing on the ground will balance only if its center of mass is directly above a point that is within the boundaries of its support points.  For example, if you lean too far, you’ll fall over unless you move a foot or hand underneath you. 

  3. An object hanging will fall and/or rotate until its support point is directly above its center of mass.  For example, if you hang from your hands you’ll hang straight down, but if you raise your legs out in front of your torso will swing back a bit. 

Questions:

  1. If your project Dancer on the Table doesn’t balance right, what can you do to change it?

  2. If a fat kid and a skinny kid use a seesaw, who should sit closer to the center?

  3. Why might it be easier to carry two buckets of 20 pounds each than one bucket of 40 pounds?

  4. Why is it harder to walk on high heels than on regular shoes?