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Balancing Rocks: A Wobbly Game

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Author: Tricia Edgar

Did you know you can make a game out of a science experiment? Play with balance and simple engineering as you hurry to create a sturdy rock tower. It's you versus gravity (and a small fake earthquake or two), and only your skills with balancing rocks can deliver the win. What do you say -- are you game?

Problem:

What makes a balanced rock tower?

Materials:

  • Rocks (roughly the same size)
  • Notebook
  • Pencil
  • Small table
  • Bubble level

Procedure:

  1. Scout out the perfect work station. You want to find a flat, open space -- preferably a backyard or a patio.
  2. Make sure the ground is flat by placing the bubble level on the ground. Are the bubbles in the middle of each liquid window? If so, then you're all set to go! If not, keep looking. This project requires a flat surface.
  3. Set up your table on the ground.
  4. Place your rocks on the table.
  5. Think about the kinds of towers you want to try out. Do you want a narrow tower or one that is wide and squat? Can you think of a special building or a skyscraper that you would want your rock tower to look like?
  6. What do you think will make the most balanced rock tower? Write your guess, sometimes called a hypothesis, in your notebook.
  7. Build your first tower. Try experimenting with seeing how sturdy and how high you can make a tower that simply stacks one rock on top of the one beneath it.
  8. Once you find a combination of rocks that seem to work, it's time for the earthquake! Set your timer and gently start shaking the table. How long does it take before the tower tumbles?
  9. Record the time in your notebook.
  10. Build your next tower. This time try forming a bigger foundation, or the lowest level of your tower. Group three or four rocks together and stop building from there.
  11. Time your fake shaking of the table again and record the results in your notebook.
  12. Make at least three more towers. Keep creating your fake earthquakes and make sure your write down all the times. Which towers did the best?

Results:

Rock towers that had sturdy foundations should have stayed up longer than towers balanced on only one rock. Generally, the bigger your foundation, the better your rock tower should have been.

Why?

The reason some of your towers did better than others probably had a lot to do with points of contact, or the areas on an object that touch another object. You can use your hands to understand points of contact. Press your palms together. What areas on each hand are touching? Probably some of the first points of contact you identified were your fingers. Now relate this to your rocks: the towers with bigger foundations had more points of contact. Lots of points help buildings become sturdier, and sturdy buildings don't fall (unless you shook your table really hard).

What other wobbly games can you experiment with? Try making a rock bridge or a rock house that can ride out a fake earthquake. Can you find other things that affect balancing besides points of contact? What about the size of the rocks or the number of rocks you use? Keep guessing, testing and balancing rocks! Real scientists don't just do one experiment and stop -- they keep experimenting and learning every day.

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