Try This: Seesaw

QUESTION

How do you balance a seesaw?

MATERIALS

Pencil

Ruler with inch markings

10 pennies, minted after 1982 (because you need them to have the same metals inside)

PROCEDURE

  • Place the pencil on a hard surface such as a table.

  • Place the ruler on the pencil so that it balances at the 6-inch mark.

  • Place five pennies at one end of the ruler.

  • Take five more pennies and find the location on the other side of the ruler that will make the ruler balance.

  • Clear the ruler off.

  • Place six pennies at the 2-inch mark on the ruler.

  • Find the location on the other side of the ruler at which only three pennies will balance the original six.

Fun Facts

Gravity on the moon is about one-sixth what it is on Earth. This means that objects fall six times faster on Earth than on the moon!

WHAT'S HAPPENING

The pencil under the ruler turns the ruler into a lever. The pencil acts as the fulcrum, or the balance point. To balance the ruler, there needs to be the same kind of force on one side as there is on the other. The force is gravity, acting on the pennies. But there's a catch! The farther away from the fulcrum the pennies are, the more their gravity counts toward balancing the ruler. For example, three pennies located 4 inches from the fulcrum (think 3 × 4 = 12) will balance six pennies located only 2 inches from the fulcrum (6 × 2 = 12). Can you think of other combinations that will balance those three pennies?1

lever: a device used to lift very heavy objects.

gravity: the force that pulls us toward the center of the earth and keeps us on the ground.

FOLLOW-UP

The next time you want to ride the teeter-totter and find that your partner is much heavier than you, see if you can figure out where you both should sit. Will this work if you ride with one of your parents? If you know each other's weight, you should be able to come up with a seating arrangement that works.2

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