Pendulum Momentum

Things You Need

• string
• 4 chopsticks
• 6" square of cardboard
• hole punch
• rubber band
• 100 pennies
• plastic cup
• role of painter's tape
• 8' rope
• basketball
• something tall and strong to hang a long pendulum from

Things You Do

Part I - Simple Pendulum

Punch a hole near each of the four corners of the cardboard. This will be your base. Put it flat on a table. Tie together all four chopsticks at one end with the rubber band. Spread them out at the other end, and put one in each of the four holes you just punched in the cardboard. You should have something like a teepee structure.

Now tie or tape a string so that it hangs from the top of your chopstick teepee. Tape a few pennies to the bottom of the string so that they hang just above the cardboard. Congratulations, you made a pendulum!

If you swing your pendulum from up high, does it go back and forth faster than if you swing it from down low? What if you add more weight (i.e. more pennies)? What if you make the string shorter or longer? Make hypotheses and experiment with all of these different variations to your pendulum.

Part II - Going Bigger

Now make a really long pendulum with string and a few pennies. Hang it from a high place and give it a swing. Does it swing faster or slower than your short pendulum?

Put all leftover pennies in a plastic cup and hang it from a string the same length as your long pendulum that has only a few pennies. Tie them both to the same high place. Pull them both back side-by-side. Before you release and let them swing, make a hypothesis. Which will swing faster, the light one or the heavy one? Why?

Have some fun playing with your tall pendulum and see if you can learn anything new just from observing.

Part III - Momentum

Momentum is when something keeps moving even after you stop pushing it. Have you ever been running very fast then tried to stop suddenly? Your body feels like it wants to keep going. That is momentum.

Try to send your roll of tape across the floor like a wheel. It keeps moving even after you push it. It has momentum and will continue rolling until something stops it. What do you think would stop it if you rolled it on the ground outside? Or would it keep going forever?

Let's experiment with momentum by making a very big pendulum. Tape the basketball to one end of the rope. Tie the other end of the rope to your tall place form Part II. You can take your other pendulums down now. You have just built a big basketball pendulum.

Pull the basketball up and hold it to the tip of your nose. Make sure the rope is stretched out tight. If you let go and keep your nose in the same place, do you think the basketball will come back and hit your face? Why or why not? Try it out...maybe start with your hand instead of your face.

Now, notice how the basketball swung in a straight line. It had momentum from the initial force of gravity pulling it straight down. What if you added another force as it was swinging? For example, what if you tapped the side of the basketball at the bottom of its swing? Do you think the basketball would keep swinging in the same straight line, a different straight line, or in a different shape altogether?

Experiment with adding different forces i.e. tapping and pushing the basketball as it swings. Remember, every time you add a force to the basketball, it will continue in that direction for a while because of momentum. You can stop that momentum by adding an opposite force. See if you can make the basketball swing in a circlular, a square, or a trianglular path.

Tips For Adults

• The speed at which a pendulum swings back and forth depends only on the length of the string/rope. Weight doesn't matter. How high the pendulum starts doesn't matter.
• For Parts II and III, two adults could hold either end of a broomstick and hold it above their heads. The long pendulums could be tied to the middle of the broomstick.