# Discover Centripetal Force

• Melissa Pinkley Home School Enrichment
• Published Aug 31, 2010

Have you ever ridden on a roller coaster that went upside down? If so, weren't you glad to have the restraining device that kept you safely in your seat! Well, the truth is that your seat belt really had very little to do with keeping you in the roller coaster when you went upside down. What really kept you there was science!

You see, God created a force called centripetal force. It was first observed and recorded by the famous scientist Sir Isaac Newton, who also first explained gravity. Newton explained that a force exists that pulls an object toward the center when it is moving in a circular motion. This force is what keeps objects in orbit around the Earth and the sun. It's also what keeps you in your seat when you're upside down on that roller coaster.

See centripetal force in action when you complete and observe the following experiments.

Experiment 1: Water Fall?

You may want to do this experiment outside. If you move too slowly, you might get wet!

Items Needed:

• 1 plastic bucket (with a sturdy handle)
• Water
• Sturdy string (if necessary)

Directions:

First, see if the bucket can easily be held at about knee height of the person doing the experiment. If necessary, add a sturdy string to the bucket handle to achieve the proper placement.

Next, place 1-2 cups of water into the bucket. Do not add too much water, or the bucket will be too heavy for the experiment. Now, move the bucket upwards and around, quickly, in a circular motion.

What happens to the water? What happens if you change the speed of your circular motion? What happens if you change the size of the circular motion, moving the bucket in a smaller or a larger circle? What did you observe about centripetal force from this experiment?

Experiment 2: Walk the Marble

This experiment is contagious! All my kids wanted to try their hand at this one. It does a great job of showing that centripetal force takes a continuous circular motion.

Items Needed:

• 1 marble
• 1 empty soda bottle

Directions:

First, place the marble into the soda bottle. Next, with the soda bottle facing upwards, twirl the bottle so that the marble begins moving in a circular movement around the inside of the bottle.

While still twirling the bottle, move the bottle to a sideways position, keeping the marble moving in a circular motion. When ready, move the bottle to an upside down position. Don't stop twirling.

What happens to the marble? Can you walk across the room with the bottle upside down and keep the marble inside?

• Add more than one marble at a time.
• Use a bottle with a wider opening.

Experiment 3: Jell-O Motion

This experiment takes a little bit more time and a few more ingredients, but it can be really fun.  You even get to eat the results.

Items Needed:

• Paper cups
• Sturdy string or yarn
• Grapes
• Two small packages of very different colored Jell-Os (such as yellow and red)
• Butter knife
• Paper plates
• Stapler

Directions:

Begin making your centripetal force test chambers by setting out 3-5 paper cups. Mix one of the boxes of Jell-O according to the directions on the box. Fill each paper cup half full with the mixed Jell-O. Place the cups in the refrigerator till they set—about 2-3 hours.

Once the Jell-O is set, gently place a grape on top of the Jell-O in the center of each cup. Then, mix the second box of Jell-O according to the directions. Pour the second batch onto the first. Be sure to leave about one inch from the top of each cup empty. Place the test chambers into the refrigerator till set.

In the meantime, you can create the centripetal force machine. Cut a length of string 12-18 inches long. Staple the ends of the sting to opposite sides of the outside of a paper cup. Make sure the ends are secure by stapling them at least 2-4 times each.

When the remaining Jell-O  has set, place one test chamber into the centripetal force machine. Hold the string in the middle and spin the machine in a circular motion 20 times. Carefully remove the test chamber from the centripetal force machine. Place a paper plate on top of the test chamber. Flip it carefully so the test chamber's ingredients come out onto the plate. You may need to carefully slide the butter knife between the Jell-O  and the inside of the cup to help it release onto the plate more easily. Observe where the grape is now. Is it in the same place? Why do you think this happened?

Repeat the experiment with the rest of the test chambers to see if your results all agree with one another. Then eat your results.

Another Fun Idea

Gather a bunch of items such as Popsicle sticks, rubber bands, PVC pipe pieces, wood scraps, pieces of toy car track, and anything else you can think of. See if you can create a centripetal force track.

Have fun doing science!

Melissa Pinkley

enjoys life with her husband, Wes. They learn a lot from their four children: Ben, Micah, Levi and Abigail. Homeschooling goes on 24/7 for the whole Pinkley family. They have been homeschooling for 10+ years.  The Lord is gracious and continues to help them follow Him.