Centripetal Force

Centripetal Force

Physics | 10-20 minutes

Students will discover the amazing power of centripetal force using jello, marbles, and a handmade Centripetal Force Generator. Please do this in an open area outside with a lot of room.

Note: If students are going to eat the Jello, please wash marbles and other objects before the activity.

Materials Needed

Per Student:

  • 1 clear plastic cup halfway full of Jello.  (Note- pour the hot Jello mixture into the cups and refrigerate until fully set, at least two to three hours)

  • 1 empty clear plastic cup

  • 1 clean marble

  • 4 feet of string (yarn, cord, etc.)

  • A few inches of duct tape or electrical tape

A few per group:

  • Scissors

  • Hole punchers

  • Flashlights

Optional:

  • Extra cups of jello for Try It! activities.

  • Various other clean objects to try instead of marbles. (Pennies, ping pong balls, etc.)

Steps:

  1. Use the scissors to punch a hole in the side of the cup approximately 1 inch from the top rim, then do the same on the opposite side.

  2. Using a small piece of duct tape or electrical tape, reinforce the top edge of the cup above each hole.  Be careful not to block the hole.

  3. Cut a four foot piece of string, and then attach it to the cup by tying each end through a hole in the cup.  Give it a gentle tug to make sure it is secure!  This cup is your Centripetal Force Generator.  Set it aside.

  4. Place a marble in the center of the cup of Jello.  Press it into the Jello just enough so that the marble is securely in place and won’t roll around.  The Jello should support the weight of the marble.

  5. Place your cup with the marble and Jello into the cup of your Centripetal Force Generator.  If you are not already outside, go outside to an open area before moving to the next step.

  6. Hold the string and twirl the generator above your head for 20 revolutions, counting each time it goes around, then stop spinning and examine the marble and the Jello.

  7. The marble should have disappeared from its original location.  If the marble is still sitting on top of the Jello, spin it harder for another 20 revolutions.

  8. Shine a flashlight through the Jello and find the marble.  Where is it?  Which way did it move?  How far did it move?  

  9. If you have trouble finding it, try backlighting it by shining the flashlight in from the back of the cup.

  10. Check with the other students.  Did all of the marbles behave the same way?

Explanation:

When you spin something in a circle, if you accidently let go, where does it go? It flies in a straight line away from you! When you spin your Centripetal Force Generator, your marble is constantly trying to fly away. We call this centrifugal force (really, this is just inertia). Centripetal force is what is keeping the generator moving in a circle. The centripetal force makes objects continue in a circle towards the middle. In our case, it’s provided by our string. Centrifugal force pushes the marble out. Centripetal force keeps the generator in. Centrifugal out, Centripetal in.

Try it!

Try replacing the jello cup in your generator with another cup and look for patterns.

Try using other objects instead of marbles.

Try putting a cup of water in the generator.

Real World:

Satellites, carnival rides, playground equipment, roller coasters, highways, and a whole lot more! Centripetal force and centrifugal force are EVERYWHERE!

Adapted from Spinning Machine by Scientific American