Key concepts
Physics
Forces
Motion
Centrifugal force
Circular motion
Acceleration

Introduction
Have you ever wondered what keeps you in your seat when you're riding a giant loop-de-loop roller coaster? Surprisingly, it's not only the seat belt! You're kept in your seat because of something called centripetal force. Centripetal force actually does much more than make a ride on a roller coaster's loop possible—it keeps satellites in orbit and you in your bicycle when taking a turn!

In this activity, you will use marbles and Jell-O to investigate centripetal force and circular motion. Which way will the marbles go?

Background
Isaac Newton, a careful observer of nature, used mathematics and science to describe natural phenomena that were not understood at the time. Newton's laws of motion are still used today, and these principles can be found in almost any moving technology. Newton discovered that for an object to move, it must be acted on by a force that makes it move in a certain direction. The object set in motion will continue this motion until it meets an opposite force. You have felt this phenomenon when riding in a car. When the car starts moving you rock backward because your body wants to stay in its stationary position. After you're moving, if the car suddenly stops you will rock forward because your body wants to keep moving at the same speed and direction.

Newton realized that when an object moves (or accelerates) in a circle, the object will move outward, away from the circle's center. For example, when you're riding in a car and it makes a turn, your body moves to the outside of the turn, away from the turn's direction. The force that makes you follow a circular path, even though your body wants to move away from the turn, is called centripetal force.

Materials
•    Transparent plastic cups, at least four
•    Scissors or one-hole puncher
•    Duct tape or electrical tape
•    String
•    Cooking pot
•    Measuring cup
•    Stove
•    Jell-O, two packages, each of a different color
•    Refrigerator
•    Marbles, at least three
•    An open area outside
•    Flashlight

Preparation
•    Use the scissors or one-hole puncher to make a small hole about 2.5 centimeters (one inch) from the top rim of one of the plastic cups. Make a second hole on the opposite side of the cup.
•    Put a small piece of duct tape or electrical tape on the edge of the cup, just above each hole. Fold over the tape so it is on the outside and inside of the cup, but not blocking the holes. This will help prevent the string from detaching.
•    Attach the string to the cup, tying one end of the string through one of the holes at the cup's top and the other end to the other hole.
•    This cup will be your centripetal force generator. Test to make sure that the string is strongly attached to the cup by holding on to the string and pulling down on the cup.
•    Ask an adult to help you make the Jell-O. Be careful when working with the boiling water. Also, be careful not to let the Jell-O spill and stain anything.

Procedure
•    Make one of the packets of Jell-O by dissolving it in the appropriate amount of water according to the directions on the package.
•    Pour the Jell-O into the three other plastic cups, filling each cup halfway full.
•    Place these cups in the refrigerator and chill until the Jell-O is completely set, about two to three hours.
•    When the Jell-O is set, in each cup place a marble on the surface of the Jell-O in the center of the cup. Gently press it into the Jell-O just until the marble is secure and will not move around. Why do you think it is important that the Jell-O is firm enough to support the weight of the marble?
•    Make the second batch of Jell-O. Why do you think the second batch should be a different color from the first?
•    Slowly and carefully pour the hot Jell-O into the cups, covering the first layer of Jell-O and the marble until the cup is almost full, leaving about 2.5 centimeters at the top of the cup.
•    Place the cups back in the refrigerator until the mixture is completely set, about two to three hours.
•    When the Jell-O cups are set, put one of them into the centripetal force generator cup (the cup with a string tied to it) by stacking the cup with Jell-O inside the centripetal force generator cup.
•    Take the stacked cups outside to an open area. (If the string on the centripetal force generator cup broke by accident, you would not want to get Jell-O all over the floors, walls and furniture!)
•    Hold the string and twirl the stacked cups around your head for 20 revolutions, counting each time the cups make a complete circle. If you were to let go of the cups while spinning them, in what direction do you think they would go?
•    After 20 revolutions, stop spinning and remove the inner cup from the outer cup.
•    Shine a flashlight through the cup with the marble. Can you see the marble? Where is it? How has it moved?
•    Tip: If you have trouble locating the marble, try backlighting the marble by shining the flashlight through the back of the cup, toward you.
•    Tip: If the marble did not move, the Jell-O may be too firm. First try spinning it around for 20 revolutions again—but this time spin it harder. If the marble has still not moved, either let the cups sit out at room temperature overnight to soften the Jell-O or repeat this activity, this time using more water to make the Jell-O.
•    One at a time, spin the other Jell-O cups in the centripetal force generator cup for 20 revolutions. Do you notice any patterns of movement? Did the marbles always move in the same direction? Did they move in the direction you thought they would? How far did they move?
•    Extra: One of Newton's other laws says that there is a relationship between the motion of an object and its mass. Try a similar activity with small objects of different weights to see if this has an effect on the amount of movement an object makes due to centripetal force. Instead of marbles, try lead fishing weights, beans, quarters, beads, etcetera. Do you notice a difference in motion between objects of different weights or sizes?
•    Extra: How fast did you swing your centripetal force generator? Is there a relationship between speed and circular motion? Try using a metronome to guide your speed of rotation, setting the metronome at fast and slow speeds. Do the cups move differently at faster speeds than they do at slower speeds?
·      Extra: There are many ways to investigate Newtonian motion. Try taking some of your cups containing Jell-O and marbles with you for some on-site experimentation. What happens to the marble when you take it on a swing, slide, merry-go-round, roller coaster, car ride, bike ride or anywhere else you are curious to find out about?

Observations and results
In each cup, did the marble move from the middle of the cup to the bottom?

When an object moves, or accelerates, in a circle, the object wants to move out, away from the circle's center. Without the push of centripetal force, the object would move in a line, flying out and away from the center of the circle. For example, if you had let go of the cups when you were spinning them, they would fly away from you in a straight line. Each marble in the spinning cups also wanted to move away from you, away from the circle's center, and so each marble should have traveled through the Jell-O and ended up at the bottom of the cup.

In this activity, centripetal force acting on the system is supplied by the tension in the string. This force kept the cups moving in a circular path. This is a "pull" force, similar to how satellites are kept in orbit due to the pull of Earth's gravity. A "push" centripetal force acted on the marble: this force, supplied by the bottom of the cup, kept the marble going in a circular path and not flying away in a straight line. A push centripetal force also keeps you in your seat on a loop of a loop-de-loop roller-coaster ride and when you're making a turn on your bicycle.

Cleanup
Throw the Jell-O in the trash when you are done, after retrieving your marbles!

More to explore
Centripetal Force from the Georgia State University, Department of Physics and Astronomy
Newton, an Apple and You from Light-Science.com
Roller Coaster Science: Marbles, Tubes and Loops from Science Buddies
Centripetal Force from Science Buddies

This activity brought to you in partnership with Science Buddies