Summary
Introduction
Have you ever wondered what keeps you in your seat when you are riding a giant loop-de-loop roller coaster? Surprisingly, it is not the seat belt! You are 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 a satellite in orbit and you in your bicycle when taking a tight curve! In this science activity, you will use marbles and Jell-O® to investigate centripetal force and circular motion. Which way will the marbles go?Materials
- Transparent plastic cups (at least 4)
- Scissors or one-hole puncher
- Flashlight
- Duct tape or electrical tape
- Marbles (at least 3)
- String, 10 inch piece
- Jell-O (2 packages, each a different color)
- Cooking pot
- Measuring cup
- Stove
- Refrigerator
- An open area outside
Prep Work
- You may need 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 touch and stain anything.
Instructions
- Use the scissors or one-hole puncher to make a small hole about 2.5 centimeters (cm) (1 inch) from the top 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 either end of the string through a hole at the cup's top. 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.
- Make one of the packets of Jell-O by dissolving the Jell-O 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 half way full.
- Place these cups in the refrigerator and chill until the Jell-O is completely set, about 2-3 hours.
- When the Jell-O is set, in each cup place a marble on the top of the Jell-O in the center of the cup. Gently press 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 by dissolving the Jell-O in the appropriate amount of water according to the directions on the package. Why do you think the second batch of Jell-O should be a different color from the first?
- Slowly and carefully pour the 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 cm at the top of the cup.
- Place the cups back in the refrigerator until the Jell-O is completely set, about 2-3 hours.
- When the Jell-O is set, put one of the cups with Jell-O into the centripetal force generator cup (the cup with a string tied to it) by stacking the cup with Jell-O into 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 your furniture!
- Hold the string and twirl the stacked cups around your head for 20 revolutions, counting each time the cups make a complete circle
- 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?
- Tips: If you have trouble locating the marble, try backlighting the marble by shining the flashlight through the back of the cup, towards you. 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 Jell-O cups sit out at room temperature overnight to soften up the Jell-O, or do this activity again but make the Jell-O using more water.
- 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?
Cleanup
What Happened?
When an object moves, or accelerates, in a circle, the object wants to move out, away from the circle's center. Without 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.
However, if your Jell-O was too firm you may have seen that the marble did not move. If this is the case it is because the centripetal force from the swinging is not enough to overcome the firmness of the Jell-O. If your marble did not move, try letting the Jell-O warm up to room temperature to soften, swinging harder, or start over with softer Jell-O (by preparing it with more water).
Digging Deeper
Sir Isaac Newton, a careful observer, used mathematics and science to describe natural phenomenon which, at the time, were not understood. Newton's Laws of Motion are still in use today, and these principles can be found in almost any moving technology. Newton discovered that to move an object, it must experience a force that makes it move in a certain direction. The object set in motion will continue this motion until it experiences an opposite force. You have felt this phenomenon when riding in a car. When the car starts moving, you rock backward because your body is not ready for a sudden forward movement. After you are moving, if the car stops you will rock forward because your body wants to keep moving forward.
Newton realized that when an object moves, or accelerates, in a circle, the object wants to move out, away from the circle's center. For example, when you are riding in a car and it makes a turn, your body moves to the outside of the turn, away from the direction of the turn. The force that makes you move in a circular path, even though your body wants to move away from the turn, is called centripetal force.
In this science activity, centripetal force acting on the system — the cup, Jell-O, and marble — is supplied by the tension in the string. This force keeps the cups moving in a circular path. This is a "pull" force, similar to how satellites are kept in orbit around the Earth 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 are making a turn on your bicycle.
Ask an Expert
For Further Exploration
- 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 a centripetal force. Instead of marbles, try lead fishing weights, beans, quarters, beads, etc. Do you notice a difference in motion between objects of different weights or sizes?
- 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?
- There are many ways to investigate Newtonian motion. Try taking some of your cups with Jell-O and marbles in them 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, roller coaster, or anywhere else you are curious to find out about?
