# Centripetal Force

 Difficulty Time Required Short (2-5 days) Prerequisites None Material Availability Readily available Cost Very Low (under \$20) Safety Adult supervision is required for making the JELL-O. Be careful when working with the boiling water.

## Abstract

What keeps you in your seat of a giant loop-de-loop roller coaster? Surprisingly, it is not the seatbelt but the seat! It works because of something called centripetal force and it does much more than make a great roller coaster. It keeps a satellite in orbit and you in your bicycle seat during a turn. How does it work?

## Objective

Investigate the movement of an object during circular motion and determine what the centripetal force is.

## Credits

Sara Agee, Ph.D., Science Buddies
Teisha Rowland, Ph.D., Science Buddies

### MLA Style

Science Buddies Staff. "Centripetal Force" Science Buddies. Science Buddies, 18 Dec. 2014. Web. 26 Aug. 2016 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p018.shtml?from=Blog>

### APA Style

Science Buddies Staff. (2014, December 18). Centripetal Force. Retrieved August 26, 2016 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p018.shtml?from=Blog

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Last edit date: 2014-12-18

## Introduction

You have probably heard the famous story about Sir Isaac Newton and the discovery of gravity where an apple fell on his head while he was sitting beneath an apple tree. Whether or not this story is true, Newton was a careful observer of the world around him. His genius was to use mathematics and science to describe natural phenomenon which, at the time, were not understood.

Newton made many discoveries: the laws of gravity, colors, prisms, advanced mathematics and motion. Newton's Three Laws of Motion are still in use today, and these principles can be found in almost any modern moving technology. Newton's First Law of Motion is often stated as "objects at rest stay at rest, and objects in motion stay in motion." This simply means that once you are still, it is hard to get moving, and once you are moving, it is hard to stay still.

Newton discovered that to get an object to move, the object must experience a force that makes it move in a certain direction. Once the object experiences this force, it is set in motion and will continue this motion until it experiences an opposite force that causes the motion to stop. 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. But after you are moving, if the car suddenly stops you will rock forward because your body wants to keep moving forward at the same speed and direction.

This example describes what happens when you experience motion in a straight line, but what about other types of movements? Newton realized that when things move in a circle that the object wants to move out, away from the center of the circle. For example, when you are riding in a car and it makes a turn, you experience this circular motion as your body moves to the outside of the turn, away from the direction of the turn.

So if your body wants to move away from the center of the circle, what keeps you moving in a circular path? This force is what Newton described as centripetal force, or a force that makes an object move, or accelerate, towards the center of a circle. Without centripetal force the object would move in a line. In the car example, the weight of the car, gravity, and the friction of the road keep you in your seat moving in a circle.

Now back to the loop-de-loop rollercoaster. The tracks are moving in a circle, and we move along with it, but what is keeping us in our seat? Gravity is a force that pushes us down toward the ground, which may help us stay in our seat at the bottom of the loop, but it probably does not help us stay in our seat at the top of the loop! The force from our seat belt may help us stay in our seat, but just mostly at the top of the loop. Centripetal force is a force that must be constantly pulling us towards the center of the loop, not just at the bottom or top of the loop. What other forces are acting on us while we move in the loop? Could it be the force of the seat itself that is holding us in our seat? Which force is the centripetal force when we ride in the loop-de-loop?

In this physics science project, you will use plastic cups, marbles, and two different colors of JELL-O® to investigate the movement of an object during circular motion and determine what the centripetal force is that keeps the object moving in a circular way. Which way will the marble go?

## Terms and Concepts

• Sir Isaac Newton
• Motion
• Newton's First Law of Motion
• Force
• Circular motion
• Centripetal force
• Gravity

### Questions

• What is force? What are some forces you encounter every day?
• If you had a ball tied to a rope and swung the rope around your head in a circle, making the ball swing in a circle, what would happen to the ball if you suddenly let go of the rope? In what direction do you think the ball would go?
• In the example of the ball tied to a rope, what force keeps the ball from flying away from you when you are spinning it above your head?

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## Materials and Equipment

• Stove
• Cooking pot
• Measuring cup
• Tap water, normal temperature and cold
• JELL-O packages (2). Use two very different colors of JELL-O, both light-colored (like cherry and lime).
• Stirrer
• Plastic cups, clear (7)
• Permanent marker
• Refrigerator
• Timer or clock
• Marbles (6)
• Oven mitt
• Scissors or one-hole puncher
• String (at least 30 cm long)
• Electrical tape or duct tape
• Optional: Flashlight
• Lab notebook

## Experimental Procedure

### Making the Centripetal Force Test Chambers

In this part of the science project, you will make 6 centripetal force test chambers using JELL-O, cups, and marbles. You will want to make more than one to have replicates, or copies, of your data.
1. Make a package of JELL-O by following the instructions on the box, except use more cold water than is recommended.
1. Making JELL-O using more water will make it less thick and liquidier. This will make it easier for the marble to move when you test it later.
2. On the stove in the cooking pot, have an adult help you boil the appropriate amount of water according to the instructions on the box.
3. Continue following the box's instructions by adding the JELL-O mix to the boiling water. Stir the JELL-O mix in the water for 2 minutes while keeping the pot on the hot burner.
4. After stirring for 2 minutes, the JELL-O should be completely dissolved and no particles of JELL-O powder should be visible. It should look clear. The JELL-O should also barely start boiling again.
5. Take the JELL-O off of the hot burner and add 1.5 times the amount of cold water that is recommended on the box's instructions. Use cold tap water and stir it in as it is added.
1. For example, if the instructions say to use 1 cup of cold water, use 1.5 cups of cold water instead.
2. Pour the JELL-O into six of the plastic cups, filling each cup a little less than halfway full. These will be your centripetal force test chambers.
1. Use a permanent marker to label these cups #1-6.
3. Place the cups in the refrigerator and refrigerate them for 4 hours.
1. Because you used more water than is recommended in the box's instructions, the JELL-O will not be very firm after 4 hours, but it should be firm enough to support the marble in the next step.
4. 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?
5. Make the second batch of JELL-O by preparing it as you did in step 1. Why do you think the second batch should be a different color from the first?
6. After adding the cold tap water to the JELL-O, place an oven mitt in the refrigerator and have an adult help you place the pot with the JELL-O on the oven mitt. Refrigerate the pot of JELL-O for 30 minutes.
1. This second batch of JELL-O will be added to the JELL-O already in the plastic cups. However, if the second batch of JELL-O is not refrigerated before it is added, it may be too hot and melt the first layer of JELL-O.
7. After refrigerating the pot of JELL-O, slowly and carefully pour the JELL-O into the cups, covering the first layer of JELL-O and the marble, until the cups are almost full. Leave about 2.5 centimeters (cm) (1 inch) at the top of the cups.
1. It is important to slowly and carefully pour the second layer of JELL-O on top of the first layer because otherwise the second layer may damage the first layer.
2. Do not worry if the two layers of JELL-O mix together a little where they meet. However, check to make sure that the marble stays roughly in the middle, between the two layers. Do not use any cups in which the marble is not near the middle.
1. If multiple cups have marbles that are not near the middle, try repeating steps 1-7 but use less cold water in step 1e, such as 1.25 cups instead of 1.5 cups.
3. Your cups should look similar to the one in Figure 1, although your layers of JELL-O may be different colors.

Figure 1. Once you have poured two layers of JELL-O into your cups, they should look similar to this one, with the marble in between the two layers.

1. Place the cups in the refrigerator and refrigerate them for 4 hours.
2. While your JELL-O solidifies, prepare your centripetal force generator. You will be testing your test chambers inside of the generator.
1. Take an empty plastic cup and use the scissors or one-hole puncher to make a small hole about 2.5 cm (1 inch) from the top rim of the cup. Make a second hole on the opposite side of the cup.
2. Put a small piece of electrical tape or duct 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, which you will attach next, from detaching.
3. Attach the string to the top of the cup, tying one end of the string through one of the holes at the cup's top and the other end through the other hole.
4. Your cup should now look like the one in Figure 2. 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.

Figure 2. Prepare your centripetal force generator cup, as shown here, by making two holes in the top of the cup, placing electrical or duct tape above the holes, and tying string through the holes.

### Testing the Centripetal Force Test Chambers

1. After refrigerating the cups, take one of your test chambers and place it in the centripetal force generator by stacking the plastic cup with the JELL-O into the plastic cup with the string.
1. Do not use any cups in which the marble is not near the middle. If multiple cups have marbles that are not near the middle, try repeating the section titled "Making the Centripetal Force Test Chambers" but this time use less cold water when making the JELL-O in steps 1e and 5, such as 1.25 cups instead of 1.5 cups.
2. Take the stacked cups outside to an open area.
3. Now hold the string and quickly twirl the cup around your head for 20 revolutions, counting each time the cup makes a complete circle, making as wide a circle as you feel comfortable with. You must spin the cup hard and fast to get enough centripetal force for the marble to move!
4. After 20 revolutions, stop spinning and remove the inner cup from the outer cup.
5. Observe the JELL-O, and the position of the marble relative to the two different colors of JELL-O. Remember that the marble started out right at the dividing line between the two colors. Where did the marble move to? Make observations, drawings, and record data in a data table in your lab notebook similar to Table 1.
1. If you have trouble locating the marble, try backlighting the marble by shining a flashlight through the back of the cup, toward you.

 Test Chamber Who swung it? Where did the marble move to? Was the test chamber spun more than once to move the marble? Other Observations #1 Student #2 Student #3 Student #4 Adult helper #5 Adult helper #6 Adult helper

Table 1. In your lab notebook, make a table like this one to record your observations and data.
2. 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.
3. If the marble has still not moved, either let the cups sit out at room temperature overnight to soften the JELL-O or repeat the section titled "Making the Centripetal Force Test Chambers" but this time use more cold water when making the JELL-O in steps 1e and 5.
6. Repeat steps 1-5 with two other cups. Then have your adult helper repeat steps 1-5 with a total of three cups.
1. If your adult helper spins the cup harder than you did, how do you think this will affect how the marble travels?
7. Did 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? Did they move differently when you spun the cups compared to when your adult helper spun them?
1. Based on where the marbles went during circular motion, what do you think is the centripetal force that keeps the marbles moving in a circular way, and prevents them from flying off in a straight line?

## Variations

• One of Newton's other laws says that there is a relationship between the motion of an object and its mass. Try a similar science project 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 experience Newtonian motion. Try taking some of your pre-made test chambers with you for some on-site experimentation. What happens to the marble when you take it on a swing, slide, merry-go-round, rollercoaster, car ride, bike ride, rollercoaster, or anywhere else you are curious to find out about?

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