Centripetal Force

Abstract

Objective

To test whether centripetal forces act by pushing an object towards or away from the center of a circular motion.

Credits

Sara Agee, Ph.D., Science Buddies

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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 3 Laws of Motion are still in use today, and these principles can be found in almost any modern moving technology. One of the most important concepts of Newton's Laws is that "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 your parent starts to drive, you rock backward because your body isn't ready for a sudden forward movement. But after you are moving, if your parent needs to slam on the breaks you will rock forward because your body wants to keep moving forward in the same 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. In our car example, you experienced this circular motion when your parent turns a corner. 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 turn, then what makes you move in a circular path? This is what Newton described as a 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? We know that gravity is a force pushing us down toward the ground. If this is the force, then our seat belt is holding us in our seat. But we also know that we are in motion towards the outside of the circle because of our weight. If this is the force, called momentum, then it is our seat holding us in our seat. Which force is it?

In this experiment, you will use paper cups, marbles and two different colors of Jello to see the movement of an object during circular motion. Which way will the marble go?

Terms and Concepts

To do this type of experiment you should know what the following terms mean. Have an adult help you search the internet, or take you to your local library to find out more!

• Sir Isaac Newton
• force
• motion
• gravity
• circular motion
• mass
• weight

Bibliography

• Staff. 2005. "Naturally Newton: Experiments to try at home." At-Bristol, Harbourside Bristol, UK. 12/29/05. http://www.at-bristol.org.uk/Newton/experiment.htm
• Staff. 2005. "Naturally Newton: Sir Isaac Newton." At-Bristol, Harbourside Bristol, UK. 12/29/05. http://www.at-bristol.org.uk/Newton/Biog.htm
• Cain, Jeanette. 2005. Newton, an Apple and You." Light Science.com. 12/29/05. http://www.light-science.com/newtonapple.html

Materials and Equipment

• paper cups
• string
• marbles
• two very different colors of Jello (like cherry and lime)
• butter knife
• paper plate

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Experimental Procedure

1. First, you will need to make several centripetal force test chambers using Jello, cups, and marbles. You will want to make more than one to have replicates, or copies, of your data. Also, you should start by making the Jello according to the instructions on the box. If your marbles will not move, then try the experiment again, making the Jello using more water so it will be less viscous.
2. Make one of your packets of Jello by dissolving the Jello packet in the appropriate amount of water according to the directions on the package.
3. Pour the Jello into the paper cups, filling each cup half way full.
4. Place cups in the refrigerator and chill until the Jello is completely set, about 2–3 hours.
5. In each cup, place a marble on the top of the Jello in the center of the cup. Gently press into the Jello just until the marble is secure and will not move around.
6. Make the second batch of Jello by dissolving the Jello packet in the appropriate amount of water according to the directions on the package.
7. Pour the Jello into the cups, covering the first layer of Jello and the marble, until the cup is almost full leaving about one inch at the top of the cup.
8. Place the cups in the refrigerator until the Jello is completely set, about 2–3 hours.
9. In the meantime, prepare your centripetal force generator.
10. Take an empty cup and staple the piece of string to the top of the cup, with each attachment point on opposite sides of the cup. Staple each side 2–4 times to make it secure.
11. When your Jello is set, take one of your test chambers and place it in the centripetal force generator by stacking the paper cup with the Jello into the other paper cup with the string.
12. Now hold the string and twirl the cup around your head for 20 revolutions, counting each time the cup makes a complete circle.
13. After 20 revolutions, stop spinning and remove the inner cup from the outer cup.
14. Slowly and carefully, flip the cup over onto a paper plate. Try to remove the cup by lifting the cup and un-molding the Jello. You may need to carefully tear the cup open to remove it without breaking the Jello.
15. Observe the Jello, and the position of the marble relative to the two different colors of Jello. 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 table.
16. Repeat with the other cups, collecting results from 3–4 more experiments.
17. 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?

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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 experiment with small objects of different weights to see of 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 objects 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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Related Links

• Science Fair Project Guide
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