Materials Included In Kit

Additional Materials Required

Safety Precautions

Although this activity is considered nonhazardous, please follow normal laboratory safety guidelines.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The materials in this kit may be saved for future use. Collect the modeling clay and store in a zipper-lock bag to prevent it from drying out. The modeling clay may be disposed of according to Flinn Suggested Disposal Method #26a.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Each experiment can be completed in a 50-minute class period.
• The two experiments that can be performed with the materials in this kit are not actually related. One experiment is more of an inquiry-based experiment constructing a rattleback bowl which shows concepts of friction, rotational motion and conservation of energy. The second experiment investigates surface tension. These labs can be performed on different days or even at different times of the year depending on your curriculum.
• Strike the rattleback on the thin side and watch it oscillate and then begin to rotate in its “natural” direction. Strike the rattleback on a fat side and watch it oscillate and begin to rotate in the opposite direction. Show this phenomenon to the students and have them hypothesize an explanation.

Teacher Tips

• To save time, break off thumb-sized pieces of clay from the sticks—enough for each lab group.
• Designing the rattleback bowl is a great activity to start the year. Students will learn the basics of experimentation and the scientific method. The additional Discussion may be provided for students who want to know the science behind the rattleback.
• Demonstrate the “real” rattleback in front of the class before students begin the experiment. This will give students an idea of how the “best” rattleback should perform.
• Challenge students to vary the mass along the edges, as well as varying the position of the mass as they design their “best” rattleback.
• The density of PVC is between 1.35–1.45 g/cm³.

Sample Data

Answers to Questions

Investigating the Rattleback

1. According to your observations, what conditions are necessary for the rattleback to switch rotational directions? Is an initial wobble necessary for the rattleback to reverse directions?

   The clay must be positioned off-center and on opposite sides for the rattleback to reverse directions. It works best when the rattleback is balanced. When no clay is on the bowl, it does not reverse directions.
   
   The rattleback, when properly made, will reverse its direction whether it is initially spun with no wobble, or if it does have a small wobble as it spins. It does not require an initial wobble in order to reverse directions.

2. Describe the motion of the rattleback as it changes direction.

   After the initial rotation, the rattleback begins to wobble up and down. As this occurs, the rotation slows down and eventually stops completely as the wobbling reaches a maximum height. Then, the wobbling decreases and the rattleback begins to spin again, but in the opposite direction. The spin is slower than the initial spin but it increases slightly as the wobbling stops. Eventually the rattleback no longer wobbles and spins smoothly in the opposite direction until it stops.

3. What conditions were required for the “best” rattleback?

   The best rattleback was the result of the clay being positioned at about 15 degrees away from the long center-line of the rattleback. A smooth initial spin also helped set up a strong wobble and the rattleback continued to spin for several seconds after it reversed directions.

4. From your experiments and observations, develop a hypothesis to explain the bowl’s change in rotational direction.

   Student responses will vary. There is no right or wrong answer when describing the mysterious rattleback.
   
   The off-center clay changes the balance of the bowl. The football-shape of the bowl also causes the bowl to spin differently compared to a bowl that is completely spherical. The combination of the offset balance and the football shape cause the bowl to oppose the rotation of motion. Instead of rotating, the bowl begins to wobble up and down and eventually the bowl stops rotating. The offset balance and football shape then transform the wobble into a rotational spin in the “natural” direction. Eventually the wobble stops and the rattleback spins in its natural direction.
   
   When the rattleback is rotated in its “natural” direction, it does not begin to wobble and continues to spin in this direction because the offset balance prefers the rotation over the wobbling when it is spun in this direction.

5. Would the same behavior occur with a completely spherical bowl? Why or why not?

   No, a completely spherical bowl has no center line and would always be balanced. Therefore, positioning the clay around the outside rim would not create a wobble and would not send the bowl in the opposite direction.

Investigating Surface Tension

1. Define the term surface tension.

   Surface tension in water creates a “skin” or “membrane” at the surface of the water that appears to stretch before it breaks.
   
   Surface tension is the result of the strong cohesive forces of water molecules below the surface and the lack of cohesive forces above the surface. The cohesive force attracts water molecules to other water molecules.

2. How did the tap water in the beaker react as more weight (water) was added to the bowl?

   As water was added to the bowl, the bowl sank further below the surface of the water in the beaker. However, the water “skin” did not break and bent along the rim of the bowl. The water in the beaker (at the surface) continued to bend as more weight (water) was added. Enough weight was added to cause the bowl to sink below the surface of the water, but the water “skin” (surface tension) did not break. Eventually, enough water was added to the bowl that put too much stress on the surface tension and caused the skin to break and the bowl to sink to the bottom of the beaker.

3. How did the soapy water in the beaker react as weight was added to the bowl?

   The surface tension of the soapy water supported a small amount of weight that was added to the boat. The surface of the soapy water did not bend much. After 4 mL of water was added to the bowl, the bowl sank.

4. Which liquid, tap water or soapy water, has stronger surface tension? Explain how surface tension helps to support the bowl.

   The surface tension of the tap water held considerably more than the surface tension of the soapy water. The soapy water supported less than half of the weight compared to the tap water.
   
   Surface tension helped support the bowl by creating a skin on the surface of the water that required a strong enough force to break. When the stress on the skin was high enough after too much weight (water) was added to the bowl, the surface tension broke and the bowl sank. Soap interfered with the cohesive forces of the water molecules and reduced the surface tension. The surface tension of the soapy water broke much more easily than the surface tension of the tap water.

Discussion

A celt’s mysterious behavior is the result of frictional forces acting on a complex, non-uniform surface. The mathematics of the behavior of a celt are complicated and still not fully understood. However, it is known that a celt must have three characteristics for it to perform its unusual act.

First, the curved bottom of the celt cannot be perfectly spherical. That is, the bottom must have two different radii of curvature. For example, a spoon has a long radius of curvature along its length and a shorter radius of curvature along its width. This produces a shape known as an ellipsoid (a three-dimensional ellipse).

The second characteristic of a celt is that the mass distribution along the two horizontal principal axes of inertia must be different. Since the plastic rattleback bowl has uniformly distributed mass, there is more mass in a section running along the length of the bowl than a section across the width of the bowl. Therefore, the plastic bowl satisfies the second characteristic of a celt.

The final characteristic necessary to produce celt-like rotation is that the principal axes of inertia must be slightly skewed from the axes of symmetry of the celt’s curved base (see Figure 6). This characteristic is created when the clay is positioned slightly off-axis from the center of the bowl. Clay placed directly across from each other keeps the rattleback balanced. This affects the bowl’s final moment of inertia, or its effective resistance to a change in its state of rotation.

A rotating celt is an excellent example of how different types of energy can be transformed from one type to another. Kinetic energy is the energy of motion. The celt experiences two types of kinetic energy—rotational kinetic energy and vibrational kinetic energy. When the celt is spun it has rotational kinetic energy. Frictional forces act on the ellipsoid bottom of the celt, and act against the direction of the spin (producing what is known as a torque on the spinning celt). On a perfectly spherical bottom, the frictional forces act antiparallel to (directly against) the spin in the horizontal direction. However, because of the ellipsoid shape and asymmetric mass distribution, the frictional forces will not always act in the antiparallel direction to the spin. The forces may be skewed slightly, thus allowing part of the frictional forces to act in other directions perpendicular to the spin, including the vertical direction. Therefore, if a celt is spun in the appropriate direction, the frictional forces transform the initial rotational energy into vibrational energy. The celt stops rotating but continues to vibrate up and down along the length of the celt. The frictional forces continue to act on the irregularly shaped bottom and eventually turn the vibrational up and down motion back into rotational motion—in the opposite direction to the initial spin. The celt will continue to spin in this “natural” rotation until friction causes it to stop completely.

Introduction

Investigate the unique object known as the rattleback. Experiment with the behavior of the rattleback to determine the conditions that are necessary for the rattleback to perform its unusual feat. Then develop a hypothesis that explains the rattleback’s behavior. The second experiment uses the rattleback bowl to investigate the surface tension of water.

Concepts

• Scientific method
• Rattleback motion
• Developing a hypothesis
• Surface tension
• Cohesion

Background

The rattleback, also known as a celt, is an object that has a “natural” rotational direction. When the rattleback is spun opposite to this natural rotation, the rattleback will begin to wobble and will eventually reverse its rotation to the natural direction. The science behind the rattleback is complex, but there are several features and conditions that can be tested to design the best rattleback. Experiments will be performed to test these conditions during the first part of this activity.

In the second investigation, the rattleback bowl will be used to study the surface tension of water. Water has an unusually strong surface tension because of its strongly polar, hydrogen-bonding water molecules. Surface tension develops because the molecules at the interface between the water and air are attracted to the water molecules below them, but not to the air molecules above (see Figure 1). The attraction of like molecules toward each other is known as cohesion. Since there are no water molecules above the surface to counteract the cohesive attraction from below, the water molecules at the surface attract each other with a stronger net force. The unbalanced forces on the surface water molecules form somewhat of an elastic “skin” on the surface of the water. The cohesive strength of water is proportional to the amount of surface area.

Soaps are effective cleaners because their chemical structure helps to break down the cohesiveness of the water molecules, and thereby reduces the surface tension. The reduced surface tension allows the soapy water to “wet” materials, such as fabric, more effectively. The “wetting” condition allows the soapy water to get into the small spaces between fabric fibers which gives the soapy water the ability to wash away dirt and soil found there.

Materials

Safety Precautions

Although this activity is considered nonhazardous, please follow normal safety precautions as directed by your instructor.

Procedure

Experiment 1. Investigating the Rattleback

1. Obtain the plastic rattleback bowl and a thumb-size piece of modeling clay.
2. Place the bowl on the tabletop and give it a gentle spin. Does it continue to spin in the same direction? Record initial observations in Data Table 1.
3. Stop the bowl and then rotate it in the opposite direction. Does it continue to spin freely in the same direction? Record initial observations in Data Table 1.
4. Break the thumb-size piece of clay into two equal portions.
5. Place each piece of clay on the edge of the bowl as shown in Figure 2. Place the clay near the “pointed” ends of the bowl. Press the clay on the edge of the bowl slightly so that it is secure and will not fall off easily.
   {13078_Procedure_Figure_2_Clay position 1}
6. Repeat step 2. How does the bowl behave now? Does the bowl’s rotation change directions? Record observations in Data Table 1.
7. Rotate the bowl in the opposite direction. Does the bowl appear to have a preferred rotational direction? Record observations in Data Table 1.
8. Move the clay along the edge of the bowl towards the center line as shown in Figure 3.
   {13078_Procedure_Figure_3_Clay position 2}
9. Repeat steps 6 and 7. Does the bowl behave the same as before? Record observations in Data Table 1. Draw a figure to indicate the position of the clay on the edge of the bowl.
10. Place each piece of clay on the same side as shown in Figure 4.
    {13078_Procedure_Figure_4_Clay position 3}
11. Repeat steps 6 and 7. How does the bowl behave? Record observations in Data Table 1.
12. Experiment with the position of the clay to develop the “best” rattleback bowl—that is a bowl with the most “influential” natural spin direction. Record the position of the clay and any observations that helped determine the rattleback’s quality, such as how quickly the bowl reverses directions, and how long it continues to spin, in Data Table 1.
13. After developing the “best” rattleback bowl, perform further experiments. For example, perform tests on the effects of the initial spin. How does the rattleback respond to different pushes—for example, is a harder push better? How does the rattleback respond with a smooth push that has no initial wobbling?
14. (Optional) Experiment with the “best” rattleback bowl on a smooth glass plate. Does it behave the same?
15. After completing the experiment, collect the clay and store it according to your instructor’s direction. Then, clean off the plastic rattleback bowl with soap and water, and dry it with a paper towel.

Experiment 2. Investigating Surface Tension

1. Obtain two 400-mL beakers, a graduated Beral-type pipet and a clean, dry rattleback bowl.
2. Fill each beaker with approximately 300 mL of water.
3. Place the bowl into one of the water-filled beakers so that it floats in the center (see Figure 5).
   {13078_Procedure_Figure_5}
4. Use the graduated Beral-type pipet to fill the rattleback bowl with water. Fill the pipet to the 1-mL graduation mark and then add the water to the bowl. Maintain a continuous record of the amount of water that is added to the bowl.
5. As the water begins to fill the bowl and a water bulge rises above the lip of the bowl, carefully add the water dropwise.
6. Observe the properties of the water. Does the bowl appear to be under the waterline? How does the surface of the water look? Record observations in Data Table 2.
7. Continue to carefully add water dropwise to fill the bowl until it sinks. Record the total amount of water added to the bowl in Data Table 2.
8. Remove the bowl from the beaker and dry it.
9. Add 1–2 drops of dish soap to one of the water-filled beakers.
10. With a stirring rod, slowly stir the soapy water until it is well mixed. Do not stir vigorously to prevent the formation of excessive soap suds on the surface.
11. Carefully place the rattleback bowl on the surface of the soapy water.
12. Repeat steps 4–7. Record observations and the total volume of water added to the bowl in Data Table 2.
13. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

13078_Student1.pdf