Fascinating Finger Balance

Introduction

What is the purpose of the long pole a tightrope walker uses when walking across the rope? Demonstrate the concept of center of gravity by supporting a device on your fingertip with the help of a heavy tube dangled from the end. Sound like a paradox? It’s a matter of balance!

Concepts

• Center of gravity
• Balance
• Stability

Materials

Safety Precautions

Follow normal laboratory safety guidelines.

Procedure

1. Obtain the acrylic balance apparatus and plastic tubing.
2. Show the balancing apparatus to your students and attempt to balance it on the tip of your finger. (It will not balance.)
3. Explain to your students that by adding a heavy object to the end of the balancing apparatus, the apparatus will now balance on your fingertip. (Students may ask how this is possible. How will adding a heavy object to the end of the apparatus will make it balance on your finger? Won’t it topple over even easier?)
4. Insert the midpoint of the plastic tubing into the slot of the balancing apparatus. Bend or flex the tubing if necessary so that the ends of the tubing hang at the same level, as shown in Figure 1.
   {13970_Procedure_Figure_1}
5. Twist the plastic tubing about a quarter turn towards the tip of the balancing apparatus. This should force the end of the tubing to extend back towards the tip of the balancing apparatus with the tubing arms curving down (see Figure 2). The friction between the tubing and the acrylic slot will keep the tubing in this twisted position.
   {13970_Procedure_Figure_2}
6. Place the tip of the balancing apparatus on a fingertip or the corner of a tabletop.
7. Release the balancing apparatus and allow it to hang as if it is suspended in midair.
8. Discuss the results with students. Note: If the tubing is not angled toward the tip of the balancing apparatus, the balancing apparatus will not balance because the center of gravity is not located below your finger. When discussing center of gravity, show the case in which the tubing is not twisted towards the tip (so that the apparatus does not balance with the hanging tube). This may help to reinforce the concept of center of gravity and why the system was balanced in the first case.

Teacher Tips

• A rigid leather belt also works well in place of the plastic tubing. The belt needs to fit snuggly in the balance apparatus slot so that the leather strap angles back towards the tip. If the belt is too loose in the slot, it may only hang straight down and the apparatus will not balance because the center of gravity will not be under your finger. Adjust the position of the belt buckle so the belt’s mass is evenly distributed and the apparatus balances and remains upright.
• Remove the protective paper covering from both sides of the acrylic balance apparatus before using.
• The Hanging Nails Challenge Guided-Inquiry Kit (Flinn Catalog No. AP7309) is a great hands-on student lab for exploring center of gravity and equilibrium.

Discussion

Gravity is the attractive force between all objects. The most familiar gravitational force is that of the Earth’s, which pulls all objects down and is more commonly referred to as an object’s weight. The more massive the objects are, the more gravitational force that exists between them.

According to Isaac Newton’s (1642–1727) laws of gravitation, the Earth attracts every tiny particle of mass of every object and pulls them toward the center of the Earth. For any specific object (composed of many tiny particles), the center of gravity of the object is the location where all the individual gravitational forces acting on the individual particles add up and result in one net downward force. It is the point where we can assume all of the mass of the object is concentrated. The location of the center of gravity is critical for the overall stability and balance of an object on the Earth’s surface. For irregularly shaped objects or moving objects, the center of gravity at a particular moment may be crucial for balance and stability. As an object’s mass shifts, so does its center of mass.

In order for an object to remain stable and balanced, its center of mass must be located directly above or below its supporting base. By adding mass to the balance apparatus in the form of a curved tube that extends beyond the tip as shown in Figure 3b, the center of gravity of the two-object system (balance apparatus and tube) has now shifted to a location below the tip supported by the finger. The center of gravity is below the point of support and therefore the two-object system remains balanced.