Materials Included In Kit

Additional Materials Required

Safety Precautions

The materials in this experiment are considered safe. Please follow all normal laboratory safety guidelines.

Disposal

The materials should be saved and stored for future use.

Lab Hints

• This kit contains enough materials for 10 groups of students. All materials are reusable. Two polygon sets are provided for students to share. Place the 10 polygon shapes on a desk or table in a central distribution area. Each group should locate the center of mass of five different polygon shapes.
• Shapes may vary slightly from kit to kit.

Teacher Tips

• Several great center of mass demonstrations available from Flinn Scientific include the Balancing Bottle (Catalog No. AP6376), Fascinating Finger Balance (Catalog No. AP6640), and the Center of Gravity Toss (Catalog No. AP7066).

Sample Data

Answers to Questions

1. Define the center of mass of an object.

   See Background information for an appropriate definition.

2. Describe a test that could be used to verify whether a point on an object was indeed the center of mass.

   If the point is truly the center of mass, the object can be held at this location and remain balanced. Or, the object could be rotated and the object will rotate around the center of mass (i.e., the center of mass will remain “stationary” while the rest of the mass spins around it).

3. Assuming that the objects all have the same uniform density throughout, rank the following objects from most stable to least stable, and explain your reasoning.
   {12509_Answers_Figure_4}

Introduction

Will a force cause an object to spin, move in a straight line or fall over? The answers to these questions depend on the location of the center of gravity of the object.

Concepts

• Center of gravity
• Stability
• Newton’s laws of motion

Background

Gravity is the attractive force between all objects. The most familiar gravitational force is that of the Earth, which pulls all objects toward the ground and is more commonly referred to as an object’s weight. The more massive two objects are, the greater 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. Consequently, the center of gravity is the point where we can assume all of the mass of the object is concentrated, and therefore is also referred to as the center of mass. The location of the center of gravity, especially for irregularly shaped objects, is critical for the overall stability and balance of an object on the Earth’s surface. An object is most stable on the Earth’s surface when the object’s center of gravity is at its lowest point, and is centered about the object’s supporting base.

In general, when a force acts on an object, it can be assumed that the force acts on the center of mass of the object. If a force is specifically applied to an object at a position other than the center of mass (i.e., to the left, right, up or down from the center of mass), then this force will cause the object to rotate about its center of mass.

When an object hangs from one corner, its center of mass will be located directly below the suspended point. This occurs because the object is most stable when its center of mass is at its lowest possible point. By drawing a vertical line through the hanging location, the line will also go through the center of mass. To pinpoint the center of mass, the object can be hung from another location and another vertical line may be drawn. The intersection of the two lines is the center of mass of the object. Hanging and drawing a line from a third location will help verify the location of the center of mass of the object.

Experiment Overview

Locate the center of mass of different polygon shapes.

Materials

Safety Precautions

The materials in this experiment are considered safe. Please follow all normal laboratory safety guidelines.

Procedure

1. Cut a piece of string approximately 30-cm long.
2. Tie a “looping knot” at one end of the string (see Figure 1).
   {12509_Procedure_Figure_1}
3. Tie the other end of the string around the washer.
4. Obtain one of the five polygon shapes from the distribution area designated by the instructor. Notice that there are holes in the corners of the polygon.
5. Set up a support stand with a clamp and two S-hooks. Hang the shape on the S-hook from one of its holes as shown in Figure 2.
   {12509_Procedure_Figure_2}
6. Hang the string from the S-hook. Make sure the support stand clamp is high enough to allow the string and polygon shape to hang freely.
7. Once the string and shape have stopped swinging, hold the string at the bottom of the polygon with one hand, and with your other hand use a dry erase marker to draw a straight line on the hanging polygon shape to mark the path of the vertically hanging string.
8. Repeat steps 5–7 two more times, hanging the shape from two different corners (holes) and drawing lines.
9. Sketch the lines drawn on the polygon shape on the corresponding shape in the data table.
10. Erase the marker lines on the polygon shape with a paper towel and return the shape to the distribution area.
11. Repeat steps 5–10 for the four remaining polygon shapes.
12. Consult instructor for proper storage procedures.

Student Worksheet PDF

12509_Student1.pdf