FINDING THE CENTER OF MASS
OF A BASEBALL BAT

Time: 30 minutes

Grade Level: 6th-12th

Objective

Students will learn how to find the center of mass (center of gravity) of an object. This is the point where the object's mass distribution is centered. For most places on Earth these terms are interchangeable. For baseballs and bats, the center of mass is within the ball or bat, but some things will have a center of mass outside the object.

Materials Necessary

• Clay or Play Dough
• Pencil
• Ruler
• Meter Stick
• Assortment of Baseball Bats
• Baseball or other Ball
• Hewitt, Paul "Conceptual Physics" chapter "Center of Gravity"
• General Physical Science Books

Teacher Background

Objects balance about a center of mass (center of gravity). You can determine an object’s center of mass very easily. A regular sphere (ball) will rotate around its center of mass (which is the geometric center). Throw a ball and watch the motion. This motion is a parabolic path. The motion is smooth as the ball moves about its center of mass. Throw a bat. The motion is not smooth, because the motion is about its center of mass which is not in the center of the bat.

Procedure

1. Support a meter stick by resting each of its ends (at 10 cm and 90 cm) on a finger.
2. Slowly slide your fingers together toward the center until they meet.
3. Your fingers will meet under the stick’s center of mass. This is at _________cm.
4. Attach a piece of clay to the stick at any place.
5. Again support the stick on two fingers, and then slide your fingers together to locate the new center of mass.
6. Move the piece of clay to anther place on the stick.
7. Again support the stick on two fingers, and then slide your fingers together to locate the new center of mass.
8. Repeat this 2 more times.
9. Complete the Data Table with the position of the clay and the center of mass of the object.
10. After you have completed the steps above, get a baseball bat and find its center of mass using your fingers. Don’t attach any clay to the bat. Find the length of the bat and then measure its center of mass from the handle end of the bat. Record your data in the table.
11. Try several kinds of bats. Are they all alike? Why? If they are different, how can you tell? Where are the center of mass of each?

Data Table

• Regardless of where you place the clay, or just bring your fingers together, they will always meet at _________ __ _____ of the object.
• Explain what center of mass means.
• The meter stick and bat are balanced at the center of mass. What does balance mean?