The Center of Mass of a Baseball Bat
A baseball bat will balance on one finger if the finger is placed under its center of mass. In addition, when the baseball bat rotates as it moves through the air it will rotate about its center of mass.
To Do and Notice
Balance a baseball bat between two fingers. Place one finger at the handle of the bat and one at the wide end.
Slowly pull the fingers together. When the fingers meet they will be underneath the center of mass of the bat.
What's Going On?
Every atom in the bat is being attracted to every atom in the earth. However all of these forces add together and can be treated like a single downward force equal to the weight of the bat acting at its center of mass. An upward force applied at the center of mass will balance the bat so that it does not rotate and fall to the ground.
The fingers automatically end up under the center of mass. As you move your hands together one hand will move relative to the bat and one will stay still. In general, as one hand moves closer to the center of mass of the bat it supports more and more of the weight of the bat and so the friction force between that finger and the bat grows while the friction force between the more distant finger and the bat lessens. Eventually the finger nearest the center of mass stops moving and the other finger starts moving. This continues until both fingers meet at the center of mass of the bat.
A bat in free fall will rotate about its center of mass.
Notice how one blinking light traces a parabola while the lights at both ends of the bat rotate about the center of mass.
The center of mass follows a parabola while the ends of the bat sweep out curves.
Scientific Explorations with Paul Doherty
11 February 2007