Center of gravity and center of mass are synonymous terms used to describe an object’s point of equilibrium. Use these “magic” bottles to introduce the topic of center of mass or improve your students’ understanding of the concept of center of mass.
- Center of gravity
- Center of mass
- Discrepant event
Center of mass and center of gravity are interchangeable terms in uniform gravity. It is the point at which all the mass is considered to be concentrated when analyzing an object's motion, the point about which gravity is considered to act. It is, in essence, the average location of all the mass in the object. For example, when one places a pencil under each end of a ruler and tries to push the pencils together, the point at which they meet is the center of mass of the ruler.
In this activity, students will discover that the center of mass of the bottle changes when a counterweight is inserted in the neck of the bottle. The bottle, without the counterweight, has a low center of mass; therefore, it will not lie down on its side. When the counterweight is added, the center of mass moves much higher in the bottle, causing the bottle to remain in place when it is placed on its side.
Center of mass bottles, 2
Counterweight rods, 2
Although the materials in this demonstration are considered non-hazardous, please observe all normal laboratory safety guidelines.
Center of Mass Bottles may be reused many times and should be stored for future use.
- Place a metal counterweight rod in each of the center of mass bottles.
- Practice slipping the counterweight out of the bottle in a mirror to ensure you do not drop it and students do not see what you are doing (see Figure 1).
- Set aside.
- With the metal counterweight rod in the bottle, show the students that the bottle will sit on its base as well as on its side (see Figure 2).
- Hand one of the bottles to a student. As you hand it to the student, hold the bottle by the neck and tilt the base up so the counterweight slips into your hand (see Figure 1 in the Prelab Preparation section).
- Ask the student to lay the bottle down on its side.
- After many failed attempts by the student, show the other bottle.
- Repeat steps 1–3 with the second bottle. As the student is trying to get the second bottle to lie on its side, take the first bottle and secretly reinsert the counterweight.
- Perform the task again with the first bottle and have students suggest possible reasons why you are able to make the bottle lie on its side and the student cannot.
- Show the students the counterweight and begin a discussion on the effect the counterweight has on shifting the center of mass of the bottle.
- Answer only yes or no to questions as the students try to understand the science behind the “magic” of the Center of Mass Bottles.
- If you want to give students clues, shake the bottle with and without the counterweight in place so the students will hear the weight in the bottom of the bottle rattling around.
- When the bottle is placed on its side, take care to lay it down gingerly to avoid allowing the counterweight to slip out.
- Handle the bottles carefully. Dropping the bottles may cause them to break.