Center of Gravity
Teacher Toolbox
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The Purpose of this Reading
This STEM Lab focuses on exploring the center of gravity (CoG) by manipulating the arm of the Clawbot to shift the position of the weight on the robot - affecting its CoG - as it drives over uneven surfaces. This first reading is intended to introduce the concept of the center of gravity by exploring how the Leaning Tower of Pisa can be tilted without falling over.
Considering the Center of Gravity
Every object has a center of gravity (CoG). It is the location on the object where gravity pulling on any one side is balanced by gravity pulling on the opposite side. Another way to think of it is the average location of the weight of an object. If the CoG stays above the base of the object (the part resting on the ground), the object will not fall over.
The picture shows the famous Leaning Tower of Pisa. This tower is known for having a very visible tilt, about 4 degrees from being perfectly vertical (straight up and down). You can compare the arrow in the picture to the columns on the tower to see how tilted it is. Although the tower is so tilted, the CoG is still above the base of the tower and so, the tower stays standing! If you further pushed the Leaning Tower in the above picture so it tilted more and more until the arrow passed the base of the tower, then it would fall over.
You can also think about a seesaw or a scale. The fulcrum, the point on the seesaw where the board rests, is directly under the seesaw's CoG. If it weren't, the seesaw would fall to one side instead of teetering up and down.
Extend Your Learning
To expand this activity, ask your students to balance a pencil on their index fingers. Tell your students to turn their hands palm-upward and balance a pencil perpendicularly to their index fingers. If/when students succeed in having it balance, they have found the pencil's CoG.
Students can also observe the center of gravity of an object such as a cup by taping a string to the center of the object and attaching weight to the bottom of the string using an eraser or other slightly weighted object. Place the cup on a book or desk and tilt it slightly. Ask the students to observe how the string moves in relation to the base of the object.
Have the students observe where the string is when the object begins to fall over. The students should notice that when the string passes the base of the object, the object begins to fall.