If your students are anything like mine, they lug every single textbook and notebook they need for the school day with them to every class in a backpack, instead of keeping them in their lockers and just grabbing the appropriate book for the class to which they’re headed. These backpacks, in addition to being stupid and adding to the gridlock in the hallways betwixt classes, also totally throw off the kids’ center of balance. Chances are good that you’ve seen a student (*cough* Kevin *cough cough*) try to stand up from his desk with his backpack on, get halfway up, then topple over backward instead, taking the whole desk down with him.
That’s about as good a demonstration of how center of gravity can affect movement and balance as you’re ever going to find. However, in the interest of not injuring any students, you should maybe show the kids in your class this Demo Science experiment instead.
This science demo requires two large, hardcover textbooks, an empty coffee can and its lid, a golf ball sized sphere of clay, and a permanent marker.
First, arrange your books to form a simple ramp. Next, put your ball o’ clay inside the coffee can and press it firmly onto the side of the can, so that it sticks in place. The clay should be centered along the length of the can. Mark the location of the clay on the outside of the can with your marker—this makes it easier to visualize the center of gravity.
Put the lid on the coffee can, then place the can at the bottom of the ramp and roll it upward. The idea is to get it all the way to the top of the ramp in one roll. (This will likely take a few tries, so maybe practice a time or two before class starts.) It rolls kinda funny, but by golly it works. How does it roll uphill?
Gravity – It = Gravy
The can can roll up the incline because the clay inside changes its center of gravity—that is, the place where all its weight is most concentrated/centered. At its center of gravity, an object will be balanced; by adding the clay to the can, its center of balance was moved, and gravity pulled the clay downward toward the Earth, even though it required the can to move upward on the ramp.