University of Minnesota
School of Physics & Astronomy

Physics Force

Mass and Inertia

Coat Hanger and Pop Cans

(Mass, Center of Mass and Inertia)

It becomes interesting to students when the teacher is balancing something on her/his head. For some reason things staying on your head are a source of curiosity and intrigue. It is possible to shape a piece of stiff wire in such a manner as to make it balance nicely on your head with weights hanging on either side. The weights, being themselves a kind of mass, will tend to stay where they are unless acted on by an unbalanced force. Not only that, we can use this demonstration to show that the center of mass tends to remain at its lowest possible point relative to the earth�s surface.

You have no doubt been amazed by some toys in which the toy seems to stand or stay in a position, seemingly to defy the law of gravity. In fact, some objects can be forced to fall down and they will spring up as if by magic.

The wire from one regular sized coathanger, two cans with bail handles on them, some water.

Bend the coathanger wire into a large "M" as in the diagram to the right. Holding the wire in the middle, hang two cans with bail handles about 1/4 full of water on each of the hooks and place the lowest point of the "M" on the top and in the middle of your head. After some practice, you can balance this object very nicely on some middle place on top of your head. Now the friction between the wire and your hair is very low, so if you were to turn to the left or right, the inertia of the cans and water will be enough to keep them in the same place. Try practicing with this apparatus until you become proficient before doing it for others. You can actually turn all the way around and the cans of water will stay where they are when you get good at it!

Tablecloth Pull


Inertia is the tendency of an object with mass to remain in its present state of motion. (Newton's first Law of Motion) When an object is at rest (just sitting there) it tends to stay there unless acted on by an outside, unbalanced force. When there are several solid objects such as dinner plates, water glasses, and silverware, these objects will remain at rest unless pushed by a sizable force. Newton described this in about the middle of the 17th century-about 1663 or so.

When you are in an auto that is accelerating forward from rest, you tends to stay where it was, and the auto has to push your body to make it move forward with the car.

You can use any glassware or metal ware or any plastic dishes you would like. A silk cloth that will have the least friction between it and the dinnerware and is about three feet square.

Place the dinnerware items on the cloth on top of a smooth, solid table like you would to serve anyone a meal. Take the edge of the cloth and wrap the edge of it around a meter stick to give you even pressure when you pull it out from under the dinnerware.

When you are ready, pull the cloth over the edge of the table top with a sharp, downward pull. You need to be sure the cloth does not lift on the objects, so pull it over the edge and downward. The cloth should have no hem on it to eliminate the possibility of catching one of the objects with this "edge" that may stick up far enough to do so.

FOSS Connection: Balance & Motion - Activity 1: Balance