Resource: Columns: Experimenting with Balloons

Columns are among the oldest and most effective building elements when it comes to resisting the pushing, or compression, that occurs in all kinds of structures. Paired with horizontal beams, these critical forms sometimes carry the weight of an entire structure. To resist the downward force of all that weight, columns apply an equal upward force.

Balloons are never used as columns in building construction. Even so, when placed in a column-like circumstance, as in this ZOOM activity, balloons encounter the same types of forces as columns and apply their own resistant forces in return. The manner in which they apply these resistant forces, however, is very different from the way a more traditional column might respond.

There's an unmistakable difference between a fluid-filled balloon and the types of materials -- metal, concrete, stone -- most often used to build columns: Balloons stretch a lot, while traditional materials stretch very little. For example, when a car drives over a bridge, the columns supporting the bridge deck flex only slightly. The dense material in the column resists compression and remains rigid and strong. Fluid-filled balloons, in contrast, compress immediately, even under small forces. As weight is applied from above, the pressure of the fluid inside the balloon rises, applying a force to the interior walls of the balloon. The rubber of the balloon stretches in response, applying equal resistance inward.

As weight is increased, or the number of columns decreased, the downward force on each column increases also. Balloon columns respond in two ways: The pressure inside the balloon increases, and the balloon stretches more, spreading out and covering a larger area. These two responses have the effect of increasing the amount of upward force the balloon can apply until the upward force once again matches the downward force. The balloon breaks when increased internal pressure and stretching finally fracture the balloon material.

Describe the forces at play when the balloons are supporting the weight of the boy.

Why do you think the water balloon supported the boy's weight better than air did?

Did the ZOOM cast members investigate the differences between air and water in a systematic way?

Have you noticed the flat place where a car tire touches the road? Do you think the size of that flat place depends on the air pressure in the tire? Was there more pressure in the water or in the air in these balloons?