Learning Lesson: Going with the Flow


Bernoulli's principle states that in fluid flow (which includes air), as the velocity of a fluid increases, its pressure decreases. The students will discover that the faster air moves, the lower the pressure becomes within that flow of air. They will see this effect by blowing between two soda cans.

TOTAL TIME 5 minutes
SUPPLIES Two (2) empty soda cans, a level surface
SAFETY FOCUS Tornado safety


  1. Lay the two cans on their sides parallel to each other, about one inch apart, near the edge of a level surface.
  2. Ask the students to predict what will happen when air is blown between the cans: Will the cans move apart or together? Have them explain why they made their prediction.
  3. Put your face down near the surface and blow lengthwise between the two cans.
  4. It will take some trial and error, but eventually the two cans will roll together.
  5. Another way to demonstrate is by suspending two cans on string about an inch apart and have the student blow between them.


Students may expect the cans to move apart as air is blown between them, however, the opposite occurs. This effect is Bernoulli's principle in action, named after the eighteenth-century Swiss mathematician and physicist Daniel Bernoulli. He discovered that the faster a fluid moves, the lower its pressure becomes. Because air is a fluid, this principle is important in understanding wind and weather conditions. It also has applications in aviation and airplane flight because it explains how planes generate lift as they increase in speed.

By blowing between the two cans, the air between them moves faster than the surrounding air, lowering the pressure between the cans. The cans roll together as the higher pressure surrounding the two cans (away from the air flow) pushes the cans together toward the region of lower pressure.

Building a Weather-Ready Nation

There is no such thing as guaranteed safety from a tornado. Freak accidents happen; and the most violent EF5 tornadoes can level and blow away almost any house and its occupants. Fortunately, most tornadoes are much weaker and can be survived.

At home, have a family tornado plan in place, based on the kind of dwelling in which you live. Know where your safe rooms are and practice a family tornado drill at least once a year. Have a pre-determined place to meet after a disaster.

Flying debris is the greatest danger in tornadoes; so store protective coverings (e.g., mattress, sleeping bags, thick blankets, etc.) in or next to your shelter space, ready to use on a few seconds' notice. When a tornado watch is issued, think about the drill and check to make sure all your safety supplies are handy. Turn on local TV, radio or NOAA Weather Radio and stay alert for warnings.

Forget about the old notion of opening windows to equalize pressure; the tornado will blast open the windows for you! If you are out shopping, know the locations the store's bathrooms, storage rooms or other interior shelter areas away from windows, and the quickest way to get there should a tornado strike.

All administrators of schools, shopping centers, nursing homes, hospitals, sports arenas, stadiums, mobile home communities and offices should have a tornado safety plan in place, with easy-to-read signs posted to direct everyone to a safe, close-by shelter area.

Schools and office building managers should regularly run well-coordinated drills. If you are planning to build a house, especially east of the Rockies, consider an underground tornado shelter or an interior "safe room".