Learning Lesson: Heavy Air

OBJECTIVE Show that air has weight.
OVERVIEW After balancing two inflated balloons, the air is removed from one balloon throwing the balance off.
TOTAL TIME 5 minutes.
SUPPLIES Yard/meter stick
Two large balloons
Scotch tape
TEACHER PREPARATION You can have someone hold the string or you may want to have a piece of string hanging from the ceiling before class.
SAFETY FOCUS Severe thunderstorm safety rules


Air is all around us. This air is composed of atoms and molecules. Despite their small size, the quantity of atoms and molecules exert weight on us known as pressure. Since our bodies are designed to live in this environment, we do not notice the pressure.

  1. Blow up both balloons so they are the same size.

  2. Tape one balloon to each end of the yard/meter stick.

  3. Tie a string to the center of the stick and adjust it so the stick balances when held by the string. Tape the sting in place to prevent it from slipping.

  4. Ask the students, "If one end were heavier, would the heavier end move up or down?".

  5. Supporting one balloon, have someone carefully deflate the other balloon. Try poking the balloon with a pin in its neck to prevent the balloon from tearing apart as it pops.

  6. Release the remaining balloon and ask the students to explain what happens.

Since the inflated balloon weighs more than the deflated one, the side of the inflated balloon will drop. Imagine the weight of air if that balloon were now 15 miles (24 km) tall. That is actually what is occurring at this moment in your classroom. When we measure air pressure, we are measuring the weight of a column of air 15 miles (24 km) high directly over us.

Fast Facts
Baseballs travel farther in moist air than in dry air. For any given volume of air, moist air (at the same temperature and pressure) has exactly the same number of molecules as dry air.

Dry air is composed of mostly of heavy oxygen (O2) and nitrogen (N2) molecules. However in moist air, some molecules are the lighter weight water molecules (H2O), rather than heavier O2 or N2 molecules.

Therefore, the air is less dense in moist air and this decrease in density equates to less resistance to the ball's motion through air. So, for two baseballs hit with equal force, the one in the moist air would travel farther than the one hit in dry air.

Live Weatherwise

Severe Thunderstorm Safety
The weight of molecules also affects the weather. One measure of the severity of a thunderstorm is the wind speed. In addition to the size of hail, the National Weather Service defines a severe thunderstorm as one containing wind speed of 58 mph (50 kts) or greater.

The weight of all of the molecules in wind of 58 mph (50 kts) can create hazardous weather conditions such a blowing down phone and power lines, trees, and make driving hazardous. When the National Weather Service issues a Severe Thunderstorm Warning it means a thunderstorms with wind gusts to 58 mph (50 kts) or greater and/or hail size of 1" or greater is occur or about to occur near you.

Discuss severe thunderstorms with your family. Everyone should know what to do in case all family members are not together. Discussing disaster ahead of time helps reduce fear and lets everyone know how to respond during a severe thunderstorm.

Take an American Red Cross first aid and CPR course to learn how to treat burns and how to give rescue breathing and administer CPR. Everyone should know how to respond, because severe thunderstorms can strike almost anywhere at anytime.

Postpone outdoor activities if thunderstorms are likely. Many people take shelter from the rain, but most people struck by lightning are not in the rain! Postponing activities is your best way to avoid being caught in a dangerous situation.

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