Learning Lesson: Crunch Time

OBJECTIVE Demonstrate the effect of heat on pressure.
OVERVIEW A plastic 2-liter bottle will be crushed by the normal atmospheric pressure in the room.
TOTAL TIME 10 minutes
SUPPLIES Two empty 2-liter bottles, hot tap water.
SAFETY FOCUS Thunderstorm safety


Pressure is not only a matter of altitude but also is dependent upon the temperature. As the temperature increases so does the pressure. The molecules and atoms that comprise the air we breath gain energy as they absorb heat. That increase in energy results in faster moving atoms which we observe as an increase in energy. The opposite occurs when the temperature decreases. As the molecules lose energy, their motion is decreased and we observe a decrease in pressure.

  1. Place two cups of hot tap water into each two 2-liter bottle.

  2. Place your thumb over each bottle opening and shake. This ensures the air inside the bottle is warmed.

  3. Pour the water out of each bottle and screw a bottle cap on only one of the two bottles.

  4. Stand both bottles side-by-side and observe over the next five minutes.

The bottle that was capped will eventually begin to collapse. This is a result of the cooling air inside that bottle. The air cools because the molecules and atoms inside the bottle lose energy as they collide with the bottle side that is exposed to the cooler surrounding air. As their energy decrease so does their velocity and therefore the pressure decreases. Since the pressure inside the bottle decreases, the force of the air outside the bottle begins to crush the bottle.

However the uncapped bottle remains unchanged. As the air cools inside, the drier outside air flows in to take up the space thereby keeping the pressure the same both inside and outside of the bottle.

Fast Facts
We can only swim down to a relatively shallow depth before the increased pressure from the water crushes our bodies. The world's record SCUBA dive is 1044 feet (318 meters) set in 2005. At a certain point, outward pressure in the lungs exceeds the structural integrity of the rib cage, and the rib cage collapses. Obviously, this would kill a human being.

However, whales can withstand this pressure because their bodies are more flexible. Their ribs are bound by loose, bendable cartilage, which allows the rib cage to collapse at pressures that would easily snap our bones.

A whale's lungs can also collapse safely under pressure, which keeps them from rupturing. This allows sperm whales, which dive to depths of 7,000 feet (2,100 meters) or more to hunt for giant squid.

Live Weatherwise

Thunderstorm Safety
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 force of all of the molecules moving at 58 mph (50 kts / 93 km/h), or more, 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 / 93 km/h) or greater and/or hail size of 1" or greater is occur or about to occur near you.

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

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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