Hot-air balloons float because the air inside the balloon is less dense than the outside air.
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# Energy Density

A tornado is the most destructive force in nature; that doesn't mean it has the most energy. Thunderstorms which produce tornadoes can have 40,000 times as much energy as a tornado! Tornadoes are so destructive because they have a higher energy density than thunderstorms; the energy is concentrated in a small area while the energy in a thunderstorm is spread-out over a much greater area.

Energy and energy density can be compared to weight and force. Energy density is energy per unit volume and force is weight per unit area. If a 100 pound person in flat shoes stepped on your foot, you'd feel it. If the same person stepped on your foot while wearing very narrow high-heels, you might end up with a broken toe. That's because the person's weight is concentrated in a small area. The same holds with tornadoes. They may not have as much total energy as thunderstorms, but the energy they do have is concentrated in a very small area. So small, in fact, that tornadoes have destroyed houses while not damaging the neighbors.

## Ready, Set, SCIENCE!: Putting Research to Work in K-8 Science Classrooms

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Tornadoes form from severe thunderstorms. They have a very high energy density which means that they are very destructive to a small area. They also don't last very long which makes them hard to study....more

## Wave Beats

Sound travels in waves. These waves have both a frequency and an amplitude. The frequency is measured in hertz, which is one wave cycle per second. A cycle is a repeated pattern of positive and negative...more

Storms chasers are different than storm spotters. Chasers travel around Tornado Alley looking for severe storms and tornadoes. This area in the Great Plains is the best for chasing. Besides having a lot...more

## Energy Density

A tornado is the most destructive force in nature; that doesn't mean it has the most energy. Thunderstorms which produce tornadoes can have 40,000 times as much energy as a tornado! Tornadoes are so destructive...more

## The Doppler Effect

The Doppler effect was named after Christian Doppler, who first came up with the idea in 1842. He determined that the frequency of sound waves would change if either the source of the sound or the observer...more