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# Air Pressure and Altitude

## The pressure exerted by the air in the atmosphere is greatest at Earth’s surface and falls as altitude increases.

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Practice Air Pressure and Altitude

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Pressure Flush

### Pressure Flush

Whoosh! A loud sucking sound comes from an airplane toilet when you flush it. Did you ever wonder what happens to the waste?

#### The Back Story

• Contrary to popular belief, an airplane toilet doesn’t just dump the waste in the sky—not even at a typical cruising altitude of 12,000 meters (about 39,000 feet).
• The waste actually goes from the toilet to a storage tank in the airplane. Once the plane lands, the tank is drained, ready to be filled again on the next flight.
• What’s interesting is how airplane toilets flush. They don’t use gravity and a lot of water, like most toilets on the ground. A different principle explains how airplane toilets work, and it involves air pressure.
• Watch this entertaining video for a dramatic demonstration of how an airplane toilet flushes: http://science.discovery.com/tv-shows/outrageous-acts-of-science/videos/monster-flush.htm

#### Can You Apply It?

1. Why can’t you use gravity and a lot of water to flush an airplane toilet as you can a toilet on the ground?
2. Describe how an airplane toilet works.
3. How does air pressure explain how an airplane toilet works?
4. Why is the air pressure so low outside an airplane at cruising altitude?

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