Contents

Insights & Connections
Vocabulary
Resources
Main Activity
Try This

INSIGHTS

If you want a sport that puts you under a lot of pressure, try underwater diving. You can even feel some of the effects of pressure in a swimming pool. Down just a few feet underwater, your ears begin to hurt. That's caused by pressure on your eardrums.

Where does that pressure come from? It's the weight of all the water--and air--above you. At the surface of the water, a column of air--stretching hundreds of miles out into space--weighs 14.7 pounds per square inch. Scientists call this amount of pressure "one atmosphere."

When you go underwater, however, you add the weight of the water to the atmospheric pressure. A 10-meter (33-foot) column of water weighs 14.7 pounds per square inch, so at a depth of 10 meters, the pressure is two atmospheres: half from the water and half from the air above it.

Pressure also influences how divers use air. At ten meters, for example, the increased pressure means your lungs hold twice as much air as they do at the surface--and you'll breathe all the air in your tank twice as fast. The deeper you dive, the more quickly you use up the air in your tank.

When you breathe compressed air through a regulator underwater, you ensure that the air spaces in your body are at the same pressure as the surrounding water. But if you breathe compressed air underwater and then ascend, holding your breath, the pressure around you decreases, so your lungs expand. Air sacs in your lungs could rupture, causing an air embolism, which means that bubbles of air enter your blood stream and block circulation to your brain.

"The bends," or decompression sickness, is another health hazard associated with pressure changes. The longer you stay down and the deeper you go, the more nitrogen dissolves into your body tissues. If you ascend too rapidly, the dissolved nitrogen comes out of solution too quickly and forms bubbles in your tissues. You could experience severe pain (particularly in joints), dizziness, blindness, paralysis, and convulsions.

Although decompression sickness is rare, divers learn they must ascend slowly and, under certain circumstances, take "decompression stops" on the way up. This allows the dissolved nitrogen to come out of the body safely.

CONNECTIONS

1. Why is it important for people to learn to dive and to spend time underwater?
2. What are some precautions people should take as they dive underwater?

• air embolism a dangerous condition in which air sacs in your lungs burst. This can create air bubbles in your bloodstream.
• decompression sickness a serious condition (also known as "the bends") resulting from nitrogen coming out of solution and forming bubbles in body tissues. It happens when divers have been breathing compressed air at depth and then ascend too rapidly.
• neutral buoyancy neither sinking nor floating; having the same average density as water
• regulator a device which delivers air for breathing at the pressure of the surrounding water
• wet suit a neoprene foam suit that will keep you warm in cold water by trapping a layer of water next to your body

Resources

• Briggs, C.S. (1985) Skin diving is for me. Minneapolis: Lerner.
• Griffiths, T. (1989) Sport scuba diving in depth: An introduction to basic scuba instruction and beyond. Princeton, NJ: Princeton Book Co.
• Open water sport diver manual (4th ed.). (1989) Englewood, CO: Jeppesen Sanderson Inc.
• McGovern, A. (1989) Down under, down under: Diving adventures on the Great Barrier Reef. New York: Macmillan.

Additional sources of information

• National Association of Underwater Instructors PO Box 14650 Mountclair, CA 91763 (909) 621-5801
• The Cousteau Society 777 United Nations Plaza, 5th Floor New York, NY 10017 (212) 949-6290

• The YMCA is the oldest dive certification organization.

Main Activity

It's important for divers to be of neutral buoyancy. That means they tend to stay at the depth they are--they neither sink nor float. You can explore buoyancy in a sink or bathtub.

Materials

• several dozen washers, in a few different sizes if possible
• some objects that float, such as wooden "kindergarten" blocks, Tinkertoys, or plastic film canisters
• paper clips
• duct tape
• an inflated and tied balloon

1. Fill a bathtub (or any other deep container, such as a sink or a waterproof wastepaper basket) with as much water as is practical.
2. Select an object. Verify that it floats.
3. Find a way to attach the washers to the object as weights. For a wooden block, make a hook out of a paper clip and tape it to the block with duct tape; put the washers on the hook. For a Tinkertoy structure, put the dowel through the washer's hole. For a film canister, put the washers inside.
4. Add washers until the object just sinks. Record how many washers it takes to sink it.
5. If you have different-size washers, try to adjust the weight on the object so that the object is of neutral buoyancy.
6. If you have different-size objects (for example, wooden blocks or Tinkertoys in different sizes), see how the amount of weight it takes to sink them depends on their sizes.
7. Look around your home for objects that float which you can sink with washers. Try some. Were you surprised by any of them?
8. Try to sink the balloon. Tie a paper-clip hook to the neck of the balloon and add washers. What happens?

Questions

1. What did you notice about the number of washers and the size of the object? Is this what you expected?
2. Can you predict how many washers you'd need to sink a balloon?
3. Divers use balloons to lift heavy things from the bottom. How do you suppose they fill them?

Invite a diver to come and talk to your class about diving. Ask the diver to bring a regulator and a tank so you can see what it's like to breathe through a regulator on dry land. What do you think it will be like?

In cold water, divers wear wet suits. These neoprene foam rubber suits trap a layer of water next to your skin. When you jump in, you feel the cold water seep into the suit, but your body warms up the water right away. Since this water isn't instantly replaced by the cold water around you, the heat--and the warm water--stay with your body. Wet suits look cold and clammy, but they're not! Visit a scuba dive shop and examine the equipment used.

Can you hold a plastic cup over your mouth and chin by sucking some of the air out of it? The air pressure outside the cup is greater than the pressure inside; that difference in pressure holds the cup onto your face. If you equalize the pressure, the cup falls off.

Imagine you are underwater and your face mask fills with water. You don't have to go to the surface. Just hold the bottom of the mask away from your face and exhale through your nose. The air will rise to the top of your mask and push the water out the bottom! This is how scuba divers clear their masks when they're underwater.

If you wear a face mask in the water and swim down a few feet, you'll feel the water pressure pushing the mask harder and harder onto your face. To relieve that discomfort, make the pressure more equal by blowing air into the mask through your nose.

Newton's Apple is a production of KTCA Twin Cities Public Television.
Made possible by a grant from 3M.
Educational materials developed with the National Science Teachers Association.