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Lesson Plans For Teachers

Feeling the Pressure?

Materials:
  • cup
  • half-gallon milk carton (not plastic)
  • quart milk carton (not plastic)
  • pencil
  • masking tape
  • deep pan (a roasting pan works well)
  • water
Background Information:
Do you know what water pressure feels like? At the water's surface, atmospheric pressure (the weight of the overlying air) is 14.7 pounds per square inch. We don't notice it because we are accustomed to it. You do not feel much water pressure when you are at the surface of a pool or the ocean, but as you dive down deeper, you will probably feel a squeezing in your head and ears. If you are wearing a mask, you may feel like it is being pressed hard to your face. This is because of water pressure. Water pressure is determined by two factors: depth and density.

Water pressure is a function of weight, which is related to depth; the deeper one goes, the greater the weight of the overlying water and the greater the surrounding water pressure. Water pressure increases equally with depth, so the pressure at 40 feet is twice the pressure at 20 feet. This marked increase in water pressure with depth affects every diver and anything that goes under water, unless it is protected by pressure-resistant walls (such as a submarine).

Density, or the "heaviness" of an object, also affects water pressure. The greater the density of water, the greater the pressure it exerts. Sea water is denser than fresh, because it contains more salt and dissolved minerals. An animal 10 feet below the surface in a fresh-water lake will feel less pressure than an animal at the same depth in the ocean.

Since water pressure increases with depth, scuba divers experience more physical changes the deeper they go. A few feet below the surface, divers will feel their ears "pop." At greater depths, the increasing pressure on the body causes nitgrogen gas in the diver's blood and body fluids to dissolve (or liquefy). If a diver comes to the surface too quickly, the sudden reduced pressure can cause bubbles of nitrogen gas to form in the blood and body fluids, as the dissolved nitrogen returns to a gaseous state -- a painful and potentially fatal condition known as "the bends." Thus, water pressure is a force to be reckoned with in ocean exploration and challenges our ability to explore life in the sea.

In this lesson, you will compare the pressure of water at different depths and gain an understanding of how increased water pressure might affect animals living in deeper waters.

Presentation:
  • Use a pencil point to make two identical holes in each milk carton. Make one hole two inches from the bottom, and the other hole three inches above the first hole.
  • On each carton, tape over the holes with one long strip of masking tape.
  • Stand the half-gallon carton in the deep pan, and fill the carton with water.
  • Remove the strip of tape. As the water shoots out, observe how the flow of water through the holes changes. Which hole squirts farthest? Why?
  • Empty the pan. Fill the quart carton with water to the same depth as the first and repeat the experiment to show that depth, not volume, affects water pressure.
What's going on?
As the water flowed out of the carton, the flow slowed down because there was less pressure forcing water out of the hole as the water drained out. The bottom hole should have squirted out farthest because the deeper you go, the more water there is pressing down from above.

Some animals in the deep sea live in water pressure that is a hundred times greater than the air pressure we feel living on land. What are some of these animals? How do these animals survive such powerful forces? Remember, only gas or air spaces inside an animal's body are compressible. Water and most oils do not compress under pressure.

Think about this...When diving, humans feel the effects of water pressure just a few feet below the surface (your eardrums will begin to ache, and can burst at greater depths unless you equalize the pressure by forcing air from your lungs into the chambers that lead to your middle ear). How can sperm whales dive to depths of 1000 meters and not suffer the effects of water pressure?

Marine animals that live in deep water have special adaptations that allow them to live in a high pressure environment. Most deep-diving whales have a flexible ribcage which allows their lungs to collapse as they dive down. When the whale returns to the surface, the lungs greatly expand. Whales also have a high volume of blood and a high lung surface area for oxygen transfer to the red blood cells. All of these adaptations allow them to overcome the increasing effects of water pressure with depth.