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  Activity: Under Pressure
Activities Index | Handout | Educator Ideas  

10 minutes to prepare bottle; 30–40 minutes second day

(per group of 4)
• 2-liter plastic soda bottle
• drinking straw
• acrylic caulk (sold in hardware stores)
• sharp scissors or awl
• long plastic tub if doing activity indoors (groups can share tubs)
• permanent marker
• meterstick
• water

Advance Preparation
Since making the hole in the bottle requires an adult's help, it may be simpler to prepare the bottles ahead of time for the whole group. Let the caulk dry overnight before the activity.

Video Connection
After the activity, show "Dam Basics" from Dams to reinforce the link between pressure and dam shape. (See the Program Description to locate this show segment.)

Try the Dam Challenge

For more information, see Additional Resources.

Illustration of milk carton with three holes, showing different strengths of flow through top, middle and bottom holes. Poke three holes in the side of a milk carton and cover them with a single strip of tape. Fill the carton with water. Hold the carton with one hand and quickly pull off the tape. The water will stream out of each hole with different force. Ask: Why does the water look different coming out of each hole? (Kids may say that the water at the bottom is "stronger.") Explain that water behind a dam is a live load pressing on the dam. The greater the amount of water built up, the greater the pressure–so the water coming from the bottom hole has more force than the water from the top hole.

Lead the Activity
• This activity is best done outdoors. If you are doing the activity indoors, have kids place the bottles inside a long plastic tub to catch spills. 
• A potential source of confusion is that the 20-cm measurement (the full bottle) models the water pressure at the bottom of a dam, while the 5-cm measurement (the nearly empty bottle) models the water pressure at the top of a dam. At the top of a dam, only a little water is pressing on the dam, as is the case when the bottle is nearly empty. Compare diagrams of a dam and the bottle to ensure that kids understand how the model relates to a real dam.

The Big Idea
Illustration of a dam, showing lines of compression forces.Water pressure increases with the depth of the water. In deep water, there is more water "piled up," which causes the pressure to be greater at the bottom than at the surface. A dam's design must enable it to withstand greater pressure at the bottom than at the top. As a result, many dams are built in a triangular shape. The wide bottom withstands the great load of the water deep below the surface, while the top of the dam can be built thinner so as not to use unnecessary costly materials.

Build on It
Water discharging from the bottom of a dam has great force. A structure called a diffuser, sometimes just a mass of large boulders, reduces the force of this discharge. Possible outcome: Kids may suggest using multiple spouts or a triangular spout to release the water, or using a diffuser to "break up" the stream of water.

Make Connections
Math Have kids graph their results. Instruct them to put the water depth on the horizontal axis and the distance on the vertical axis.