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  Activity: Under Pressure
Activities Index | Handout | Educator Ideas  

How great is the water pressure?
The most important load that a dam must be able to support is the water behind it. How much the water pushes on the dam is called water pressure. The water pressure pushes on the dam from the side, putting it into compression. A dam's shape can help it to withstand this compression.

What You Need
• 2-liter plastic soda bottle
• drinking straw
• acrylic caulk
• sharp scissors or awl
• permanent marker
• meterstick
• water

Make a Prediction
Predict how water pressure at the top of a dam compares to the pressure at the bottom of the dam.

Try It Out
1. To make a model dam, ask an adult to drill a small hole near the bottom of the bottle using scissors or an awl. Cut a short piece of straw and insert it in the hole. Squirt caulk all around the straw to make the hole watertight. Let it dry overnight.  
Illustration of a bottle, with a hole near its bottom, placed next to a ruler which will measure distance of flow of liquid out through the hole. 2. With a permanent marker, draw marks on the side of the bottle at the following distances up from the straw: 5 centimeters (2 in.), 10 cm (4 in.), 15 cm (6 in.), and 20 cm (8 in.). Label the marks. 
3. Place your finger firmly over the end of the straw. Fill the bottle with water. 
4. With your finger still over the straw, place the bottle somewhere that can get wet (such as on the ground outside or in a tub). Your partner should place the meterstick under the straw so that one end lines up with the open end of the straw.  
5. When you're ready to record your results, remove your finger from the straw. When the water level inside the bottle reaches the 20-cm mark, say, "Twenty." Your partner should record the distance at which the stream of water is hitting the meterstick. Repeat as the water level in the bottle reaches each mark.

Explain It
Was the water pressure greater when the dam was full (20 cm) or nearly empty (5 cm)? How could you tell? How do your results affect how you would design a dam?

Build on It
How could you reduce the force of the water coming out of the bottom of a dam? Make a prediction and test it using your model dam.