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Dam Design

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Key Concepts
Potential energy, kinetic energy, hydrostatic pressure, hydroelectricity, gravity
Ben Finio, PhD, Science Buddies
A milk jug filled with water with two small holes, allowing small streams of water to come out


You are probably used using water for drinking, cooking, washing, and swimming. Did you know that water can also be used to make electricity? Try this fun activity to learn about hydroelectric power.

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.


All objects and materials have potential energy due to gravity when they are raised off the ground. When these objects fall back towards the ground, the potential energy is converted to kinetic energy, or energy of motion. In the case falling water, it can be used to turn something called a turbine, which looks kind of like a propeller. This turbine is connected to an electric generator that makes electricity. This type of electricity generated by falling water is called hydroelectric power ("hydro" means "water").

How do we harness the energy of falling water? In order to generate a lot of electricity, you need a lot of water that is falling very fast in one place. That means rain is not a good way to generate hydroelectric power, because it is spread out over a very large area and occurs at unpredictable times. Waterfalls have lots of water falling in one place, but many waterfalls are very beautiful and are considered natural landmarks and tourist destinations, so people would not be happy if we built ugly power plants on top of them. Our solution to this problem is to build dams, or large structures that hold back a stream or river. This causes a deep lake to form, and the water can be used to generate electricity when it flows through the dam. In this activity you will investigate where the turbine should be placed to generate electricity.

Note that hydroelectric power is considered a type of renewable energy, but it does have some environmental impacts. The large lakes formed by dams can destroy existing animal habitats, and the catastrophic failure of a dam can cause severe flooding and loss of life. If you are interested, you can do your own research on the costs and benefits of hydroelectric power.


  • Milk jug or 2 liter soda bottle
  • Small nail
  • Tap water
  • Duct tape
  • Chair, stool, or stepladder
  • Bathtub, large sink, or outdoor area where you can spill water
  • Optional: tape measure


  1. Make sure you can do this project in an area where it is not a big deal if you spill water on the ground, like outdoors or in a bathtub.


  1. Use the nail to punch three small holes in one side of the milk jug: one near the top, one in the middle, and one near the bottom. Important: make sure to offset the holes from each other horizontally by a few centimeters (the holes should not be directly above each other in a straight line).
  2. Use pieces of duct tape to tightly seal over the holes.
  3. Fill the milk jug with water. Make sure you fill it past the top hole. Look carefully to see if the duct tape seal is holding in the water before you carry the milk jug to your testing area.
  4. Set the milk jug on an elevated surface like a stool or stepladder, in an area where you can pour water on the ground.
  5. Get ready to quickly remove the duct tape. What do you think will happen when you remove the duct tape? How will water flow out of the holes? Will the water coming out of the three holes behave the same or differently?
  6. Quickly peel off all three pieces of duct tape. Watch closely. What happens? Where do the streams of water hit the ground?
  7. Continue watching as the water drains out of the milk jug. Do the streams of water continue to hit the ground in the same place, or does it change as the jug drains?
  8. Pretend that your milk jug is a dam, and the water inside it is a lake. Where would you place a turbine to extract the most electricity from the water?

Extra: use a tape measure on the ground to record how far the water shoots out. How does this distance change if you change the height of the milk jug off the ground? How does it vary between holes?

Extra: try the experiment with different shapes of containers, with holes punched at the same height from the bottom. What happens if you compare a milk jug and a two-liter soda bottle? Does the shape of the container affect how far the water shoots out?

Extra: use different size nails to punch holes, or have an adult help you user a power drill to make holes. Does the size of the hole affect how far the water goes or how fast the milk jug drains?

Observations and Results

You should observe that the water from the hole at the bottom of the jug shoots out the farthest, followed by the middle hole, then the top hole. This is because the hydrostatic pressure (the pressure applied by the water at a certain depth) at the bottom of the jug is the highest, causing the water to shoot out with a higher velocity. This high-velocity water has more kinetic energy to turn a turbine and produce electricity, so the bottom location would be the best place to put your turbine.

Depending on where you poked it and how much you filled the jug, the water from the top hole may just trickle down the side of the jug and not actually shoot out horizontally. As the water level drains, the water will not shoot out as far from the holes. This happens because hydrostatic pressure is determined by the difference in height between the hole's location and the surface of the water, and the surface gets lower as the water drains.

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  1. If necessary, clean up any spilled water with a towel.

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