Humans have always been fascinated by hydraulics, the study of the movements of fluids. Simple experiments and projects can be done that demonstrate how a fluid behaves. No special fluid or expensive equipment is needed. Common household items and water demonstrate ideas well. These projects also make excellent science projects, and all involved will gain an understanding of how fluids move. If these projects are done by children, adult supervision is always recommended, to provide guidance.
One of the first principles of hydraulics is that a fluid always seeks to stay on the horizontal. The plane of the surface is always parallel to the horizon. To demonstrate this, fill a clear glass half full of water. Gently tilt the glass back and forth. Notice that the surface of the water always stays level. This fundamental law of fluids is what liquid-filled compasses in boats are based upon. A compass floats on top of a liquid. No matter how much the boat tosses and turns, the compass will always stay steady.
Arrange two glasses, one higher than the other. Fill the top glass with water. Fill a clear plastic aquarium tube with water and insert both ends in the cups. Notice how the water flows from the top glass to the bottom glass. This is a siphon. Because the water pressure is higher at the bottom of the tube, it draws water from the top glass. Siphons are used extensively to draw water from a higher spot to a lower one, such as from the Sierras Mountains to Virginia City in Nevada.
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It is a well-known fact that steel sinks in water. However, many boats and ships are made of steel. The reason they float is because the ship is sealed, and the air inside the ship displaces water, which is heavier than air. To demonstrate this principle, fill a bucket about halfway with water. Float an empty can inside the water. Start filling the can with water. Notice the level of the water in the can will be the same as the level of the water outside the can. This is because the air only displaces the space above the water.
This experiment demonstrates that fluid is not compressible. You will need a potato, a rigid drinking straw and a skewer. Cut a slice of potato, about a quarter of an inch thick. Push one side of the drinking straw into the cut slice of potato so a piston is formed inside the straw. Push the piston halfway in the straw with the skewer. Fill the straw with water and push another piece of potato into the straw, trapping the water. Push on the new piston formed with the skewer. Notice the other piston moves too. This is because the trapped water cannot be compressed. Automotive braking systems rely on this principle. When you push on the pedal, it moves a piston. The fluid inside the brake lines push on another piston, inside the wheels, which moves the brake pads.