For this simple science fair project, you need a 2-liter soda bottle, a bowl that holds more water than the bottle, water and a marking pen. Fill the bowl halfway with water and place a mark on the bowl at the water level. Fill the bottle halfway with water and write with a marker to indicate the water level. Push the soda bottle into the bowl and observe if the water levels change on either container. The water in the soda bottle will remain unchanged, however, the water in the bowl will rise higher than the mark. This is due to displacement, where the water in the bowl was moved by the soda bottle. The difference between the new level and the old one determines the water displaced. Water displacement is how boats float. If an object weighs less than the water it displaces it will float. The soda bottle weighed less than the water it displaced so it floats in the bowl.
For this science fair project you'll only need a few simple items; aluminum foil, a bucket, water and a hammer. Shape a boat from aluminum foil then place in the bucket which is filled with water. Observe what happens. Next take the aluminum foil boat and with an adult's supervision, or help, hammer the boat into the smallest ball possible. Put the ball into the bucket of water and note what happens this time. The boat floated because it weighed less than the water it displaced. However, the aluminum foil ball while weighing the same, was smaller and could not displace enough water to allow it to float. Because the ball took up a smaller volume the amount of water displaced wasn't enough to allow the buoyancy principle to work here, as the gravitational force was stronger and made the ball sink.
Shape a boat from aluminum foil making sure to fold the edges up around the sides. Place the boat in a bowl of water. Watch how the boat floats. Now place a penny in your boat and observe whether it floats. Estimate how many pennies the boat will be able to carry before beginning to sink. Slowly add pennies, one at a time, to determine the penny cargo limit for your aluminum foil boat. The pennies are trying to weigh down the boat due to gravity while buoyancy is pushing the boat upward. At some point, the weight of the pennies will cause the buoyancy force to be overwhelmed by the weight and therefore the gravitational pull exceeds the buoyancy force.
For this project, assemble a variety of materials to be used to build boats. Some ideas include Styrofoam bowls, containers or small plates, wood scraps, egg cartons, empty paper towel or toilet paper tubes, plastic containers, straws, foil and an assortment of different papers. Using glue or tape, have students construct boats and then attempt to float them in a large tub of water. Once they have found some materials and shapes that successfully float, have students fill the boats with small items of varying weight to determine which items will still allow a boat to float. A boat floats whenever the water it displaces weighs more than the boat itself.