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Clay Boat
What you need:
Sink or Swim
Ever wonder how a 200 ton ocean liner manages to float? By using modeling clay and two aquariums you can clearly demonstrate the principle of buoyancy.
Fill both aquariums three-quarters full of water. Mix just enough food colouring with the water to give it a bluish tint without clouding or darkening it. Divide the 500g of clay in half, weighing each half to make sure you have two equal parts. Mold one chunk of clay into a ball and another into a hollow boat with tall sides. Drop the ball into an aquarium and watch it sink to the bottom. Place the boat in the other aquarium and watch it float. Remember, boat boat and ball were made from the same amount of clay. Do you notice anything different about the water level in the aquarium that contains the boat? Remove the boat and squash it into a ball, dropping it into the aquarium again. It sinks! Remove the ball from the other aquarium and shape it into a boat. It Floats! The water level is always higher in the aquarium that contains the boat.
Density and Displacement
Substances denser than water, such as metal or modeling clay in our experiment, sink because they weigh more than the upward thrust of water they displace. But dense substances can be made to float by reshaping them to increase the volume of water they displace, thereby increasing the upward thrust of water. The hull of a 200 ton ocean liner is carefully designed to maximize upward thrust and stabilise weight, so that it floats despite its huge size. Invite onlookers to model the clay into their own boat shapes. Some boats float while others do not, since everything depends on good design.