The biggest sphere you will make-the one with the largest volume-will have a lower density than the smaller spheres, since the mass (the amount of aluminum) will be the same for all the spheres. You will gradually form different sizes of spheres (balls) out of the same piece of aluminum. In this hydrodynamics (the study of fluid in motion) science project, you will work with a different kind of metal, called aluminum. This trapped air lowers the density of the ship, so that it can float, just as air holes in a kitchen sponge give it a low density and allow it to float. They also make the density of the ship lower than water by encasing air inside the hull. This creates a large upward buoyant force. They design the part of the ship that goes in the water, the hull, with a shape that displaces (pushes out of the way) a lot of water. Ship-builders have figured out how to make steel ships float. The sponge will float, while the block will sink. The block has a higher density than the sponge-there is more matter packed into the same volume. Now imagine a block with exactly the same width, height, and depth as this sponge, but made out of solid metal. Think of a rectangular kitchen sponge, which is airy and light, with lots of holes in it. In other words, it describes how much "stuff" is packed into a volume of space. Whether an object sinks or floats depends upon its density compared to the density of water. If, on the other hand, the object displaces a lot of water, then there will be a large buoyant force pushing upward. So, if the object that you place in the water moves just a little bit of water out of the way, the weight of that small amount of water is small, and so the buoyant force is small. The strength of this upward acting force is equal to the weight of the water that was moved out of the way. (NASA, 2010.)Īrchimedes discovered that there is a buoyant force that pushes up on an object when you place it in the water. This photo shows a sculpture of the great Greek scientist Archimedes. You will determine the diameter at which the lifting force is just not strong enough to keep them afloat.įigure 2. In this hydrodynamics science project, you will make little spherical "boats" out of aluminum foil and find out at what point they can't push away enough water, causing them to sink. He used this idea, called Archimedes' principle, to help the local king figure out if his crown was made of pure gold or not, and engineers use his principle today to help build ships of steel that can float. Have you noticed that, too? The weight of the water that is pushed out of the way is equal to the lifting force on that object. What was he so excited about? He had discovered that when objects, like his body, are placed in water, water is pushed out of the way. Eureka! He went running through the streets without even bothering with his clothes. Where do you get your best ideas? At school with your friends? When you are out for a bike ride? Over 2,200 years ago, a scientist named Archimedes got one of his best ideas when he sat down in his bath. Now, we can find the volume of the crown by dividing the mass of the water displaced by the density of water (ρ_water = 1 g/cm³ = 1000 kg/m³).Create Assignment Create Announcement Abstract Mass of water displaced = Buoyant force / Gravity We can find the mass of the water displaced by dividing the buoyant force by gravity. The buoyant force is also equal to the weight of the water displaced by the crown, which can be calculated using the mass of the water displaced and gravity.īuoyant force = Mass of water displaced × Gravity We can do this using the buoyant force, which is the difference between the weight in air and the weight in water.īuoyant force = Weight in air - Weight in water Now, we need to find the volume of the crown. We can do this using the buoyant force, w. We can do this using the weight in air and the acceleration due to gravity (g = 9.81 m/s²).
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