Which floats better

This is a feature of flat-bottomed boats: they require careful balancing of the cargo and passengers, or else they become unstable and prone to tip and take on water. A distinct advantage of flat-bottomed boats is that they have a shallow draft, meaning their hulls do not extend very far down below the surface of the water compared to other hull shapes see Figure 1.

Flat-bottomed boats are thus desirable for moving around in shallow water. Their simple shape also makes them the least expensive type of boat to build. Flat hulls are typically found in small utility boats such as Jon boats, and were commonly used in the last century as barges to transport goods on the quiet waters of canals in this country and in parts of Europe.

The more contemporary use for flat-bottomed boats is as high-speed runabouts for recreational purposes. In this case the flat hull is designed to rise up and ride on top of the water rather than cutting through the water, thereby encountering the reduced friction of moving through air instead of water see Figure 2.

Although it takes a lot of engine power to get the hull up, at which point the boat is said to plane, it can then travel at very high rates of speed. A disadvantage of flat hulls is that they give a rough ride if any waves are present, because the entire width of the boat's bottom is in contact with the water. Even when planing, the back, or stern, of the boat is still in the water. Some students may try making boats from their clay that are shaped more like canoes, with tapered ends and rounded hulls.

Tapered ends certainly let a boat move through the water more efficiently than a bowl-shape, since water can easily flow around the front bow of the boat if it is tapered. The rounded hull, however, presents a problem because such boats roll easily and take on water or capsize. Large sailboats, fishing trawlers, and cargo ships, which do have rounded hulls, generally also have keels.

A keel is a narrow V-shaped extension of the hull along the boat's centerline that helps prevent excessive rolling see Figure 1b.

Because the keel extends down into the water, these boats cannot travel in shallow water the way boats with flat bottoms can.

With their complicated hull shapes, these boats are also expensive to build. Multi-hulled boats, such as catamarans, trimarans, pontoon boats, and some house boats, are very stable due to their wide stance in the water. Each of the hulls can be flat, but usually they are either round or V-shaped.

Multi-hulled boats are usually the most expensive to build. The hull shape is the main determinant of how the boat interacts with the water, but real boats carry structures and cargo above their decks, too.

Structures such as cabins, masts, cranes, booms, and communications towers that are found above the deck are known collectively as the boat's superstructure. All of these affect the boat's center of gravity. Ask students how they think a tall superstructure would affect a ship when strong winds blow from the side. Also ask how a tall superstructure would affect a ship if it rolled to one side due to large waves. If there is time and student interest, you could provide materials such as Popsicle sticks and white glue, and challenge students to make the tallest floating superstructures they can for their boats.

Then you could blow on the boats from an arm's length away to test each boat's seaworthiness. Students should be able to realize that it is necessary to keep the center of gravity as close to the midline of the ship as possible. Once the center of gravity is beyond the deck of the ship, it will tip over just as the towers tipped over once their centers of gravity got beyond their bases.

Ask students where they think heavy cargo should be placed on a ship. Point out that ships carry ballast, or extra weight usually in the form of scrap metal , in their keels for the purpose of keeping the center of gravity low and along the midline of the ship.

You can also ask students to speculate on the comparative keel depths of ships with lots of superstructure versus those with little superstructure. Regardless of the amount of clay students use in the second activity, Buoyant Boats, they should find that in both cases the mass of the water displaced by their clay boat is equal or close to equal to the mass of the boat itself. They are asked to repeat the procedure using a different amount of clay the second time in order to generalize the phenomenon.

This is the principle of buoyancy, also known as Archimedes' Principle. When an object floats, it displaces a volume of water whose mass is equal to the object's own mass. If it can't displace this much water, the object sinks. After completing the activity, students can look back at the water levels they marked on their beakers to verify that the floating boat displaced more water than the sunken lump of clay did, a result that may have surprised them.

Clay, therefore, can be a floater or a sinker, depending on its shape. It is denser than water, so ordinarily it sinks. But it can also be molded into a shape designed to displace a lot of water. Archimedes' empirical observation is interesting, but as an explanation for how something floats it is very limited. It tells us that something has to happen, but it doesn't give us a mechanism that explains why that something happens. In order to really understand what is going on with buoyancy, it is necessary to understand the idea of water pressure.

Think of a large container of water. Water, because it is made of atoms and molecules, has mass, and the mass of the water near the surface pushes down on the water near the bottom. In other words, the water below is under pressure due to the mass of the water above. Actually, the water at the surface is also under pressure, due to the mass of the atmosphere pressing upon it, but this pressure is much lower than the pressure at the bottom of the container. One thing that is interesting about fluid pressure is that it is a type of force that acts in all directions at once.

In contrast to gravity, which only acts downwards, water pressure pushes against any object it contacts, regardless of the orientation or location of the object within the fluid.

What this means is that if an object such as a block of wood is placed in the water, gravity acts to pull downwards on the block tending to make it sink , but at the same time, water pressure acts upwards against the block. The water pressure counteracts the force of gravity -- in accordance with Newton's Laws of Motion -- and allows the block to float. The water pressure provides the buoyant force. To understand this concept, it may help students to think about three blocks, each in the shape of a cube that is one foot on a side.

One block is made of solid wood, and a cubic foot of wood weighs about fifty pounds. Another block is also made of wood, but it has been hollowed out in the middle, so it weighs only 10 pounds. If you put all three blocks in a pool of water, they would all float, since they are all less dense than the water. However, the blocks would not float in quite the same way. The solid block would ride low in the water, as shown in the figure below.

The foam block is so light that only a small amount of water pressure is needed to balance the mass of the block and let it float. The water pressure is slight up at the surface of the water, but since very little pressure is needed, the foam block does not sink very deeply into the water.

The hollow wooden block, however, has to ride lower in the water in order to encounter enough water pressure to keep it afloat. There are hundreds of science fair projects dealing with physical science topics. Use your imagination to try to think of others you may enjoy doing. Manage My Favorites.

Excerpted from. This book contains great information for different kinds of science fair projects. Buy the Book. FutureFit IN. Will a bottlecap really float in salt water? Students will learn about density and surface tension with this printable science activity. They compare how well objects float in fresh water compared to salt water. Looking for project-based learning?

Check out our collection of hands-on projects that combine math, ELA, and science concepts with 21st Century and social-emotional skills! Lesson Plans. Featured Middle School Resources. In other words, if you are thin and muscular and have a low or even normal body fat percentage, you are more likely to naturally sink. Whatsapp: how to tell if your mobile has spyware installed on it.

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