Displacement explains why objects sink or float

Displacement explains why objects sink or float. Displacement occurs when you place something in a fluid, or any substance that flows, and it moves the fluid out of its way. You can watch displacement at work when you drop an object in a cup of water and the water level rises. Gravity pulls the object down, but the difference in pressure above and below the object causes an upward force. The object pushes the water out of its way, making the water rise. An object will sink if it weighs more than the water it pushes away, and an object will float if it weighs less than the water it pushes away. The Greek mathematician Archimedes discovered that the amount of water displaced by an object depends on the mass of that object. Mass is the amount of matter in a substance, and dense objects have more mass than less dense objects. Dense objects that do not displace much water will sink, while less dense objects that displace a lot of water will float.

Shape can also help an object float. A ball of clay will sink, but a canoe shape made from the same amount of clay can float because it displaces more water. A canoe shape can push more fluid out of its way in relation to its weight. The amount of air inside of an object can also help it float. Boats can float despite the heavy and dense materials used to build them because of the large amount of air inside the hull. Hollow objects, such as table tennis balls or an empty plastic bottle, are able to float better than solid objects.

Many children believe that heavy objects sink and lighter objects float. Point out that some heavy objects like boats float, while relatively lighter objects such as coins sink. Encourage them to hypothesize why objects sink and float. Have them draw on prior knowledge and describe observations they have made of sinking and floating objects. Encourage them to make comparisons and connections.

Most children can easily observe and recognize objects that sink or float, but understanding buoyancy at a scientific level may be difficult for young children. We recommend watching the Sink or Float movie more than once, doing demonstrations or experiments, completing the activities and features together, and leading class discussions to reinforce the ideas in the movie. If your children are more advanced and ready to explore sinking and floating further, we recommend screening BrainPOP’s Buoyancy movie.

http://www.brainpopjr.com/science/forces/sinkorfloat/“http://www.brainpop.com/science/motionsandforces/buoyancy/”
buoyancy puzzlers
http://www.pbs.org/wgbh/nova/lasalle/buoyancy.html

Types Of Hulls Flat bottom boat - These boats are generally less expensive to build and have a shallow draft (the part of the boat that's under the water). They can get up on plane easily but unless the water is very calm they tend to give a rough ride because of the flat bottom pounding on each wave. They also tend to be less stable and require careful balancing of cargo and crew. Examples of flat bottom boats might be Jon boats, small utility boats, and some high speed runabouts. Vee bottom boat - The vee bottom tends to have a sharper entry into the water which provides for a smoother ride in rough water. They do, however, require more power to achieve the same speed. Many runabouts use the vee-bottom design. Round bottom boat - These move easily through the water, especially at slow speeds. They do, however, tend to roll unless they are outfitted with a deep keel or stabilizers. Many trawlers, canoes and sailboats have round bottoms. Multi-hull boat - Catamarans, trimarans, pontoon boats and some house boats use a  multi-hull design. The wide stance provides greater stability. Each of the hulls may carry any of the above bottom designs.

Log boat stone

http://www.onr.navy.mil/focus/blowballast/sub/work2.htm
Procedure

1. Fill your basin, sink, or pool 1/2 full with water.

2. Survey the lightweight materials available to you.

3. In your science journal, draw a design of a boat to float in

the water by using only the available materials.

4. Following your design, build your boat.

5. Place your boat in the water and see if it floats.

6. If necessary, modify your boat design until it floats.

7. Record your observations and design changes in your science journal.

8. Using paper clips, pennies, or other objects, add weight to the boat by adding one object at a time.

Continue until the boat no longer floats.

9. Retrieve the objects from the water and dry them.