Topics: Buoyancy, Density, Archimedes Pages: 12 (4888 words) Published: February 8, 2013
1.A Little About Buoyancy
oIn order to understand how a boat can float in water we must first go over one of the principles behind such a feat: buoyancy. This principle in physics was discovered by the mathematician Archimedes about 2,000 years ago, so it has been taught and understood for some time now. The basics of buoyancy involve the relationship between the weight of the object in question, in this case a boat, and the weight of the water it displaces. If the object weighs more than the water it displaces, it is going to sink to the bottom. This object is negatively buoyant. If the weight of the object is the same as the weight of the water it displaces, it is going to hover suspended in the water. This is called neutral buoyancy. Finally, if the weight of the object is less than the weight of the water it displaces, such as in the case of a boat, it is going to float. It is positively buoyant. A Little About Displacement

oNow that we understand what buoyancy is, let's take a look at water displacement, since it has so much to do with buoyancy and the floating of a boat. Archimedes is said to have discovered this scientific principle as he lowered himself into a bathtub one day. No doubt you have seen it yourself. When you sit in a bathtub, or lower anything into water, the water level rises, because some of it is being displaced. An object will always displace an amount of water equal to its weight or its volume, whichever comes first. This principle, coupled with the principle of buoyancy, reveals how boats are made to float. How Boats Use These Principles to Float

oA boat is specially designed to displace a great amount of water without being so heavy that it will sink. For one, boats are relatively hollow; much of the inside is air. This makes them much less dense than most objects their size. The large surface area at the bottom of the boat also displaces more water, but does so in a way that the boat does not have to sink too far into the water. Because boats displace a great deal of water and remain light, the weight of the pressure the water exerts on the boat is much less than the pressure the boat exerts on the water. As long as a boat is not overloaded with people and supplies, it will remain buoyant and float.

Buoyancy Activities & Experiments
Buoyancy is an object's ability to float in a liquid. Floating is caused by an upward force known as the buoyant force, exerted against the object in the liquid. Buoyant force is a result of the difference in pressure at the top and bottom of the object. The pressure at the bottom must be greater than the pressure at the top for an object to float. Children can learn about buoyancy by conducting simple experiments using common household items.

A cork will float because it contains air pockets that cause it to weigh less than the water it displaces. 1.
Float the Boat
oA ship must weigh less than the water that it displaces in order for it to float. The air within the ship helps lower the actual weight of the ship so that it weighs less than the water. To demonstrate this, roll a piece of clay into a ball and drop it into a bucket of water. It will sink because it does not have any air in it. Take another piece of clay and form it into the shape of a small cup or bowl. Form a small keel, or a ridge at the bottom of the cup to act as the spine. Place it in the same bucket of water. It should float because it has air pressing against the top of the cup. Orange Peel

oDrop a whole orange into a small bucket or a cup of water. Note where the water is before placing the orange in the water and again after the orange has been dropped. The water should rise and the orange should float. Now remove all of the orange peel and place it back in the water. The orange should sink with the peel removed because the peel contains air bubbles that help it weigh less than the water it displaced. Sink to Float

oFill a clear cup or glass three-quarters of the way full...
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