# Buoyancy. Why some things float and others sink

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Why is it that some objects float while other objects sink? Whether the objects floats or not depends on its density, or how much mass it has, compared to the density of water. The force of buoyancy is greatly influenced by the type of fluid that the object is floating in. That is because the buoyancy force is related to the density of the fluid and why an object that is buoyant in one fluid may not necessarily be buoyant in another fluid. As such, the fluids with greater density will have greater buoyancy forces versus less dense fluids. Which is why it may be easier for you float in the salty ocean, then it is in the local swimming pool or freshwater lake.
The buoyancy force can be given an indication of being negative, where the object is denser than water, and indicates that the object will sink. Or positive, where the object is less dense than water, and indicates that the object will float. That's why a bowling bowl sinks in water, while a beach ball with the same volume floats. The bowling ball is much denser than water. The beach ball is filled with air, which is much less dense. The line that indicates the level at which the object floats within the fluid is the neutral buoyancy, where the upward force of the fluid is equal to the downward force of gravity on the object. When an object is at the neutral buoyancy it appears to be hovering in the fluid, think of a SCUBA diver in the ocean. These types of buoyancy also explains how submarines are able to go up and down within the ocean, as they change the density of the boat by moving sea-water into and out of tanks that the submarine has along the outer edges of the boat.
The series of experiments will examine these relationships of buoyancy of objects and densities of fluids. In which students will get a chance to determine the difference in buoyancy offered by two different types of fluids and the buoyancy of an object based on the orientation of the object within the water, lying flat on the surface or pointing at an angle to the surface. And then will get a chance to design and develop a device to hopefully improve the buoyancy of the object in one of the types of fluids tested.
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7 pages
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Teaching Duration
4 days
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