buoyancy

what is buoyant force

Buoyant force is an upward force that fluids exert on any object that is placed in them. The ability of fluids to exert this force is called buoyancy. What explains buoyant force? A fluid exerts pressure in all directions, but the pressure is greater at greater depth. Therefore, the fluid below an object, where the fluid is deeper, exerts greater pressure on the object than the fluid above it. You can see in the Figure 1.1 how this works. Buoyant force explains why the girl pictured above can float in water. Q: Youve probably noticed that some things dont float in water. For example, if you drop a stone in water, it will sink to the bottom rather than floating. If buoyant force applies to all objects in fluids, why do some objects sink instead of float? A: The answer has to do with their weight.

weight and buoyant force

Weight is a measure of the force of gravity pulling down on an object, whereas buoyant force pushes up on an object. Which force is greater determines whether an object sinks or floats. Look at the Figure 1.2. On the left, the objects weight is the same as the buoyant force acting on it, so the object floats. On the right, the objects weight is greater than the buoyant force acting on it, so the object sinks.

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density and buoyant force

Density, or the amount of mass in a given volume, is also related to the ability of an object to float. Thats because density affects weight. A given volume of a denser substance is heavier than the same volume of a less dense substance. For example, ice is less dense than liquid water. This explains why the giant ice berg in the Figure 1.3 is floating in the ocean. Q: Can you think of more examples of substances that float in a fluid because they are low in density? A: Oil is less dense than water, so oil from a spill floats on ocean water. Helium is less dense than air, so balloons filled with helium float in air.

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instructional diagrams

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questions

a fluid exerts pressure only in an upward direction.

a. true

-->  b. false

fluid pressure is greater at greater depth.

-->  a. true

b. false

the fluid below an object exerts greater force on it than the fluid above the object.

-->  a. true

b. false

an object floats in a fluid when the

-->  a) buoyant force acting on the object is greater than the objects weight.

b) objects weight is greater than the buoyant force acting on the object.

c) objects weight is the same as buoyant force acting on the object.

d) buoyant force acts only under and not above the object.

a denser substance will

a) float on a less dense fluid.

-->  b) sink in a less dense fluid.

c) become less dense in a less dense fluid.

d) become more dense in a less dense fluid.

diagram questions

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